Bjorn’s Corner: Fly by steel or electrical wire, Part 6

By Bjorn Fehrm

August 30, 2019, ©. Leeham News: In our series about classical flight controls (“fly by steel wire”) and Fly-By-Wire (FBW or “fly by electrical wire”) we now discuss the flight control laws which are used for Classical flight controls and FBW systems.

Figure 1. The Boeing 737 artificial feel unit operating over right rod increases roller pressure on feel unit cam, by it making displacement of both left and right rods over Elevator Control Quadrant harder (the arrows depict an elevator up command). Source: Boeing.

The control laws of Classical versus Fly-By-Wire control systems

A Classical mechanical system, as exemplified by the Boeing 737 system, employs flight laws where the pilot moves the control surfaces proportionally to his movements of Yoke and Pedals.

To avoid overcontrol at higher speed (when the surfaces are more effective and as the system has 100% control boost) the system introduces an artificial force increase to displace the controls at increased speed via an “Elevator Feel and Centering unit”, Figure 1.

An open-loop FBW system like the Embraer E-jet’s adds displacement gain control and other enhancements to a “Control via Electrical Wires” base system. The Flight Control Modules (FCM, the system’s computers) work in parallel and inject their analog “modification” signals to the Yoke and Pedal analog displacement signals to adjust their function.

The FCMs can thus assist with pitch Angle-of-Attack (AoA) protection, Engine thrust and Configuration change compensation, Mach trim and for the rudder Yaw damping and Turn coordination.

The system requires the pilot to trim the aircraft when the speed changes. For the Pilot, the feel is a conventional flight control system with some workload taken away (Thrust and Configuration compensation, Turn coordination) with an added high AoA protection.

A feedback FBW system like the Airbus A320 (plus all Airbus aircraft after the A300 and A310) can go further in helping the Pilot. Now the connection between the Pilot and the control surfaces is via computer software. The software’s flexibility and the aircraft’s sensors are used to isolate the Pilot from the varying effectiveness of the control surfaces and changing aerodynamics within the flight envelope.

In an A320 the Pilot in normal flying gets the same pitch load factor for the same stick deflection, irrespective of speed and altitude. In roll, he gets a pitch rate. The pedals are footrests, as there is no need to kick rudder in normal flight, all turns are 100% coordinated. Further, you don’t apply pitch up in a turn, you just deflect the stick sideways and the aircraft does a clean turn without losing altitude.

It is then logical the FBW does the trimming for you. When you release the stick the aircraft remains in this attitude until the Pilot commands an attitude change. This means the system handles all Speed, Altitude, Configuration and Mach changes.

As the aircraft remains in attitude, a pitch up attitude which is not compensated by a thrust increase can lead to low speed and stall. Therefore, auto-trim FBW systems have stall protection. The system warns when stall approaches on the speed tape, Figure 2, and switches to AoA control of the stick instead of load factor at “Alpha Prot” in Figure 2, then it increases thrust at “Alpha Floor” