A Basic “MCAS” System was installed in the Boeing 707 in the 1960s

By Oliver Stuart Menteth

Fintech Aviation Services

Special to Leeham News

Oliver Stuart Menteth

Nov. 1, 2019, © Leeham News: It was simplistic in design, had a graduated and limited input into the pitch control system and details were included in the relevant training, operational and maintenance manuals.  It worked seamlessly and because of this its existence, and reason for installation, has eluded most people in the industry.  The system was installed not as a result of a recommendation or regulation imposed by the FAA but at the insistence of the Air Registration Board (now the Civil Aviation Authority) of the United Kingdom.

But first, some relevant background.

The 737 Program

The grounding of the 737 MAX aircraft will have permanent and wide ranging repercussions throughout the global aviation industry as regulators seek to understand in greater detail the underlying assumptions utilised by the FAA and Boeing during the certification of the MCAS (Maneuvering Characteristics Augmentation System) architecture.   The regulatory system governing the flow of information required by the international bi-laterals when certifying new products will, and already has, come under intense scrutiny, with the result that delays and additional oversight of other programmes will now invariably result.

The 737 program has long been viewed as Boeing’s dependable cash cow and over the years it has permitted the Company to develop and invest in other very cash hungry models which serve relatively small markets.  Launching and delivering the MAX was an absolute priority to the Boeing company not only from a financial perspective but also because its competitor, the Airbus A320neo, was amassing orders and increasing its own market share.

In order for the MAX to be accepted as a certified 737 variant it had to retain similar flying characteristics and, to a large degree, the same systems to ensure that a separate type certificate was not deemed necessary. If the FAA determined that there was not sufficient commonality between the MAX and the NG, its predecessor, then Boeing would have been forced to spend billions of dollars certifying what on paper was, except for notable exceptions, another 737 variant.

This re-certification exercise would have vastly reduced the amount of profit per delivery position compared to the current 737 model, as a significant proportion of the 737 MAX research and design costs had already been amortised throughout the 737 NG production run.  If it were determined that a new type certificate was required, then this would have most likely rendered the current programme un-economical and Boeing would have been forced to re-consider an all new short haul design.  Consequently, from a commercial perspective the need to retain as much commonality as possible, yet provide a competitive model, was absolutely critical.

MCAS

Much has already been published about the 737 MAX MCAS system which was introduced to effectively nullify the control handling differences between the NG and MAX, at specific attitudes, weights and C of G.  Compared to the 737 NG the MAX requires increased thrust, which in turn dictated a larger engine nacelle.  Boeing rejected the idea of extending landing gear oleos so the larger engines could only be accommodated by extending and raising the pylon.  The net effect of this displacement resulted in a corresponding forward shift in the Centre of Pressure at higher angles of attack, typically encountered when approaching a stall.  We understand that this effect, which only occurred during specific flight conditions, has the tendency to implement a pitch up motion, which was not experienced on the NG.

This particular aerodynamic ‘pitch up’ effect on jet transport aircraft is not new.  One should remember that the 1950s and 1960s, which saw the introduction of the high speed wing coupled with different aircraft configurations, represented a period when aerodynamicists and pilots, sometimes to their own cost, learned practically all there was to know about high and slow speed flight.   At that time, as an industry, we also learnt about how those systems which assist pilots to operate throughout the flight envelope in a safe manner, can be implemented.

Additional Certification Requirements to the 707 by the UK CAA

Prior to the introduction of the European Aviation Safety Agency (EASA) each country, should it elect to, was responsible for approving each model and variant destined to be operated by their own domiciled carriers.  As a result of its own aircraft manufacturing industry the United Kingdom (UK) was in reality the only other country in the world that could effectively complete an independent airworthiness review of an aircraft.   Each foreign aircraft that was to be registered with the UK authorities underwent a review of its design and, equally important, was also subject to rigorous flight tests.

In his book Handling the Big Jets (first published in 1967) D. P. Davies, formally head of the flight test department of the UK Air Registration Board (ARB), now the CAA, describes at length the different stalling characteristics exhibited by aircraft with different wing and tail configurations.  He personally test-flew every new jet aircraft model that was destined to fly with UK operators, such as BOAC and BEA.  Davies also describes the various control systems that were incorporated on both British and American aircraft that reduced the possibility of a line pilot entering a stall.   Details of flying characteristics and the use of automated control systems at high angles of attack are also included.

Davies is best known for mandating the fin of the Boeing 707 be lengthened by approximately 36 inches in order to achieve published Vmcg speeds (minimum control speed on the ground).  Boeing ultimately accepted Davies recommendation and, we understand, halted production of the 707 for 10-11 months, whilst a new fin was designed and installed.  All existing 707s were then retrospectively modified.  A ventral fin was also installed on some variants to prevent ‘fully stalled take-offs’ which the Comet was also susceptible to.

Whilst test flying the 707-400 series at Renton, Davies noted that with the first stage of flap selected, the aircraft had a tendency to pitch up just prior to entering the stall.  This was determined to be caused by the inboard leading edge devices, peculiar only to the 707-3/400 series which, when extended, effectively retained lift to higher angles of attack, and moved the Center of Pressure forwards, causing the nose to pitch up.   Davies was not comfortable with this pitch up tendency and insisted that all UK certified 707 series aircraft were modified with the installation of a ‘stick nudger’ system.

US Air Force VC-137s (modified 707) designated Air Force One, Pima Air & Space Museum. Source: Author

In his book Davies stated that “The stick nudger introduces a small force into the elevator circuit which imposes positive stick free stability and removed the otherwise self-stalling tendency”.  He goes onto to say that “as its input is so small all the runaway cases are completely innocuous.”

The ‘stick nudger’ should not be confused with a ‘stick pusher’, which is an entirely different system with a different objective.  The stick pusher is typically installed on aircraft with T tail configurations which are susceptible to “super deep” stalls, such as the Bae 146 and Bac 1-11.  All those Boeing 727s on the UK register were also modified with a stick pusher.

The B707 System

Reference to the relevant B707-320 Operations Manual, dated January 1974, reveals the following schematic which details the anti-stalling systems, which also included a Stick Shaker.

In the case of the B707 the “stick nudger” consisted of an electrical linear actuator connected to the rear elevator quadrant.  When activated at higher angles of attack, as sensed by the underwing vanes, the actuator would begin to wind and effectively back drive the control column forward, resulting in a maximum force of 15 lbs (7Kg).   The system did not have any cut out switches. In the event that the actuator needed to be isolated a single 5A DC circuit breaker was connected to the Essential Radio Bus.

B707-320B/C Operations Manual January 1974. Source: CX Flight Ops Manual B707 1974

The system was energised when the flaps were not up, i.e., the opposite to that of the MCAS system installed on the 737 MAX, which only works when the flaps are up.  The system was activated when the stick shaker relay was energised on the Captain’s side.

System Differences

There are fundamental differences in the design of this system compared to that of the more advanced 737 MAX.

  • The stick nudger system required the input of both Lift Transducers (vanes located on the lower surface of each wing).
  • The system was implemented into the elevator control circuit and not the horizontal stabilizer system.
  • The nudger “activated” once only. There were no repeat control inputs potentially magnifying the loading on the control column.
  • After the nudger had activated the forces acting on the control column could effectively be trimmed out by adjusting the horizontal stabilizer. The pilot could also easily physically overcome the control column forces.   Should the electrical actuator suffer from an un-commanded signal the limited control input permitted ensured that the pilot would be able to easily recover from the resulting downwards pitching moment.
  • When activated the nudger’s forward movement of the control column was un-mistakable. Given that the 707 had stabilizer trim wheels, which provided visual confirmation of the activation of the stabilizer trim, it would be very difficult for the pilots to confuse the two systems.
  • The nudger system was checked as part of the crew’s pre-take off stall warning test.
  • The nudger system was presented and incorporated into all relevant aircraft operations, training and maintenance publications.
Summary

The article illustrates how an un-wanted aerodynamic effect, which only occurred in particular variants of the B707, during a particular phase of flight, was eradicated in a safe and logical manner.   The two aerodynamic anomalies experienced on the 707 and 737 MAX are not exactly the same however, given the information released to the public, it is fair to assume that there are similarities between the two resulting effects.

It also demonstrates that although regulatory bilaterals should ease the administrative burden and reduce the overall cost of certifying a new product to varying regulations, there is no substitute for competent authorities to physically test fly each aircraft.  Flying an aircraft and understanding its handling characteristics, and how they compare to another variant, is very much a tactile and individual experience, and should be undertaken by more than just one authority.

In our opinion, the MAX will be the last iteration of a very successful 737 program.  The continued grounding of the MAX variant has proven to Boeing, and the industry at large, that in order to continually produce a technological, cost efficient and commercially competitive product it is simply un-realistic to expect iterative re-designs to provide the ultimate solution.

In this respect Airbus has a distinct advantage with the A320neo family, who’s digital flight control software can be amended, to a limited extent, to take into account varying aerodynamic profiles.   Airbus’ decision to incorporate a fly by wire control system in the 1980s was a bold but ultimately wise decision.

It is still not clear when the 737MAX will return to service or what additional modifications will be required by the FAA and EASA.   The various investigations underway internally within Boeing and separately by the US Government and associated parties, such as EASA, will ultimately bring about seismic changes to procedures.  The lessons learnt during this period will only make current and future generation aircraft safer for the travelling public however we should also remember that many of the problems encountered today were first resolved 60 years ago.

Oliver Stuart Menteth

Managing Director

ISTAT Certified Senior Appraiser

Fintech Aviation Services Sarl

www.fintech.aero

 

 

127 Comments on “A Basic “MCAS” System was installed in the Boeing 707 in the 1960s

  1. Much weight was put on No Interface changes ( to the pilots ).
    Beyond other reasons ( customer expectations ) to avoid certification cost.

    In a contrasting move ( coming to light via various hickups ) Boeing seems to have revamped a vast amount of details large and small on the MAX vs NG.
    Tail structure, engines, partitioning of avionics, pickle forks, …. ( what else ?).

    Were these changes ( obviously requiring certification attention ) just “hand waved away”?

  2. IMO, Boeing is reaping the consequences of a strategic planning failure that has been 30 years in the making.

    This is a comment I wrote in this blog on February 25, 2015:

    In short, Boeing IMJ is now reaping the consequences of poor strategic planning. Soon after Airbus launched the A320, Boeing started to market the 7J7 which was targeted squarely at killing the A320 with technology radical by the standard of the 1980s and 1990s. Realising by the early 1990s that the A320 was a serious threat, Boeing was forced to respond. The strategic mistake, IMO, was to launch the 737NG instead of an all new single aisle family, however successful the 737NG family turned out to be. The major mistake IMO was the senior Boeing management’s failure to envision that their single aisle platform would be inferior to the born-to-be-re-engined-with a-singnificantly-larger-fan — A320 — if advances in technology in the not too distant future would allow for much higher by-pass ratios for short-haul, single aisle aircraft.

    The original 737 was born in an era** in which high by-pass ratios were not looked at as very desirable for short haul aircraft. It may look as if Boeing’s senior management believed that would continue to be the case for a very long time when they decided to go ahead with the 737NG. Of course, other factors played a role as well, Southwest Airlines wanted maximum commonality with their installed fleet of 737-300s, while Boeing probably wanted to maintain the fuselage commonality with the 757. Perhaps one could say that the streak of strategic mistakes for Boeing started with the 757, by not designing it in the first place, with an all new and wider fuselage. Why go through all that hassle of designing what was basically an all new aircraft, and then insisting on using a fuselage that was designed at the beginning of the jet age, while not being able to provide the same 18-inch seat width as the 767, which was being developed at the same time. Yet, Boeing still managed to design the 737NG; a platform that would have very little commonality with the 757.

    https://leehamnews.com/2015/02/24/order-cycle-may-have-peaked-for-mainline-single-aisles-but-smaller-jet-cycle-booming/#comment-93698

    Comment further downthread:

    There was nothing IMJ that would have prevented Boeing from developing an all new single aisle, A320 competitor, in the 1990s. It should IMO have been fully fly-by-wire and developed with full cockpit commonality with the 777.

    What Boeing’s managers seemingly decided to do instead — back then — was, of course, not much different from any other business that follows the Anglo-Saxon shareholder value model, where the sole legitimate purpose of business is to create shareholder value. So, saving a few billions up-front makes investors happy, but doing so may lead to some long term negative repercussions for the company.

    • Boeing did create the B757-200 in the single aisle market which was somewhat successful full and certainly respected. There was a B757-100 study for approx 160 passengers. Despite what would have been spectacular performance I suspect the airframe, having over 4000nm range, would have been too heavy to be optimal for the kind of routes the B737 mostly flies which would be under 1000nm.

  3. Excellent article.

    Stick pushers are well documented. If the MAX was subject to a stick pusher I would have kept my mouth shut. But then 346 people would still be alive if MAX was simply subject to a stick pusher.

    So we come back to the elevators. Stick pushers use the elevators. MCAS uses the trim stabiliser. We have not been told why? We can guess the elevators don’t work. But Boeing should tell the truth.

    The real question for me is when the pitch up tendency starts. We were first told that it was at ‘extreme’ AoA. Then ‘high’ AoA. Now the word ‘elevated’ [AoA] is being used. In other words, the pitching moment [curve] is not typical, it’s atypical. But how atypical? Again, Boeing should tell the truth.

    I’m not comfortable with atypical pitching moments. If it’s minor and in the extreme part of the envelope then a stick pusher works. But I still prefer aerodynamic changes. If it’s a more severe atypical, pitching moment, I think aerodynamic changes are necessary.

    But then I come to fail-safe redundancy. Stick pushers have fail-safe redundancy because the elevators have fail-safe redundancy. The MAX trim stabiliser does not have fail-safe redundancy and the changes to MCAS will not introduce fail-safe redundancy, at least not according to the true definition of fail-safe redundancy. The true definition requires end-to-end duplex/triplex control systems with failure conditions isolated to allow change over. The action/reaction loop must be timely . Not there even with the new MCAS.

    I will end by pointing people at FlightGlobal. FlightGlobal are now saying there are e-mails addressing concerns about the fail-safe redundancy of MCAS in 2015. This is before the power of MCAS increased by a factor of 4.

    • @ philip and others:

      The reason MCAS uses the horizontal tail and not the elevator is that the elevators on the 737 under certain flight conditions (such as a jerky go-around) are too small to have sufficient authority to provide pitch control (this is true for the NG, and the MCAS system tried to addressed this issue by inducing more control authority by changing tail incidence angle. MCAS also improves longitudinal stability/control for the airplane at higher speeds). Simply put, the elevators alone lack the control authority to pitch the nose of the airplane down under certain flight conditions, leading the airplane to stall at the worst possible moment (near the ground while attempting a TO/GA). This has led to at least two 737 NG crashes that until recently were chalked up to being a product of pilot error made by “third world pilots”.
      One could hope that sooner or later this issue will be discovered and addressed with a real and robust fix, instead of the current dubious software fix. I would avoid flying the NG or the MAX in the meantime.

      • Your assuming it’s at the extreme parts of the envelope. What if it’s not? All of the indicators are that it isn’t in the extreme part of the envelope.

        But we don’t know, Boeing won’t tell anybody. Hence, EASA demanding they fly the airplane with MCAS off.

        Anyway resize the stabiliser and elevators if what you are saying is true. It would be done and dusted by now.

      • Please respond to this observation. This seems extremely significant! Is it correct?

        • I can’t argue with A Jones. I offered a theory awhile back saying the same thing. But I think the MAX is worse than the NG.

          So I think, with regard to the MAX, the conditions don’t have to be extreme.

          • All indications are this is only at the extreme edge of the envelope.

            Any opinions to the contrary disagree with aerodynamic experts.

            Now Steven Hawkins could argue with Einstein, but I sure would not.

  4. Can I also thank the writer for pointing out that the issue is the Centre of Pressure (Centre of Lift) not the Centre of Gravity. I’ve been alone on this web-site with regard to correcting everybody.

    And then I was also reminded that there were aerodynamic changes to the 707 that stopped production 10-11 month. Why didn’t Boeing repeat that behaviour with the MAX? Specifically, Boeing should have accepted a production delay until a new bigger stabiliser and new bigger elevators were designed and certified. Modern design and production systems would have kept the weight down and improved the aerodynamics.

    So we come to Boeing’s history. The MAX isn’t Boeing’s history. Boeing need to return to their history, a history of engineering excellence.

    My message to Boeing. Take the hit. Implement aerodynamic changes. Make the MAX safe.

    • “” the issue is the Centre of Pressure (Centre of Lift) not the Centre of Gravity “”

      But MCAS moves the stabilizers and that turns the MAX around the CoG.

      If Boeing tested MCAS on a MAX-7, the effects are bigger on a MAX-9, because the distance between stabilizer and CoG is bigger.
      I hope EASA won’t only flight test a MAX-7. I wonder how much more powerful MCAS is on a MAX-8 and MAX-9 than on a MAX-7.

      • I agree. So yes I agree the CoG changes with AoA. That’s basic maths. But AoA changes with Centre of Pressure/Centre of Lift (CoP/CoL). So it’s the CoP/CoL that governs stability with the same wieght distribution.

        • So yes I agree the CoG changes with AoA.

          This is fundamentally wrong. The CoG is in the same location as the CoM since we can safely assume that gravitational variations are negligible over the size of the airplane. CoM is a mass property which does not change with AoA. It only changes if the distribution of mass on the aircraft changes, e,g. if the configuration changes (flaps, slats, gear), fuel is consumed/moved, passengers move about, cargo shifts etc..

          But AoA changes with Centre of Pressure/Centre of Lift (CoP/CoL).

          In my opinion this is not the conventional way of looking at it. The conventional way is to say that CoP/CoL changes with AoA. This is how aerodynamic results are presented when AoA is the independent variable. Changes in AoA ultimately result from changes in CoL because the pitching moment changes with CoL and those changed moments will rotate the aircraft around its CoM.

          But AoA changes with Centre of Pressure/Centre of Lift (CoP/CoL).

          I agree that this is correct, even though how you got here started with something fundamentally wrong.

          • Darn it! The last block quote in my above comment should read:

            So it’s the CoP/CoL that governs stability with the same wieght distribution.

            I agree that this is correct, even though how you got here started with something fundamentally wrong.

          • Give it a break.

            The horizontal weight distribution is dependent on the horizontal. The horizontal weight distribution is dependent on the AoA for that changes the horizontal.

            The centre of gravity is not the point of rotation. The neutral point is the point of rotation. Try and remember that. The neutral point is behind the centre of gravity and close to the centre of lift. The neutral point is the centre of lift with a symmetric body.

            Please remember gravity must be considered a vertical force regardless of rotation. But lift is a force dependent on rotation.

            One day you will get there, but not today.

            But thanks for the lecture

          • I have learned the following: As the airfoil angle of attack changes, the pressure field changes. Due to this, the center of pressure changes with variation in the angle of attack. In the airplane’s normal range of flight attitudes, if the angle of attack is increased, the center of pressure moves forward; and if decreased, it moves rearward.

            I believe to have read that on the MAX the need for MAX is limited to far aft COG and very light loaded aircraft. If this is correct the need for MCAS is nor present in normal passenger operations. It should be possible to operate without MCAS provided (very limited) loading restrictions are imposed.

            If you remember the July 19 LHN story about the A321 with ‘pitch up tendencies’, it was a similar operating problem. The A321 had all the talked about redundancies (!) in place, – but the software (!) was inadequate/wrong for the model. The problem was temporarily solved with imposing loading restrictions – Lufthansa, for example, removed the aft seat row. Software fix comes next year.

          • Philip,

            The centre of gravity is not the point of rotation.

            Stop the presses!! We have to re-write all the physics and engineering books because you say the CoG is not the point of rotation. Remember that an aircraft is not constrained in motion.

            The neutral point is the point of rotation. Try and remember that.

            In the context of aircraft longitudinal stability, the only time the neutral point is the point of rotation is when the CoG is at the neutral point. The aircraft will be neutrally stable when the CoG is at this point, hence the name neutral point. The aircraft is stable if the CoG is forward of the neutral point and unstable if the CoG is aft of the neutral point. In general the neutral point is not the center of rotation. When an aircraft is in flight (not constrained by e.g. having gear on the ground), the CoG is the center of rotation.

          • Mike Bohnet

            I’m having so much fun.

            On the basis that your right. If the nacelle is providing a significant amount of lift, what happens. Remember the engines are well ahead of the CoG. The CoG is typically 25% chord.

            You can tell me that I’m rewriting the laws of physics, again, if you want.

            I just want you to admit the detrimental affects of the nacelle.

            Moment curves can be typical or atypical.

            The CoG does move aft with AoA if the axis is the direction of travel. It all depends on axis. I was using the world axis to keep it simple.

            Finally agree, the CoP/CoL controls stability. Only took a year for I’ve been banging on about it since just after the Lion Air crash.

            Have fun. You do deserve to be tormented.

          • I frequently travelled on the Fokker F28
            If plane is not full passengers were requested to take a seat in the rear of the cabin to displace the CoG
            once take-off was done crews will allow us to go to our assigned seat
            When plane was full (not so often in those days early in the 80’s) the take off was OK

          • The hoot is it makes no difference.

            If you actually mange to stall a 737, A320 (or an A330) the answer to a real pilot is the same, dump the nose.

            Per a Boeing Sim Trainer, the mantra is Unload, unload ,unload (dump the nose)

            And there is NOTHING that says that does not work in the MAX, The Jenny or even the original Wright Flyers (as iffy a plane that was )

    • @philip: You have not been alone on the COL matter. I have been discussing that question several times in various threads. I believe that a key problem is the position of the engines IN FRONT of the wing. This means that with increasing AOA the flow over the central part of the wing is substantially disturbed/ changed/ impacted, making the MAX feel really “funny” on the elevator. (Please bear with me as I try to translate my German pilot speak into English). And this weird flying characteristics over a large part of the normal flight envelope would certainly not be acceptable, which is probably also the reason why EASA was not offered to test-fly the MAX without MCAS. The whole world would then know that the king has no clothes.

      • I think you’re right. This is a head on picture of a MAX from Bjorns corner:
        https://leehamnews.com/wp-content/uploads/2018/11/737NG-vs-MAX-nacelles.png
        Here are some side views:
        http://www.b737.org.uk/737max.htm
        At zero angle of attack the top engine nacelle is definitely over and above the stagnation point of the wing leading edge. At 10 degrees AOA the intakes would be well above the wing. At that angle these huge bell mouthed intakes would be scooping in huge amounts of air. The effects would be quite complicated.

        I’m curious as to what would happen if Pratt & Whitney produced w PW1100G with NOR increased from 12 to say 18 and Airbus did the same thing, Mount the engines ahead and above the wing how well the A320 would cope. It might require an all flying tail plane.

    • Mike Bohnet

      I was going to wait for your reply, but I’ll put an end to the fun.

      All you do is try and trash me. And I got bored with it.

      I clearly stated that the axis of rotation is around the CoG for I clearly stated it was for the same weight distribution. You had a go at me.

      So I deliberately reversed it. I said the axis of rotation is around the CoL. You still had a go at me.

      Indeed you said I was rewriting the laws of physics. No I wasn’t. If ‘A’ is said to rotate about ‘B’ then ‘B’ can be said to rotate about ‘A’. That’s the laws of physics!

      But I accept that the reference axis for longitudinal stability is around the CoG. I’ve accepted it for 40 years.

      But I don’t have to. I could move the reference axis to the centre of the universe if I wanted to and measure rotations from there. But then I don’t know were it is.

      Anyway you contribute nothing to the debate about the 737 MAX. All you do is trash anybody who says there is a serious problem with the 737 MAX.

      My posts have been entirely vindicated by the JATR report and now by the Lion Air crash report. The Ethiopian Airlines crash report will be no different.

      Remember if ‘A’ rotates about ‘B’ then ‘B’ can be said to rotate about ‘A’. In other words I can mess with you anytime I want to.

      Contribute to the debate, don’t trash.

      • Philip,

        So someone responds to your comment because they probably wanted clarity:

        “” the issue is the Centre of Pressure (Centre of Lift) not the Centre of Gravity “”

        But MCAS moves the stabilizers and that turns the MAX around the CoG.

        And you answer with this:

        I agree. So yes I agree the CoG changes with AoA. That’s basic maths.

        I call you out on it, and then you claim that the neutral point is the center of rotation.

        The centre of gravity is not the point of rotation. The neutral point is the point of rotation. Try and remember that.

        Along with a personal attack:

        One day you will get there, but not today.

        I then point out that the neutral point really has nothing to do with the point of rotation and you respond:

        The CoG does move aft with AoA if the axis is the direction of travel. It all depends on axis. I was using the world axis to keep it simple.

        Along with another personal attack:

        Have fun. You do deserve to be tormented.

        So which is it? The “direction of travel axis” whatever that is (I’m guessing you mean that it moves with the aircraft), or the world axis which does not. For your world axis, the CG of most aircraft I’ve ever seen from the ground moves forward at considerable speed. And your “direction of travel axis” would not work out very well for high performance aircraft where AoA’s can approach 90°.

        Then you say you were clear:

        I clearly stated that the axis of rotation is around the CoG for I clearly stated it was for the same weight distribution.

        But you first responded to me by saying this:

        The centre of gravity is not the point of rotation. The neutral point is the point of rotation. Try and remember that.

        So much for clarity.

        And then finally you claim you were just messing with me all along:

        So I deliberately reversed it. I said the axis of rotation is around the CoL.

        Also:

        Remember if ‘A’ rotates about ‘B’ then ‘B’ can be said to rotate about ‘A’. In other words I can mess with you anytime I want to.

        You say you accept that the reference axis for longitudinal stability is around the CoG. Well that axis is also fixed on the aircraft as well (rotates with the aircraft), not fixed in the direction of aircraft travel.

        It doesn’t seem at all like you clearly answered the other commenter’s question or that you were messing with me from the beginning. It rather seems to me that you answered it incorrectly, and then backpedaled when I called you out on it.

        You say your posts are completely vindicated by the JATR and LionAir crash reports, and will be by the Ethiopian crash reports when it comes out. Anyone can claim this if they don’t clearly enumerate why its true.

        You say I’m trashing you, and that I don’t contribute to the discussion but just trash others? I guess your definition of trash must be when someone points out that another person is wrong? If that’s the case then you better put on your big boy pants. When someone responds to disagreement with personal attacks, like you’ve done toward me and others repeatedly in the past, I relish calling them out when they are wrong about something.

        Thank you. Watching you backpedal and squirm has been an entertaining diversion.

        • I say it for a year and you failed to agree with me. I reverse it on purpose and you still didn’t agree with me.

          Sorry, no squirming. Just needed to prove that you simply behave like Muilenberg. You say whatever is needed to trash people.

          Explain to the world why you told me I was wrong for an entire year.

          • Philip,

            I’ve always only ever maintained that the slope of the pitching moment curve determines stability according to the well documented textbook longitudinal static stability analysis that can be easily found on the web or in actual textbooks. Since the position of the CoL is one of the determining factors of the pitching moment curve slope, the CoL position obviously determines stability.

            I haven’t been telling you you’re wrong about the role of CoL position over the last year although I feel you’ve been wrong about a lot of other related things. Since you’re accusing me, I suggest you get cracking and find the evidence that supports your accusation. I’ll be patiently waiting.

        • For a solid body the principal axes of interia goes thru the center of mass, so it gives that aero forces moves the Aircraft around the center of mass/gravity, the only way to have center of mass/gravity to move is to shift mass around, not by changes in pitch unless the mass of fuel moves aft. Boeing introduced the MCAS because of non linear stick forces at aft c.g. limits and angles of attack over a certain limit. We don’t know at what aft c.g. and alfa it gets outside FAA certification limits. We will see if Boeing software and MCAS run horisontal stabiliser can compensate for a too small elevator in the end.

    • Back then you did not have 1000’s of software engineers telling management “It is just a software fix”. PWA tried a software fix for their stalling PW4000 before they did deep surgery and solved the problem with a combination of hardware and software. There is a chance one of the other parts of “United Aircraft corp.” need to do the same.

  5. Engines placement on Max is said to be due to short main landing gears ..
    Please explain me why Boeing did not extended the main landing gear therefore installing the engines in a safer COG situation !!
    Such extension is included in the revised design of this element for MAX-10 to help safe take-off !!

    • MAX-10 via trailing lever articulation has a softer spring constant for the MLG.
      i.e. with the same amount of lift (compensating weight partially) it will extend the gearleg further than the regular oleo.come rotation the fuselage will sit nn inches higher already.
      ( The interesting trick is to fold the trailing arm when the gear leg is folded into the belly. )
      Wheels in the wheelwells nearly touch when folded.
      Longer legs without intricate mechanics would demand the attachment point being moved outwards. This is undesirable for keeping the historic certification environment.

    • in order to maintain a number of existing 737 certifications, extending the length of the MLG at rest is not viable:

      1: the 737 is certified with no escape slides for the overwing exits, because the wings are low enough to the ground that with flaps extended, the flaps act as a slide. raising the gear would necessitate inflatable slides with the associated cost, weight and certification penalties

      2: lengthening the primary MLG leg structures would require major changes to wing and gear bay structure as well as a full redesign/recert effort for the MLG

      3: customers would lose a lot of the parts/maintenance commonality benefits between 737NG and MAX

  6. Basically, the fundamental cause of Boeing’s troubles today lies in the high management’s desire to keep the Boeing share ascending instead of giving priority to safety. End of story. Now, the deep damage inflicted to the brand will never , never be corrected, whatever corrective action Boeing tries to implement. Very, very sad for a superb company.

    • I don’t know how ignoring security issues makes shares ascending in an aircraft company…
      It is clear that they put a product out they did not even know how it will behave.
      This not some sneaky malevolence but blatant incompetence. This is a school level issue of how to manage a project. The rot is much worse than just greedy $.

      • To fight against the dangerous A320 Boeing had to decide quickly after American Airways ordered a bunch of A320
        Decide fast Yes … but also design and produce very fast with seamlessly EIS = minimal training of crews
        In short do not spend monies on “trivial” things
        It save money for the company and makes shareholder so happy that they buy many shares so the price goes up !!!

        • How an airplane can fly without falling from sky ” not spend monies on “trivial” things” ?

          They are not even competent to to make share prices go up.

    • Don’t think Boeing management want to compromize safety, they are just not as skilled engineers as in the older days and took safety for granted while pushing stock price, speed to certification and cost reductions. They did not see the long time effects on safety for a series of decisions made since the 1980’s. If EASA had as skilled and experienced engineers/test pilots as D.P. Davies during the 707 certification they would have caught it on the 737MAX. But this time I don’t know if EASA even flew the 737MAX or just rubber stamped what FAA did as per bilateral political agreements.

      • Bean counters at the wheel?

        They are paid big buck to run the com,pan, that means knowledge about the product and none of them are aircraft virgins.

        Did they deliberately try to kill people, no.

        But, they laid the groundwork for it to occur and to this day they insist despite all the evidence and panels that say others, that the FAA relationship is just fine.

        I find it hard to argue a razor point that when you know better and insists you want to do things the same way then you truly have pushed over the edge into deliverable, all that fake lovely farm stuff aside.

        Was it the farm MO to cut corners?

        • Think both Boeing and FAA really tried to move fast and help each other but the top level management engineering skills at Boeing Comercial Aircraft needed to check the evolution of the 737 one more step was not really there at Boeing nor at the FAA. The Boeing CEO don’t have that much influence besides making sure internal funding is available, customers are happy to pay the asking price for its delivery slots, FAA is happy and that Boeing quality checks that all internal rules and policies are followed.
          If the chief engineers are not up to the task is not something a “regular MBA CEO” can discover. You need someone like Dutch Kindelberger, Kelly Johnson or maybe Norm Augustine for that.

    • ” When I say I changed the culture of Boeing, that was the intent, so it’s run like a business rather than a great engineering firm. It is a great engineering firm, but people invest in a company because they want to make money.

      Harry Stonecipher, 2004, former CEO of The Boeing Company,
      reflecting on the late 1990s as found in the book

      “Emerging fromTurbulence ” which is a 20 year follow on to the iniktial book “Turbulence “about 20 years previous. The book is/was a description- survey of Boeing after the mcDouglas buyout.

      A lot of info- interviews from the worker -bees and some mnagment of
      Boeing

      • “.. but people invest in a company because they want to make money. ”

        Wagging the dog.
        Boeing did all moves to _appear_ as a successful producer of perfect airplanes.
        Lots of things are done for faking looks and not for good engineering solutions.
        Share buy backs are supposed to be done when you have an overflow of money and no good sink beyond buying shares.
        Looking back Boeing could have been better of spending that money in new, better products.

        Same for selling the prototypes at all cost: a very expensive move to hide hideously expensive specimen in the deferred production basket.

        • Bubba:

          Build it and they will come.

          You put out a great product (including safe) and oleo literal flock to your company.

          The essence of an Aircraft Company has to be safe, otherwise its a business wreck.

          Software /tech companies get a free ride due to the law they imposed and mostly non one is killed (or in low numbers, Tesla comes to mind)

          To argue its just another business is a fallacy. It lives and or dies on safety and the bar has gotten nothing but higher and higher.

          250,000 people killed a year due to hospitals, ho hum

          346 in two air crashes, world ends.

          Lesson: if you just want to run a business and not worry about killing people , hospitals are a good one to be in.

  7. Would a stick nudger have passed the two stall tests MCAS was designed for?
    Did the MAX have to do a stall tests with flaps down like this 707?
    How many different stall tests are required other than the two associated with MCAS?

    • Adding a stick pusher would be an interface change, right?

      Would that be accepted without proper simulator time for crews?
      I think not. return to START 🙂

  8. One of the questions I have is: Boeing is using a MAX7 for the test flights to regain certification. But both crashes were in MAX8s.
    They are both fitted with MCAS, new engine and pylon, and should be quite similar. But not aerodynamically the same.
    In the example, not all 707s exhibited the same behaviors. Why is the FAA allowing the MAX8 to be ‘proven’ with a MAX7?

    • Source for the MAX-7 as certification airplane?
      Also, why is the FAA not demanding to fly the MAX with MCAS off?

      • I read that flight tests are made with a MAX-7, it makes sense because it’s cheaper.
        I read Boeing repeated 30% of the MAX-8 flight tests with the MAX-9. In service were mostly MAX-8 and only few MAX-9.
        If 0.6 and 2.5 degrees MCAS were tested on the MAX-8, the degrees could be smaller on the MAX-9, but I never read about different degrees, only 0.6 and 2.5. These degrees have less effects on the MAX-7 because the distance between stabilizer and CoG is smaller.
        It’s the same with this article, only the 707-300 and 707-400 were effected, not the smaller ones.
        If Boeing wants to hide critical behaviours it offers a MAX-7 for certification flight tests because 0.6 and 2.5 degrees MCAS have less effects on a MAX-7.
        EASA should test with a MAX-9 and since Boeing is hiding with other variants too.

        • They are only test flying the Max 7 as it hasnt been certified yet and need to complete that program as well. The new software will have to be tested for all the Max variants, including much longer 9 and 10.

          • The same logic applies across the line of builds as there is no difference in how MCAS 2.0 will work.

            Dump the nose and no issue.

        • Leon,

          These degrees have less effects on the MAX-7 because the distance between stabilizer and CoG is smaller.
          ———————–
          If Boeing wants to hide critical behaviours it offers a MAX-7 for certification flight tests because 0.6 and 2.5 degrees MCAS have less effects on a MAX-7.

          By the same token the destabilizing contribution of the engine placement is worse on the MAX-7 than on the MAX-8 or MAX-9. So, which model is truly easier to certify, or slip past the regulator as you seem to be suggesting.

          • Wings and engines are the same on MAX-7, -8, -9. so the CoL at level flight should not be very different.
            The distance between the stabilzer and CoG is much bigger than the distance between CoL and CoG.

            EASA should start testing with the MAX-7 with and without MCAS, so they know what to expect and might not crash into the sea, and then check MAX-9.

          • Leon,

            All other things being equal, a shorter distance between the tail and the CG means less stable.

          • Mike,

            the distance between stabilizer and CG on the MAX-7 must be stable enough, otherwise bigger stabilizers and elevators would be needed. The MAX-7 is even longer than the 737-700.

            The powerful MCAS was the killer, each degree on the stabilizer has much more effects on the MAX-9 than on the MAX-7.

            Better would have been with bigger stabilizer/elevator and without MCAS. We know why this wasn’t chosen.

        • One obtains real data at the least at the extremes, intermediate real data at inflections of calculation or likely problematic areas, and leaves for interpolation only if safe to do so. At best MAX-9 is not a prioritary, non skipable, data collection point. Under the circumstances, we are, or should absolutely be, beyond time or cost issues.

  9. Always get some one esle to check your measurements.
    At some point Boeing must have realised that they were gambling.
    Another excellent article.

    • One mans major stalling problem was another mans no issue.

      The Brits build some of the worst handling aircraft of WWII, the Moquito in the air was a wonder, taking it off with two engine turning the same direction, a killer. The US solved that easily on the P-38 (good engine design , miner change in assembly and you were countered rotation)

      Tempest was lethal as well.

      Bureaucracy trying to prove it was relevant in the post WWII years.

      Without MCAS 1.0 we would have had no crashes.

      • The Mosquito had excellent handling despite the fact that it the propellers rotated in the same direction. You are probably thinking of the Beaufighter which had a nasty swing on take-off. Everyone knew that counter rotating the props would improve handing, few were willing to pay for the production costs this would create though as engine powers increased over 2000hp it became unavoidable. British aircraft don’t seem to have been difficult to handle in general.

  10. “We understand that this effect, which only occurred during specific flight conditions, has the tendency to implement a pitch up motion, which was not experienced on the NG.”

    Again, after almost 8 months of grounding and countless analyses on websites like this, people still have to guess, and try to ‘understand’. When is Boeing finally, finally providing precise information what are the precise ‘flight conditions’ that need correction/support by a system like MCAS?

    • Probably because their lawyers tell them not to.

      Why? Because it would become obvious that Boeing has forced an unsafe plane through certification. Remember “Yedi-tricks”?

      Look back at the MAX story to the point in time when they figured that the MAX was actually a really bad idea and could not be made to the specifications it was sold at. Just picture Boeing to stop the development of the MAX right at the start of production. Just think of the mess. How to handle the giant order book? The production and supplier pipeline? No, they meant to avoid all that and a solution had to be found at all cost. And as it wasn’t, the cheating and lying started. And it still goes on.

      So no, Boeing will never publish the “flight conditions” on free will. It probably needs the DOJ to wrangle it out of their hands and bring it to daylight. Well, we’ll see if and when that happens.

  11. There seem to be more than a few misconceptions regarding 737 MAX stability and MCAS swirling around LNA’s comments sections in the past year. This is happening despite Bjorn’s thoughtful and thorough series of articles on pitch stability explaining these very issues and the MCAS descriptions given repeatedly by Boeing and other technical sites on the web. These MCAS descriptions are echoed in the the KNKT final report on JT610 which also contains the NTSB System Safety and Certification Specialist’s Report in Appendix 6.2 (pg. 245).

    I would like to address some of these misconceptions as best I can.

    Misconception 1: The 737 MAX is longitudinally (pitch) unstable
    The corollary to this is is that the MAX might be stable at lower AoA’s but becomes unstable above some non-zero positive AoA. To be clear, I am talking about longitudinal static stability which is another way of saying natural pitch stability. This is different from longitudinal dynamic stability where rotational inertia, rates, and accelerations must be considered.
    If the MAX was truly unstable in pitch then the control column would be very “touchy” in pitch at all AoA’s, not just large AoA’s, and in both directions, not just in the ANU direction. This would render the aircraft virtually uncontrollable by all but the best pilots. Also, a feature like MCAS can never stabilize a statically unstable aircraft. This is because MCAS is an open loop control law (note 32 at the bottom of the NTSB System Safety and Certification Specialist’s Report within the KNKT final report pg. 247) and it acts through the stabilizer incidence angle.
    Static stability depends on the slope of the pitching moment vs. AoA curve as Bjorn has stated several times in his Corner’s. This slope can never be changed by varying the stabilizer incidence angle or even the elevator angle. The slope is affected by the static margin, the tail length, the stabilizer area, etc, but not by the stabilizer incidence or elevator angles. Changing these angles only changes the trim AoA, i.e. slides the pitching moment curve up and down. Since the slope is not changed the static stability is not changed. This applies to all AoA’s, even if the aircraft is stalling. For an unstable aircraft, the column will be very touchy whether MCAS is active or not.
    Since the NTSB stated outright that MCAS is an open loop control law, MCAS cannot be used to give the MAX artificial pitch stability, like the control systems on modern fighter jets can. Besides, the jackscrew on the 737 stabilizer is much too slow for that.
    Finally, to my knowledge, I have not heard any reports that the MAX pitch control is overly touchy, nor have I heard of any reported incidences of severe MAX pitch control problems outside of the flights and crashes when MCAS misfired due to faulty/damaged AoA vanes. Because of all this, the MAX is statically stable in pitch over all AoA’s short of stall.

    Misconception 2: MCAS activates at lower AoA’s that are encountered during normal flight
    The fact that the MCAS activation schedule is not publicly known and Boeing has not disclosed it in detail does not mean that MCAS activates at lower AoA’s than Boeing implied in its public statements (AoA’s closer to stall).
    Since Boeing hasn’t publicly revealed the AoA’s where MCAS is active, instead of appealing to the unknown to prove my point I’m instead going make an educated estimate about the AoA’s where MCAS cannot be active. Bjorn states in Part 3 of his Pitch Stability series that normal flight of a clean aircraft (no slats or flaps) is flown at 2° – 4° AoA during cruise and 6° – 8° AoA in a turn or when waiting for landing at moderate speed in a circling pattern. I firmly believe in his numbers here because he calculates these types of parameters regularly when using his aircraft performance model to evaluate various aircraft for paying customers of LNA.
    For cruise and long duration turns like circling patterns, the pilot would undoubtedly trim the aircraft for those situations. When the aircraft is trimmed, any MCAS action under these conditions would appear as un-commanded AND trims any pilot would immediately notice. So, why are there no reports surfacing about the MAX suffering unexplained repetitive AND trim events happening about 5 seconds after manual re-trim by the pilot during normal flight, other than the flights where MCAS misfired due to faulty/damaged AoA sensors? After all, it has been a year since the first accident and many people have come forward since then, whether anonymously or not, to share/leak their information about the MAX MCAS development and certification. It is because MCAS does not activate at AoA’s that are encountered during normal flight, which is up to 8°. This is my educated estimate. My guess would be that MCAS doesn’t activate at AoA’s below 10° accounting for AoA’s that are on the high end of what is considered normal flight. The NYT article published June 1, 2019 supports this. “But a few weeks later, Mr. Wilson and his co-pilot began noticing that something was off, according to a person with direct knowledge of the flights. The Max wasn’t handling well when nearing stalls at low speeds.

    Misconception 3: The 737 MAX has an atypical pitching moment vs. AoA curve
    I can find no credible basis to support this notion.
    As far as I can tell the logic to support this misconception goes something like this:
    1) The Leap engines are larger and placed higher and farther forward relative to the wing on the MAX vs. the NG.
    2) This engine placement causes some mysterious aerodynamic effects to occur that Boeing can not or will not explain, and experts like Bjorn Fehrm also can not explain.
    3) These mysterious aerodynamic effects result a pitch up moment for the 737 MAX that is disproportionately larger than any other previous commercial twin jet
    4) These mystery effects and lousy pitch up tendencies only happen on the MAX, and not on any other commercial twin jet
    5) Boeing had to hide these nasty tendencies from everyone so they increased the power of MCAS hoping no one would catch on
    5) The fact that MCAS has these “stealth” purposes proves Boeing must be hiding something awful
    6) Because Boeing is hiding something awful, it must mean that the 737 pitching moment vs. AoA curve has to be the weirdest and most atypical one ever seen
    This is supported by cultish statements that highlight how atypical the MAX pitching moment curve must be, especially compared to the Airbus gold standard:

    Airbus don’t use atypical moment curves. That is a lie and a total lie. All Airbus airplanes are naturally stable. They have typical moment curves.
    I do admit that Bjorn does claim Airbus do what Boeing do. Not true. Airbus don’t do what Boeing do. They can’t be further apart. It’s a lie to suggest Airbus do what Boeing do.
    https://leehamnews.com/2019/10/25/bjorns-corner-analysing-the-737-max-crashes/#comment-287065

    In aerodynamic reality, relative changes in effect scale reasonably with relative changes in configuration, not an order of magnitude more. This is very true at lower speeds where compressibility effects are relatively small. At higher subsonic Mach numbers, the relative changes in effect are larger but again, not an order of magnitude larger. The diagrams overlaying the NG with the MAX illustrate that the configuration changes are relatively small.

    • Now regarding these pitch versus AOA curves, I think they are for a specific tail setting at a specific speed, for the aircraft in level flight? I would assume there are an infinite number of curves. I guess the ones that are pertinent are the ones as the MAX approaches stall, 10 degress through 15 degrees.
      According to the low speed stall test, it begins with the stab set to a given in level flight. Then the elevator brings the nose up as the aircraft slows. At every degree, it should be stable. I’m guessing this is not the case as MCAS must reconfigure the stab trim in the middle of this sequence to provide a different configuration through the last few degrees. A config. with less stab down force, and higher deflections of the elevator and higher forces on the elevator.

      • Hi Ted,

        When I talked about the pitching moment vs. AoA curve in my above comment, I actually meant the pitching moment coefficient vs. AoA curve. It doesn’t really make a difference for the discussion in my post above, so I omitted it because I didn’t want to repeatedly type it out. However, because you asked about the speed dependence, I need to make myself more clear.

        The pitching moment coefficient, Cm, is related to the pitching moment as follows:

        Cm=M/(q*c*S)

        where M is the pitching moment, q is the dynamic pressure, c is the mean chord length of the wing ( average distance from wing leading edge to trailing edge), and S is the wing area.

        Dynamic pressure, q, is related to the aircraft velocity as follows:

        q=(1/2)*rho*V^2

        where V is the free stream velocity and rho is the free stream air density.

        After all this, I agree with you that each configuration (eg. stabilizer incidence angle, elevator angle, or thrust setting, etc.) has it’s own pitching moment coefficient vs. AoA curve. However, the velocity is accounted for by the equations given above, and the flight path does not need to be level. An aircraft with a given configuration can have essentially the same pitching moment coefficient curve and still fly at different velocities.

        Assuming a static analysis where nothing changes with time, a slow aircraft will descend and a fast aircraft will ascend. In between will be the exact velocity needed for the aircraft flight path to be level. All this is for essentially the same pitching moment coefficient curve because the configuration hasn’t changed.
        ——————

        At every degree, it should be stable. I’m guessing this is not the case as MCAS must reconfigure the stab trim in the middle of this sequence to provide a different configuration through the last few degrees.

        I think that MCAS must reconfigure the stab trim in the middle of the sequence like you describe because there are AoA’s (“the last few degrees”) where the aircraft becomes less stable, not unstable. There are degrees of stability. Steeper (more negative slope) moment coefficient curves are more stable than shallower (less negative slope) ones. My contention is that the MAX pitching moment coefficient vs. AoA curve all the way up to stall is always stable (negative slope) and never neutral (zero slope) or unstable (positive slope). Changes in the degree of stability still need to be compensated for. The existence of MCAS does not prove instability.

        • Yes, thanks for that info. I believe you are correct after reading Bjorn’s series and crowd sourcing this by asking the question on A.net. The credible response seemed to be that there needed to be a somewhat linear and consistent stick force until stall.
          For example if it took 90lbs force to get to 9 deg., and 100lbs to get to 10 deg., if it took 100lbs and 1 ounce to get to 11 deg., that would not pass the intent of certification although it would still have negative slope (very low), so by definition of slope, stable (by magnitude, minimal).
          My question above is if this is solvable with the elevator, as the article talks about a stick nudger for the elevator? If the elevator deflects one degree for an increase in a degree of AOA, but when the AOA gets to 10 deg., suddenly a 1/10th degree change in the elevator equates to a whole degree change in AOA, then AOA at high angles is too sensitive to small changes in elevator postion. Having a nudger move it back to 9 deg. solves the problem of helping stay out of a dangerous realm, but it would not seem to help pass the test. In order to do that, at high AOA before stall, the AOA has to be dependent on larger movements of the elevator. The only solutions to me, are a larger stabilizer to provide a counter upward force to the elevator down force. Or the MCAS solution, which provides a higher incidence to the stabilizer, which I assume provides a larger range of counter upward force.
          If the upward force before MCAS is 0 to -4 deg. upward at 10lb per degree, but post MCAS going from -2.7 to -6.7 upward is 40lb per degree, then I could how it could make the elevator four times less sensitive before stall.
          Here’s another question. Once MCAS activates once, hoding the exact same force on the control column, how much does MCAS reduce the AOA? On the pitch curve the line goes horizontal to the left two degrees, then starts on a steeper negative slope than before?

    • @Mike,
      your entire post is based on assumptions and on refuting other peoples logic, knowledge and experience. Yet the most important facts are still missing. No independent agency has flown the MAX with MCAS switched off. Boeing has not published the flight/stability characteristics of the MAX. This means you don’t know at all how the plane handles without MCAS and any AOA.
      You also ignore the fact that no airliner has ever been built and operated with such a “special” configuration of a giant, up and forward engine. Engines are placed underneath the wings to NOT interfere with the flow of air over the wings, because that has a major impact on the flight characteristics in different ways at different speeds and AOA.
      Give me a reasonable explanation why Boeing A) does not publish the planes changing COL over AOA and speed and B) doesn’t offer test flight to EASA with MCAS switched off. These both points should end a lot of discussions that are probably not only going on here but also at the offices of many airlines.

      • Gundolf,

        Are you really criticizing my post because it contains assumptions? Every post ever made in this forum contains assumptions.

        You say that my entire post is based on refuting other peoples logic, knowledge and experience. That is only true for part of the last 1/3 of my post, Misconception 3: The MAX has an atypical pitching moment vs. AoA curve. I did this because I think this misconception is based primarily on faulty logic and several wrong assumptions including, misunderstood aerodynamics (assumed overly exaggerated aerodynamic effects), assumed facts that are not quantitative and not proven, and assumed motives that are not proven. This misconception is highly subjective and almost anything can be used to support or disprove it, including over the top emotional arguments.

        You clearly ignore the first 2 parts of my post (Misconception 1 and Misconception 2) which are based on quantitative facts, concepts clearly explained by Bjorn in his Corner series, detailed explanations of the MCAS function in the NTSB report, logic, and my knowledge and experience with aerodynamics and aircraft stability.

        You say I’m missing the most important fact: No independent agency has flown the MAX with MCAS switched off and Boeing has not published the flight/stability characteristics of the MAX. Because of this you say I have no idea how the MAX handles without MCAS at any AoA. I would counter by saying that you also have no idea about how the MAX handles without MCAS. Yet you claim that the MAX cannot safely fly in any part of the envelope without MCAS. Where is the data to support your claim? You think Boeing is hiding something. This is not data, it is an unproven motive, the weakest of all data in a technical discussion. I don’t need guesses about motives or unpublished data to make my arguments.

        You say I am ignoring that the MAX is some special configuration because of engine size and placement. I disagree. I freely concede that the MAX engine placement is different from the NG. The engine is obviously larger and placed higher and more forward relative to the wing leading edge. I also freely concede that this changes the MAX aerodynamics relative to the NG aerodynamics. The question is not whether the MAX is different from the NG, rather it is how much does it differ from the NG. You and others are claiming that the MAX is so different from all other aircraft that it must not handle well in any part of the flight envelope. I disagree with this unsupported guess. There are many military and commercial transport aircraft where the engines significantly disturb the flow over the wings. The MAX is not at all unique in this regard. Is the MAX aerodynamically different enough from the NG that it required MCAS to be certified? Yes. Is the Max aerodynamically different enough from the NG that it must not handle well in any part of the flight envelope? No.

        A) Boeing has not published the MAX stability curve or CoL vs. AoA curve, or CoL vs. speed curve because there is no need for them to publish it. The regulators are the only ones who actually need to see it. You and I and the general public don’t really need to see it. How can you claim to know what information Boeing has or has not provided to the regulators? If you have any special insight into this, we would all love to know it.

        B) How do you know that Boeing won’t conduct a test flight with MCAS switched off? Do you have any evidence at all the Boeing is refusing to do this? Again, if you do we would all love to see it. I think Boeing will perform this test and it will be for the FAA first because the MAX is a US product. The FAA will then share the data with EASA.

        • “A) Boeing has not published the MAX stability curve or CoL vs. AoA curve, or CoL vs. speed curve because there is no need for them to publish it. The regulators are the only ones who actually need to see it. You and I and the general public don’t really need to see it. How can you claim to know what information Boeing has or has not provided to the regulators? If you have any special insight into this, we would all love to know it.”

          Didn’t EASA complain that Boeing only provided incomplete and fragmented information?

          • Didn’t EASA complain that Boeing only provided incomplete and fragmented information?

            All these events surrounding Boeing fixing the flight control software and their communication with almost every major regulator around the world are literally unfolding as we speak. The picture is constantly changing. A complaining regulator one week might be a partially satisfied regulator this week. Who knows?

        • There are quite a few commenters here and at other aviation forums who seem to believe that the major airframe manufacturers should be publicly publishing proprietary technical documentation for the amusement of forum readers. Yes, Boeing owes full disclosure of required information to all regulatory authorities making certification decisions in affected states. Those disclosures typically include agreements for protecting proprietary information and we will not be able to FOIA all of Boeing’s filings from the FAA. The same is true for Airbus information filed with the FAA. But such protection of confidential information is taken as evidence of further evil intent. Sure.

          We do know from FlightAware the Boeing has conducted quite a few test flights with the 737MAX, some of them with very… interesting altitude graphs. But again we do not know who, and from which organizations, may have accompanied Boeing test pilots on those flights. The working assumption in this forum is that no one did, but there is null evidence on that question. Using a null premise one can of course derive anything one wishes.

          • And what is the action they were supposed to take if MCAS ran away?

            Hint, two switches, turning it off!

            Or now it turns itself (MCAS 2.0) off if AOA disagree!

            And the fix originally was the same as runaway stabilizer (turn it off).

            Yep, I may not agree with all MB has to write or say (nor should I nor should he with me) but he has that one spot on facts.

          • It also needs to be kept in mind that Leeham flat said, you cannot land an A320 (or any FBW I assume) without one of the computers working.

            At best you can struggle to stay in the air.

            So clearly FBW are not stable.

            However, they take extraordinary steps to ensure at least one computer is working.

            Someday there will be a gap found and one will come down as a result, but its going to be rare, the logic is well developed to stop it.

      • Gundolf,
        You make an excellent point. A lot of information is refuted. So, the conclusion? The Max is stable. It flies fine without MCAS. So, what is MCAS for? Will computers that turn it off in cases of angle of attack disagreement present another lurking danger? Is it really just a “safety” feature directed at limiting pilot training, enabling them to move seamlessly between the NG and the Max, as the CEO says Will that make it the safest airplane in the sky?

        • “” So, what is MCAS for? Will computers that turn it off in cases of angle of attack disagreement present another lurking danger? “”

          MCAS was needed to get certification. I think we all are sure about this, otherwise Boeing would have taken it out after the groundings and wouldn’t have lost $10 billions.

          MCAS is needed for a linear flight behaviour, so how can it be allowed to be turned off. It can’t be allowed and therefore the MAX can’t get certification.

          EASA will check this, also because Ethiopia asked for help and Ethiopia don’t expect independant help from the US. This independant EASA report is important to sue Boeing and not allow the US to cover it up.

          For this, EASA needs to wait first to see what FAA will do and then EASA will let us all know.

        • RealSteve,

          A lot of information is refuted. So, the conclusion? The Max is stable. It flies fine without MCAS. So, what is MCAS for?

          Like I told Gundolf, I only refute information in the last 1/3 of my comment. This is because I find the assertion that the MAX pitching moment curve is somehow unlike any other aircraft before it to be a baseless misconception. And I’m being polite here.

          My first two arguments don’t involve refuting information but rather involve using concepts gleaned from Bjorn’s articles along with reports by authorities and in the media, then synthesized by my own knowledge and experience with this subject matter to deduce what hasn’t yet been directly reported on. Yes, I discount information if I don’t think it is credible. If you think I’m not giving certain information a fair shake, then name it and we’ll discuss it.

          You mischaracterized my conclusion. I do think the MAX is stable in pitch, and I do think it flies fine without MCAS in the normal flight envelope (vast majority of the flight envelope). However, the aircraft has to be certified over it’s entire operational flight envelope (includes all non-normal parts of the flight envelope). The MAX needs MCAS to cover these areas.

          Finally the question of safety. What is safe vs. unsafe? If we define safety as compliance with the certification rules, then I would say that the MAX without MCAS is safe to fly in the normal envelope but unsafe to fly in the non-normal parts of the operational envelope. I think a good pilot could handle the MAX without MCAS in the entire operational envelope but that is not our working definition of safety in this discussion.

    • Of course your comments disregard the JATR report and the Lion Air crash report. After all they don’t represent evidence. According to you.

      • I’ll go further, the JATR report and the Lion Air crash report are the most damning indictment of an aviation OEM in the history of aviation.

        But you put it down to just bad software. As Bubba said the software engineers just did what the aeronautical engineers told them to do. Or is that the wrong way round.

        I agree with Bubba, MCAS was put in place because of aerodynamics. But I’m sure you will say that’s wrong

        • Something is deeply wrong if a discussion between engineers takes on the polarization of the current political climate. What must be wrong is that they have not agreed on the “givens” or the givens are absent. But at first the issue seemed clear cut. Soon after the accidents, the Seattle Times published articles observing that the placement of the newer, larger and more powerful engines on the aging 737 air frame negatively altered its aerodynamics. MCAS, we learned, was created as a software patch for this aerodynamic problem. Since then, through a major PR campaign, circular reasoning, and obfuscation the focus went from the aerodynamics of the Max to problems in the software patch itself; which, we are told, just needs some tweaking. You engineers and tech types need to get it together and stop allowing politics, ideology and PR to cloud your judgment. There shouldn’t be polarizing debate about whether the engines on the Max altered its aerodynamics and whether MCAS was designed to compel it to straighten up and fly right. These are clear cut issues that were in the basic design of the aircraft and should be known. If they are not, you should be asking why not.

          • RealSteve,

            Please point out to me where politics, ideology, and PR are infecting my technical arguments. Please point out where and how I’m obfuscating in my technical arguments. You are making very broad claims that you are not supporting.

            I never once claimed that the aerodynamics of the MAX weren’t altered by the larger engine placement. Just because they are altered, doesn’t mean they cant be compensated for by software. This has been done many times before on cable controlled commercial aircraft.

          • In my world software writers were a horror. They did not understand the equipment and just wrote stuff to the spec.

            Not sure how it works in the Av world.

            But a good chunk of my career was spend underwriting what the software guy did and making them do it so it worked.

            Worse they would argue with you. Then you would have to beat into their heads how it worked so they would make it right.

        • Philip,

          I’ll go further, the JATR report and the Lion Air crash report are the most damning indictment of an aviation OEM in the history of aviation.

          Agreed.

          Much of the JATR report focuses on the bad assumptions in the MCAS safety assessment, the insufficient MCAS testing, and shortcomings in the FAA certification of MCAS. I would say that the software did have a lot to do with it.

          I totally agree that MCAS was put in place because of aerodynamics. There is no other reason for it to be there. However, this fact doesn’t at all mean that a properly designed software function can’t safely compensate for the aerodynamics.

          • “” However, this fact doesn’t at all mean that a properly designed software function can’t safely compensate for the aerodynamics. “”

            Obviously software only can’t fix this, otherwise EASA wouldn’t care about flight tests without MCAS.

          • You refer to MCAS as an open loop system. I’ve done the same but well before the NTSB report

            So please tell what an open loop system is?. You can then explain to yourself and the rest of the world why software alone is dangerous. But, if you think software alone isn’t dangerous then explain it.

            If you can’t I will. But I may trick you again.

            Somethings you can’t Google for the issues are specific to the 737 MAX!

          • Didn’t Muilenberg himself deny this was the reason for MCAS at the hearings?

          • I did not accuse you in particular of obfuscation and circular reasoning. But it troubles me when a discussion of the basic laws of aerodynamics turns into a polarized debate using mathematical formulas and undermines the attempt of normal people to grasp a highly technical issue. I’m further troubled by the mystifying formulas being used to make arguments that the CEO of the company itself is not making. According to Muilenberg MCAS was not “put in place for aerodynamics.” Instead, he said, it is a “safety feature” for pilots that produces equivalent handling characteristics between the Max and the NG. This whole argument about the unsafe safety feature is built on circular reasoning. I think people on this site and others are not clamoring to know proprietary information about how Boeing safely added more powerful and more efficient engines to the Max, making it a less expensive competitor to the Neo. But we do want to know what went wrong in the crashes of two, brand new airplanes. Very good journalism has indicated that Boeing used a software patch to rectify an aerodynamic problem cause by the new engines. And it seems that patch caused two plane accidents. Yet, a PR campaign and less reputable journalism has muddied the issue, blaming pilots, mechanics and the culture of other countries. I began reading this blog and the comments section, not for partisan-style name calling and baiting, but for a sophisticated discussion of an issue that fascinates me: the interface between the physical world and the digital world. As for obfuscation, Boeing does indeed owe us — the people who buy stock, who buy airplane tickets, and who have unwittingly put their lives on the line boarding the Max — an explanation. The regulators, by the way, work for us. Yet, it also seems that people on this site have had to conjecture about so much — the very aeronautical stability of a passenger plane, which simply can’t be all that proprietary! — because it seems like the manufacturer has not been open about providing this information to the regulators. In any case, those regulators ended up being themselves. Didn’t EASA complain in September that Boeing was not providing the information it needed to evaluate MCAS? EASA Director Patrick Ky’s report stated that there was: “Still no appropriate response to Angle of Attack integrity issues.”

          • It must have been this type of heated discussion that led to the creation of the Max in the first place.

            Realist: “You can safely compensate for the aerodynamic problems caused by the placement of the engines using software. It will be cheap and you will beat Airbus! This is a win win!”

            Quixotic: “Are you crazy! It can’t be done like that. You have to redesign the tail (based on CoL, CoG, CoP???) and at least giving more authority to those tiny elevators. Otherwise this software patch will be like a ticking time bomb. You have to slow down. Spend more money. Better yet: design a new aircraft from the bottom up! Let’s take the time to do it right.”

          • Philip:

            You can’t fool controls people, only yourself.

            And Open Loop system has no feedback.

            Now, you can argue what feedback actually is.

            AOA returning to normal could be construed as feedback.

            I think thats a bit nebulous but it is feedback on affect of the action.

            I had one guy try to convince me a heating control loop had no feedback. But it did as it was a room temp sensor to reset it.

            Now if I set a system to supply 55 degree air and it never changed, that truly has no feedback from the point of the areas served.

            Room could be too hot or too cold and likely not just right.

            I tend to feedback is any form of adjustment to an output that otherwise would stay fixed.

          • Mike Bonhet

            So no explanation of an open loop system. There’s a surprise. You are going to tell me you won’t sink to my depth.

            An open loop system does not receive feedback directly but can receive feedback indirectly. The usual reason is a gap in the loop, a gap with no physical connection. In the case of MCAS the gap is between the alpha vane and the stabiliser. But there is air, so very often it’s called an air gap.

            The loop starts with the alpha vane. MCAS processes the alpha vane input to move the stabiliser. This causes the alpha vane to move because the stabiliser changes the AoA. The loop starts again.

            So there is feedback but the feedback is indirect.

            The problem with the loop is that it does not have fail-safe redundancy and it’s slow. Some articles say the loop takes 3 seconds. That’s very slow. The norm is less than 1 second.

            The slow response of the loop is probably the reason why MCAS is/was crushing. It doesn’t get two chances. So nudging the stabiliser bit by bit isn’t on. It must apply one crashing blow.

            Maneuverings control loops must be fast not slow.

            To everybody, I’m not playing tricks. I think my words are right.

      • Uh no, I clearly use evidence contained the NTSB System Safety and Certification Specialist’s Report which is contained within the KNKT final report. I even used bold font to highlight it. Specifically I use its MCAS description as evidence.
        I didn’t draw from the JATR report to make my arguments because I didn’t need to. Feel free to cite any details you find in the JATR report that you feel invalidate any of my arguments.

  12. I’m not sure I entirely understand the difference between a “stick nudger” and a “stick pusher”. I do understand the Stick pusher. The Stick Pusher as developed by the British in the 1950’s for the Trident and BAC 111 because two complete flight crews died as a result of deep stalls whereby the main wing blanks of the T tail horizontal stabiliser and the engines stall thereby preventing a power out from the stall by increasing airspeed. The Stick Pusher fired of two sensors, was powered by compressed nitrogen and acted directly on the control column. It could over power the pilot so it could be over ridden, there is no doubt that the pilot would be aware that the stick pusher had activated. Autolight was also added to the engines to restart them without pilot intervention. Some stick pushers do not over power the pilot and are, I suspect, more an evolution of artificial feel units.
    By contrast the ‘Stick Nudger” adds in a force directly to the elevator by some mechanical mechanism (differential gear or a moving fulcrum leaver at the elevator quadrant). Thus the stick nudger would deflect the elevator down slightly even if the control column were held steady. A side effect would be that the elevator would then feed a forward control force to the pilots column. It’s an elegant solution to tame a stall characteristic. (Note B707 did not have powered flight controls, it used geared spring tabs to alleviate force)
    It’s worth noting that the Vickers Super VC10 and the Ilyushin IL62 both did not suffer from super stalls. This is because the higher sweep angles of these fast cruising wings would generate sufficient pitch up effect from the wing tips to recover from the stall. The problem of premature tip stall due to span wise flow lengthening the boundary layer and separating early was dealt with by small dog tooth leading edges to interrupt the span wise flow and start a new boundary layer towards the wing tips.
    The invention of the swept wing goes to delay shock wave formation back to German aerodynamicist Adolf Busmann in 1932. The advantages when combined with jet engines was quickly recognised but it’s worth noting that much of the effort of the next 10 years went into taming the low speed characteristics of swept wings. Due to the sweep and the fuselage swept wings have an span wise flow and due to the increased travel of the air the chances of boundary layer separation and therefore stall increase towards the tips which would cause a deep stall and a loss of roll control The usual practice of twisting the wings at the tips is limited since the twist generate shock waves at high speed. The first solution was the well known slat but soon others came out. There was forward sweep (inward flow is less severe and more easily handled), leading edge flaps (which work poorly on thick wings but work well on fine wings), Krueger flaps. Geometric twist where the outboard wing section is cambered more than inboard so as to delay stall longer. The most interesting one is ‘area ruling’ which was an attempt by Dietrich Kuchemann to waist a fuselage to reduce span wise flow but which was found to area rule the aircraft.
    The efforts of British industry and regulators is to be commended. They developed incredible technical abilities and understanding and forced stick pushers and nudgers onto B727 and B707. Despite the comet 1 disasters the industry learned very much.

    • What it amounts to is its a non issue as to whats used as long as it does what was intended (safely)

      There is a lot of gobbledygook spun out on aerodynamic and hidden issue, but its just alternative universe stuff, it has no legs to stand on.

      You can suspend reality and come up with anything you want. But its not real.

  13. Very clear and succinct.

    Did these changes (fin length aside) percolate throughout much more of the 707 and 727 fleets or rather remain largely focused on UK registered/operated aircraft?

    • I had asked myself the same question. All information I have found say that nether Boeing nor McDonald Douglass equipped any B727, B707, DC9 with stick pushers or stick nudgers outside of those required by the British Regulator. The BAC111 had a stick pusher, the Trident-100 had a stick pusher and the Trident-200 was accepted with a stick nudger. Maybe the Canadians or Australians who drew of British experience required them. All posts I found on Airliners.net or pprune.org specifically state non of these American aircraft received stick pushers though they did have stick shakers. Early MD80 did not have a stick pusher but it had very bad superstall and so received one. It wasn’t until the B767 that a Boeing aircraft seemed to get a stick pusher. The B747-100 had a benign stall with much pre stall buffet and supposedly didn’t need one but may have had a nudger . B747-8 received a pitch augmentation system that lowers the elevator without pilot input.
      It is a pity as they really do save lives. Having said that they don’t always work. A classic problem is taking of without flaps set in which case the crew is disoriented already and typically the Pilot Flying over powers or over rides the stick pusher by deactivating it.
      One 8 year old Airliners.net post I examined said the B737NG already trimmed down the stabiliser when approaching a stall, in other words a MCAS system already likely exists in the B737NG! See roseflyers post:
      https://www.airliners.net/forum/viewtopic.php?t=768675
      I also found this useful
      https://www.pprune.org/archive/index.php/t-270567.html

      Here are some supertall crashes:
      https://aviation-safety.net/database/record.php?id=20050816-0
      https://en.wikipedia.org/wiki/British_European_Airways_Flight_548
      Note T tail aircraft can be designed to escape supertall by aerodynamic measures.

      • William,

        Thanks for you interesting comments here.

        Regarding:

        One 8 year old Airliners.net post I examined said the B737NG already trimmed down the stabiliser when approaching a stall, in other words a MCAS system already likely exists in the B737NG!

        It’s the speed trim, not MCAS (although on the MAX the MCAS function is implemented within speed trim). It uses info from the ADIRU, mainly speed, but other info as well (that I forget), but it does not use AoA measurements from the vanes. Satcom Guru uses several of his posts to describe it in detail. Here is a link to the speed trim introductory post.
        https://leehamnews.com/2019/11/01/a-basic-mcas-system-was-installed-in-the-boeing-707-in-the-1960s/#more-31553

  14. Mike Bohnet

    I believe I’m a nice person. But you attack me as if I’m your prey. But those who go on the hunt became the hunted. So you are my prey. From now on you are my prey.

    Sometimes the prey needs to be brought out into the open. That means giving you the food. I gave you food. I contradicted everything I said. You loved it and you are basking in it.

    But I did it on purpose.

    You said that I was contravening the laws of physics. No true. But what I did say wasn’t right. But my purpose was to bring my prey into the open. My prey was you, Mike Bohnet.

    In response to you, Mike Bonhet, I agreed with those who said it was all about the CoG when it’s about the CoL/CoP. I don’t believe that. I said the opposite for a year. I still say the opposite. It’s about the CoL/CoP.

    So I reversed my position. The reason. I wanted my prey in the open. I wanted you, Mike Bohnet.

    You jumped on it. You are now in the open. My prey is in the open. You now need to explain. Why did you agree for an entire year that it was the about the CoG then all of a sudden say it’s about the CoL/CoP. You need to explain that.

    Remember you told me I wrong for a year. You flipped when I agreed I was wrong. But you flipped. Indeed you where glorious in telling me I was wrong even though I had been right all along.

    I am though perplexed as to why addressing the problem by fixing the CoL/CoP is contrary to the laws of physics. It’s wrong. I accept that. But contrary to the the laws of physics. Really? No. It’s wrong, but not contrary to the laws of physics.

    So please explain to humanity why it’s about the CoL/CoP and not about the CoG. Remember you spent an entire year saying the opposite.

    Can you also explain why resetting the axis from the CoG to the CoL/CoP contravenes the laws of physics. That I’m really interested in. Really, I do mean explain that one.

    I do mean explain. You don’t explain. Every body has picked up on it. Explain.

    We wait with baited breath. My prey is in the open for I’ve lured my prey by offering food.

    You have treated me as your prey. You are now my prey.

    • Wheee, I guess I am right in there with Mike there then. I have given up reading the details because they made no sense so a reverse would just get lost in the noise as well.

      Frankly the term fantasy comes to mind with your weird takes, but Leeham does allow fantasy no matter how misplaced.

      I like learning and I am happy when proven wrong (as has happened more than once) despite good intentions.

      I like facts not fantasy. If I had managed my work on fantasy I wold soon have been in the unemployment line. I worked steady 1982 on (construction prior which was seasonal) –

    • Philip,

      So I’m your prey now all of a sudden? What about all the other times throughout the last 9 months when you engaged in both direct and indirect personal attacks against me and others? From my perspective, nothing is really new here so I’m not really expecting anything different from you.

      I’ll re-iterate here what I’ve already said in response to one of your earlier comments to me:

      I’ve always only ever maintained that the slope of the pitching moment curve determines static pitch stability. This is accordance with the well documented textbook longitudinal static stability analysis that can be easily found on the web or in actual textbooks, several of which are sitting right here on my shelf. Since the position of the CoL is one of the main determining factors of the pitching moment curve slope, the CoL position obviously drives the pitch stability. I’ve never claimed any different, now or over the last year.

      Are you sure you’re not confusing me with someone else? We’ve only interacted about the subject of pitch stability in 9 LNA articles over this year starting in March. None of those times have I ever said you were wrong about the role of CoL position. I definitely thought (and stated) you were wrong about other various issues related to stability, and conversely you thought (and stated) I was wrong as well.

      So, stop with this fabrication about how I disagreed with you on this particular point and than you fooled me into agreeing with you. It’s utter BS and I feel no need to respond to you any more on this issue. Since you’re accusing me, the onus is on you to prove that what you claim is true. I’m certain that’s going to take a very long time.

      • For the record, it was Bjorn who first said that the issue was about the CoG. He said it just after the Lion Air crash. I corrected him then. I made it clear it was about CoL/CoP and explained why for I always explain why. On the basis of memory, I said the nacelle was extending the mean chord of the wing forward, thereby moving the CoL/CoP forward. Those words are correct. I’m sure Scott can go into his archives for the link.

        So it’s been a year. If you knew all along then why didn’t you support me? Why didn’t you challenge Bjorn? Why did I continue to get the crap from you for a year? So don’t play the innocent.

        I can take it from Transworld, sPh, Bilbo and others. They don’t have a clue. But you clearly did look up static stability and do have some understanding of it. Not a lot for what I wrote isn’t contrary to the laws of physics. But it was wrong. What I wrote was my explanation as to why people were thinking it was about the CoG for it is theoretically possible to fix the axis about the aerodynamic centre. But no airplane as been developed that does that. The aerodynamic centre is dynamic, the centre of gravity is more static. Note the use of the word static.

        I admit I didn’t expect you to correct me. I expected to have to correct myself.

        You said elsewhere that it was about the aerodynamics and not about the software. Well look up the Coanda effect. Bilbo with your support tried to trip me up by saying I needed to look up the Coanda effect. In other words you and Bilbo gave me crap.

        I said I didn’t need to look it up. I also said the effect surported the debate about the nacelle and forward lift. For those who don’t know, the Coanda effect is about airflow following (sticking to) a convex surface. It’s actually the reason why pylons are necessary. The MAX has a pylon that causes the nacelle airflow to go over the wing not under it. That’s causing the forward lift, but it’s there at low AoA. The forward lift increases with AoA. That will cause an atypical moment curve. How atypical? That’s up to EASA, for Boeing aren’t saying.

        In the above do you note that I’ve explained myself. So I haven’t just said something without explanation. Elsewhere I’ve explained an open loop control system.

        Try it. Try and explain yourself. I’ll leave it at that.

        • Everybody tone it down. This is not a personalized debating society.

          Philip was the first to focus on the issue.

          Hamilton

    • Philip,
      get a grip on yourself. You are the one who flipped. If you want people to take you seriously, you should stop grandstanding and displaying your self-righteousness (accompanied by a significant degree of weirdness), and instead learn some humility, as you were told months ago.
      Stating an erroneous fact and later maintaining that it was by intention – that’s the kind of stuff we’re used to hearing from a certain president, but not from serious people.

      • Did it all the time when I interviewed people. Why? Some candidates knew the words but didn’t understand them. So say something wrong and let them correct what is wrong. That way you know the candidates that understand and those who don’t

        My record on the matter is repeatedly set out on this web-site, starting just a few days after the Lion Air crash.

        I might add the correction made by Mike Bonhet also corrected Bjorn for it was Bjorn who said that it was about the CoG just a few days after the Lion Air crash. I corrected Bjorn then. Why didn’t I get support from Mike Bonhet then? Fair question!

        For the record I explained why it’s the CoL/CoP and not the CoG, not Mike Bonhet. Having the decency to explain why is just that: decency. But apparently that is grandstanding.

        • So this is an exercise in fooling people?

          My capability in aerodynamics is limited, I understand the underling (E=MC2 I get) but have me do the math on its, that is what Einstein is for, he didn’t keep generator and fire pumps running and I don’t step on his Cape (coldn’t even but …..).

          Getting hung on up what the Plank constant value really is? Kind of (well really) so what – they all say expansion. Is my base for it better than your base? Anyone really care?

          It matters not a white that MCAS was for flight smoothing or stall avoidance. They were told they had to do something about pitch up at stall which was deemed more than allowed (of which I have to see relative values, 5%, 10%, 20%?)

          Boeing put in a crap written piece of Software, people died. Bottom line.

          No one that has an credentials has stated the 737 is unstable (though Philip has extrapolated a small problem at an extreme of flight only seen in a Simulator) into the Sky is Falling.

          He has the audacity to say its Bjorns “opinion” its stable. While I disagree with Bjorn on AOA, that is not an opinion, its a take from two different professional pilots perspectives from whole different background. I understand why its relevant to him. I can understand he does not get its lack of relevancy to me.

          So Philip just gets tuned out as an annoying buzz to a serious contributer./

          Prey? Really? This is supposed to be about aviation not stalking.

      • I will add. Telling a lie very often exposes the truth. A lesson in life.

        Bjorn said in the aftermath of the Lion Air crash that it’s about the CoG. I corrected him.

        But the media still believe it. The Wall St Journal, the New York Times, the Washington Post and the Seattle Times are US examples.

        Bjorn has never corrected what he said. Indeed he reinforced it with his article on the A320 pitching issue. The faithful supported him. We know the faithful.

        The issue with the MAX is about the CoL/CoP. It’s causing an atypical moment curve. Boeing are not telling. That is clear from the JATR report and the report into the Lion Air crash.

        EASA want to know. We must all support that. The families of those who died have that right.

        With regard to telling a lie to expose the truth. The lie must be believable. It also needs someone to take the bait. No better than Mike Bonhet.

        I will set out the issue in very simple terms. It will mean I talk about windmills, children’s playgrounds and submarines. In advance:

        A moment curve defines the change of moment to balance the moment.

        I need to think about simplifying those words for I don’t want to grandstand. So windmills, children’s playgrounds and submarines.

        Believe the author of this article. Not Bjorn.

        • Philip:

          I believe you are the worlds best dead horse beater.

          You can play with numbers all you want, the bottom line is the MAX is a stable aircraft. All your silliness is attempting to prove its some kind of vicious monster.

          By jet standards its still stable despite the pitch up. An A320 is not stable. Its managed all by computers and software.

          A MAX will still fly without any of it.

          Is one better than the other? So far FBW has not failed so we can say no.

          With all this your contention winds up trying to prove the world is flat.

          MAX simply has one aspect at stall that is deemed not so good and a method to deal with that was required. If you do any aircraft research there have been far worse issues and successful aircraft that have flown.

          Bait all you want, that is purely bizarre. Pary on someone. Phew, talk about arrogance and ego to match Muilenber.

          So we know you lie, you have just proven nothing you say can be believed. Well done.

  15. It seems to matter to the CEO, Muilenberg, who is also an aeronautical engineer whether “MCAS was for flight smoothing or stall avoidance. ” For him it’s a “safety feature” destined to flatten the learning curve for pilots moving between two iterations of the same aircraft. But if I have followed the evolution of Philips argument during these last few months, MCAS is not a separate, independent system, or even just a poorly written piece of software operating independently within the Max control system economy. At issue is whether what looks like a fly by wire piece of software can work within the connected hydraulic, cable economy of the Max in isolation, I think. Before going back to the stability of aerodynamics, people here were actually discussing the digital architecture of the Max and whether its processors could handle the proposed changes. On one level, the problem seems to be that using programing to solve this particular type of problem with this particular aircraft had a cascading effect, introducing problems in other aspects of the system, or being subject to problems in other aspects of the system. Finally, if I was reading Phillip’s observations correctly, he was also suggesting the the aerodynamic problems were not just on the lift of the wing, but affected the control surfaces of the tail. Again, before moving back to this whole center of gravity/center of lift discussion; a robust discussion was taking place concerning the authority of the elevators. At one point I believe Phillip said that the stabilizer, as configured on the 737 is too slow to do what it is possibly being asked to do to perhaps account for the troubled aerodynamics. He saw this as a problem and something to investigate. I was surprised to read, however, that M. Bohnet used this same question as a somewhat circular argument. The stabilizer couldn’t be asked to do this because it is too slow to do this therefore it wasn’t being asked to do this. “Since the NTSB stated outright that MCAS is an open loop control law, MCAS cannot be used to give the MAX artificial pitch stability, like the control systems on modern fighter jets can. Besides, the jackscrew on the 737 stabilizer is much too slow for that.” It seems that Phillip agrees with this, yet he is not sure whether the stabilizer is indeed being asked to behave like it is part of a fly by wire system, or even a much more supple system (he mentioned the tail of the L-1011). In short, I think the issue of disagreement is that the givens are still unknown. What is MCAS designed to do? M. Bohnet argues that the Max must be stable because there have only been two crashes. The Max flies and it flies well. Therefore MCAS is not designed to correct a significant aerodynamic problem. It simply needs better implementation. Phillip seems to be arguing that MCAS was designed to correct problems within a delicate ecology that can only safely be amended physically, otherwise too many unknowns are introduced into the system. Bottom line, I think he observes, the Max needs a different tail. These are fascinating, and very serious issues involving life and death and perhaps the economy of a nation. I hope the engineers working on them have better temperaments; otherwise God help us.

    • Your new to the discussion. But quick.

      I’m actually more sure than I’m saying. I want to know the moment curve with MCAS off. But then the regulators want the same. We need to support that.

      When I first brought this up, shortly after the Lion Air crash, you have no idea of the abuse. Especially as I contradicted Bjorn.

      I do try and keep my temper but it’s hard when you are being systematically abused. It’s necessary to return fire to maintain your dignity.

    • Real Steve,

      I have lost track of what Philip actually believes, its changed dramatically from his first post.

      Nor am I an aeronautical engineer, I am a commercial pilot with an instrument rating. Basically that is an operator vs an engineer/designer.

      The reality is that the MAX first flew without MCAS of any kind.

      What Philip contends and tries to stuff illogical in, is that a change on the engine has turned the 737 in its MAX variation into a dangerous monster.

      As it passed all the flight tests and ops prior to MCAS ever coming up, that is thinner than the thinnest negligee on the planet. In fact its simply silly.

      During desing tghe engineers saw the issue but due to limitation of software, it did not look to be the degree needing any aciton.

      These are the same people that desingd the 787. So they can desing one aircraf as stable and a miner mod they can’t asess?

      And that ignroes all the flights made in test and the commerila flight before it was gournded.

      There is no logic in the argument. I can say the world is flat and make myself stupid by doing so, saying someitn does not make it an issue nor does sayingsomethin reveal anyting other than your mouth was moving (or the keys on the keyboard were activae4d) .

      This is not the days of thwoing women into Voanoes to make them calm down.

      If you want to have a seirous disu9ion you have to have serious foundation.

      Thjere is no foudnaiton of any kind, prooof etc that the MAX has any issue other than a pitch up at stall nor that MCAS does anyt9ing other than put the nose down despite its crappy progrrmin.

      And in the end that is all it is. Crapyy progamin because Boeing did not want to have to do any trining for peopel flying the MAX.

      And while tehcialy a feel vs a need ala MCAS is different, the reality is that it was alloowe in its crapp-y form and killed pole.

      But if you read WHY they did it, it was based on a stall behavior they wanted corrected.

      So loose terms or not it made and makes no real difference – they were told to do it, they did it horribly and people died.

      How to stop that in the future is a discusable point

      Should an old manual control aircraft be allowed to be grandfathered in is a disscusable point.

      But the stability of the MAX other than at the point of stall and pitch up (and even that is easily dealt with) is not in question and to put it there is no more than flat earth or sun rising in the West.

      Simply put the MAX is as stable as a C-150. I tried to do myself in twice int hat aircraft, but it had nothign to do with the aircraft.

      • Transworld,

        I will repeat my believes. I believe

        1) There is a pitch instability issue caused by forward lift from the nacelle. The JATR report and the report into the Lion Air crash have confirmed the pitch instability issue but have not said it’s due to the nacelle.

        To resolve the issue aerodynamic changes need to be made. The changes need to remove the instability AND make the elevators work.

        2) The Flight Control System does not exhibit fail-safe redundancy as per duplex/triplex control systems with full failure isolation.

        Resolution is stated by the problem

        3) Manual trim needs power assistance.

        Resolution is stated by the problem

        All three I’ve made clear on a number of occasions.

        But I end with the issue of obscuration. I’ve spent a year reading that MCAS was developed because of a CoG issue. That has completely obscured the debate for it as nothing to do with the CoG.

        I ended telling a lie to prove a lie. I entirely contradicted what I said over the past year. It’s nothing to do with the CoG. It’s the CoP as stated by the author of this article.

        With regard to the first issue, the regulators need to know the moment curve. Boeing are not telling. That why EASA have demanded to do their own flights. I think the moment curve is atypical.

        Hope that clarifies my position, a position I have repeated numerous times.

        • This is what I understood. I had forgotten to mention a small issue that you mention that everyone here, and EASA agree on, but may be the elephant in the room — the inadequacy of the manual trim wheels. This is also a problem with the NG and pilot training since it seems that Boeing might have been dishonest in simulating their efficacy in the simulators.

          • from the JATR Report p21

            * Recommendation R3.13: The FAA should ensure that simulation devices that are used for certification credit have the required level of fidelity for the associated test.
            º Observation O3.13-A: During evaluation in the Boeing engineering simulator (E-Cab), the JATR team observed that the device does not incorporate control loading on the manual stabilizer trim wheel. As a result, control forces on the manual stabilizer trim wheel are not representative of the aircraft.

            (my emphasis)

            Was this the reason that made Boeing cancel, mere days before, the scheduled visit with APA to the Miami MAX simulator mentioned by Dan Carey back in June before the transp. committee?

            How could, Ethiopian Airlines, which has 1 of the 4 existing MAX simulators in the world, ever train their pilots on the November released AD wrt Runaway Stabilizer Checklist, if the simulator they were employing would never reveal the ultimate failure of the second MAX tragedy?

            from Ethiopia AAI’s Preliminary Report p25

            The crew performed runaway stabilizer checklist and put the stab trim cutout switch to cutout position and confirmed that the manual trim operation was not working.

            I note this was already after a video, announced on this site, then made private, later sanitized, but still faithfully revealed to the public at large, the manual trim wheel issue in full on an NG real training simulator.

            How come NG simulators don’t lack the proper control loading ability mentioned in the JATR report? Or better put, why are MAX simulators lacking this ability?

            These are very, very serious questions… indicting Boeing behaviour, not just before, but after the accidents!

          • @Vasco

            Even worse as it was the ‘Boeing engineering simulator (E-Cab)’, if that one is not representative of the real aircraft, how on earth could the commercial simulators be representative of real flying MAXs ?

          • JakDak,

            There is no reported change from NG>MAX stabilizer surfaces, a copy over of control loading data until final update is available should be at least indicative, but why leave this issue pending on e-cab? and if it happened to be an APA finding on the Miami sim would be simply unbelievable.

    • @RealSteve@

      Well Said

      “Bottom line, I think he observes, the Max needs a different tail. These are fascinating, and very serious issues involving life and death and perhaps the economy of a nation. I hope the engineers working on them have better temperaments; otherwise God help us.”

      I believe we cannot rely on Phillip, Mike Bohnet, or even Bjorn – we have to rely on the regulators, after all its their job to certify the aircraft as safe. The situation has now reached the point to do the effectively they must be seen to act independently – which means the FAA, EASA, etc show each conduct their own reviews and test flights.

      Having read the crash report the JATR’s report, the EASA’s Boeing 737 TCDS APPENDIX ISS 10 and given the state of both Boeing’s and the FAA’s tattered reputations the international regulatory bodies should also publish results for public scrutiny.

      • No one is sayi9ng Bjorn has anything to do with what the MAX is.

        We can rely on him to have a vastly superior assessment of the whole picture.

        In other words, where my knowledge falls off the edge of the cliff, if I had to make a my life depends on it decision, Bjorn has it hands down vs Philip.

        I will happily fly in a MAX when its back in the air.

    • RealSteve,

      I was surprised to read, however, that M. Bohnet used this same question as a somewhat circular argument. The stabilizer couldn’t be asked to do this because it is too slow to do this therefore it wasn’t being asked to do this.
      —————-
      “Since the NTSB stated outright that MCAS is an open loop control law, MCAS cannot be used to give the MAX artificial pitch stability, like the control systems on modern fighter jets can. Besides, the jackscrew on the 737 stabilizer is much too slow for that.”

      No, my argument is not circular. I start with the fact that MCAS is an open-loop control law, as established by the NTSB. I then state that an open-loop control law cannot be used to give an aircraft artificial stability. I declared this as a fact. You may disagree with this fact for whatever reason, but I chose to forego the lengthy discussion required to support this fact. I then added a supporting fact that the stabilizer is too slow to be used for artificial stability. Thus MCAS, which acts through the stabilizer, cannot be used to give an aircraft artificial stability. I never addressed the question of whether Boeing engineers used or thought they could use MCAS this way. For the record though, I think the Boeing engineers are too competent for that.
      ————

      What is MCAS designed to do? M. Bohnet argues that the Max must be stable because there have only been two crashes. The Max flies and it flies well. Therefore MCAS is not designed to correct a significant aerodynamic problem. It simply needs better implementation.

      This is not my argument at all. I argue that the MAX must be stable because of the following facts:
      1) A control law such as open-loop MCAS acting through a slow stabilizer can never be used to give an unstable aircraft artificial stability.
      2) Resetting the stabilizer incidence angle, which is what MCAS really does, can never be used to change the aircraft basic static stability. It can, however, be used to compensate for less stable portions of the flight envelope, but only portions that are at high angles of attack when they are arrived at from a lower angle of attack trim condition without the pilot re-trimming. This is a whole discussion on its own.
      3) There is a complete absence of MAX control incidents where the pilot reports “touchy” controls or control reversal at low to moderate angles of attack. These are the type of incidents one would expect from an unstable aircraft. The two crashes and the one previous LionAir flight don’t count because they resulted from a malfunctioning MCAS.

      MCAS was absolutely designed to correct a significant aerodynamic issue. I never said anything that would contradict this. How significant is a matter of interpretation. Bottom line is the MAX needs to comply with all the certification rules over the entire operational envelope (which includes the non-normal portions), not just in the normal envelop. MCAS is needed for this. I contend that the MAX handles fine without MCAS in the normal flight envelope (vast majority of envelope), but does not handle well enough for certification in the non-normal parts of the envelope.

      Implementing a better MCAS is not simple or trivial. MCAS is not and never was a software “patch”.

      • Ergo:

        An open loop control law can be used for stability provided it is
        responsive.

        You are assuming all open loop control laws are slow. That’s an assumption not a fact. In other words the definition of an open loop system does not include the word “slow”.

        Hence the circular argument.

        Real Steve’s a bit bright.

        • Please highlight the words in my above comment that show I assume an open loop controller must be slow.

        • OK. Got me. It was Steve not you.

          But the same applies. You have said that an open loop system cannot be used for “artifical” stability.

          But that’s how airplanes are controlled. To compare the elevators to the stabiliser.

          When a pilot uses the yoke to move the elevators the elevator send a DIRECT response to the yoke that they have moved. But that response is that the elevators have moved. The response doesn’t mean the airplane has moved. The alpha vane gives the INDIRECT response that the airplane as moved.

          In contrast:

          When a pilot uses the stabiliser trim wheel the stabiliser sends a DIRECT response that the stabiliser has moved. In this case the DIRECT reponse is to the stabiliser dial that shows the deflection of the stabiliser. But again, it doesn’t mean the airplane has moved. The alpha vane sends the INDIRECT respnse that the airplane has moved.

          So what’s the difference? The physical movement of the elevator is closed loop through the yoke. The physical movement of the stabiliser is closed loop through wheel and dial. But the physical movement of the airplane is determined by the alpha vane in both cases.

          Elevator/stabiliser movement and aerodynamic movement are two different conditions. The elevator/stabiliser movement is closed loop as there are DIRECT responses to their movement. The aerodynamic movement is open looped for the response is INDIRECT through the alpha vane in both cases.

          This comes to MCAS. MCAS is just computer automation. The autopilot and auto throttle are also computer automations. So the automation has nothing to do with open/closed loop systems.

          So it’s the word “artificial” not the word “slow” that’s the circular argument. Airplane movements are controlled by open loop systems. So what do you mean by “artificial”.

          But then perhaps the issue is the definition of open/closed loop systems. Closed loop means that all parts of a loop are synchronised. Open means that some part of the loop isn’t synchronised. In pitch, it’s the part between the elevator/stabiliser and the alpha vane.

          Please note, the F22 has negative stability. So it’s controlled by computer automation. So the F22 is subject to “artifical” stability, altough I don’t like the word “artifical”. Please take that into account.

          So I mis-read Steve’s comments as your comments.

          But he is right, your argument is circular. But it’s the word “artificial” not “slow”. You’re saying it can’t be done. Ergo the MAX is stable. But it can be done. The F22 is proof of that.

          No tricks this time.

  16. I just realized that the most compelling evidence yet that the 737 MAX can be flown safely without MCAS operating (essentially off) was provided courtesy of LNA’s own Bjorn Fehrm in the previous article.

    https://leehamnews.com/2019/11/01/bjorns-corner-analysing-the-lion-air-737-max-crash-part-1/#more-31559

    During several flights before LNI043, which was the flight before LNI610 when the aircraft crashed, LionAir flight crews reported intermittent speed and altitude flags on the left side instruments declaring that the speed and altitude data was invalid. These got progressively worse on subsequent flights until the flight right before LNI043 when the unreliable AoA sensor was replaced with the now infamous AoA sensor that had a +21° bias fault that went undetected.

    The original unreliable AoA sensor was tracked down and examined as part of the LNI610 crash investigation and found to have an intermittent open circuit in the resolver that inputted to the ADIRU/FCC. This open circuit caused an invalid AoA value to be sent to the ADIRU which is why the left side instruments received increasingly frequent speed and altitude invalid flags. Pitot and static port data has to be corrected for AoA otherwise it is invalid.

    Here is the kicker. The complete disappearance of the AoA signal did not trigger MCAS because the crew did not report any trouble with unusual AND stabilizer movements. But most importantly, they did not report any trouble with touchy controls or control reversal. Since the crew alert level was heightened because of the left side speed and altitude faults, I seriously doubt they would miss any control problems if they had occurred.

    During the flight right before LNIo43, the intermittent AoA failures were occurring often enough that the AoA sensor problem could finally be diagnosed in the post flight maintenance check when the sensor was finally replaced. This means that there were large portions of the flight right before LNI043 where MCAS was effectively shut off. Again there were no mentions of any control problems like unusually severe AND trim changes, touchy controls, or control reversal.

    This means the MAX is pitch stable at least over AoA’s that are encountered in the normal flight envelope. Otherwise the flight crew would’ve noticed pitch control problems.

    • Finally, the while discussion seemed to move backward to the stability issue and the definition of stability. There did seem to be another issue and it related to the responsiveness of the elevators, which seem to have been affected be the engines. I’m not sure what happened to that.

      • I thought they were wondering if with the stab trimmed full nose down, should the elevator be able to counter that and keep the aircraft level at any speed of load? I doubt that is a reasonable request for any aircraft. If this was the case, when the Alaska DC-9 got the stab stuck nose down by the jackscrew, they could have kept it level with the elevator.
        Or other way, if the stabilizer gets stuck full nose up, will there be enough elevator authority to keep it from stalling? Probably not.

  17. M. Bohnet, I apologize if I seem to have caricatured your position. My main issue is that I think the CEO of Boeing doesn’t agree with it. He says MCAS is not designed to address a major aerodynamic issue. Meanwhile, I think I remember some language in the JATR report that questioned an unusual or novel use of the stabilizer by Boeing engineers. But I see your point. For the most part, flying the line means going up, making minimal turns, flying straight, and coming down. This, I think, is the normal flight envelope. And the Max can perform these motions without assistance, you assert. But why is the company leaving so much in the dark — to the point where flight attendants and pilots (who must gamble with statistics by spending their working like lives on this plane) have lost confidence in it?

    • RealSteve,

      I believe you are referring to this finding.

      Finding F2.7-A: The FAA was not completely unaware of MCAS; however, because the information and discussions about MCAS were so fragmented and were delivered to disconnected groups within the process, it was difficult to recognize the impacts and implications of this system. If the FAA technical staff had been fully aware of the details of the MCAS function, the JATR team believes the agency likely would have required an issue paper for using the stabilizer in a way that it had not previously been used. MCAS used the stabilizer to change the column force feel, not trim the aircraft. This is a case of using the control surface in a new way that the regulations never accounted for and should have required an issue paper for further analysis by the FAA. If an issue paper had been required, the JATR team believes it likely would have identified the potential for the stabilizer to overpower the elevator.

      “… an issue paper for using the stabilizer in a way that it had not previously been used. MCAS used the stabilizer to change the column force feel, not trim the aircraft. This is a case of using the control surface in a new way …”
      I agree this is true for the 737, but I’m not certain this was never done before on any other aircraft. I’ll have to do some checking and think about this a bit before I comment further.

      • Well, I guess I would have to argue semantics with the JTAR team on “not trim the aircraft”. That was the trick that made MCAS work, it retrimmed the aircraft in the middle of the low speed stall test. The test given is: set aircraft to trim, and slow down and increase elevator until stall, and it must be able to stop at a stable angle at any point all the way there. That didn’t work, so the MCAS solution was to retrim part way through that event, and with that reconfiguration, the elevator maintained control of the AOA as required. By requiring higher elevator loads and deflections than before through the final angles of attack before stall.
        Was that against the rules of the test? No
        Was that against the intent or spirit of the test?
        Have any other FBW aircraft used this trick?

        • Excellent stuff. It comes back to my theory that the elevators lose their authority at ever increasing AoA. So they switched to the stabiliser.

          The big, big question is when? If it’s in the pre-stall region, who cares. But I think it’s well below the pre-stall region.

          You said part way through. But was does part way through mean.

          Is your post fact or is it the grape-vine? If it’s fact is there a reference? Have I missed something in the JATR report?

          • I believe it is a fact that there are two stall tests.
            And, that MCAS has to fire once for the 2.5 unit retrim of the stab in order to pass these tests.
            I don’t have any reference, so grape vine.

      • I am not an engineer — far from it. I’m not a pilot. I’m not a computer programmer. I deal in images and ideologies. But up to this point, before the discussion returned to the semantics of the word “unstable,” I think I was beginning to understand something. Here is what I understood: The tail of the native 737 has some funky aerodynamics. The elevator has too little authority, but it works. At certain points in the flight envelope, the new engines on the 737 Max exacerbate this problem, undermining even further the authority of elevators that didn’t have much to begin with. MCAS steps in under certain conditions and tilts the stabilizer to restore proper airflow over the elevator, restoring its authority — reasserting control, giving the control column its feel. That’s what I was understanding. And my sense is that MCAS is a cheap workaround of putting a larger tail that includes larger elevators on the Max. The reason MCAS isn’t a problem on the 767 is that it already has a proper sized tail, and that the system was used to account for the very slow, shifting balance of a fueling operation, not the loss of authority of the elevators. This is what I was beginning to think that I understood until the discussion began switching back to the semantics of the word stability. I liked the old discussion because it used the word “authority” and that’s a concept that I can work with and understand.

        • Remember also that the tail on the MAX has been modified compared to the NG.

          The tail cone has been modified to make it more aerodynamic. The vortex generators are gone!

          Have these changes affected the airflow over the stabiliser / elevators ?

Leave a Reply

Your email address will not be published. Required fields are marked *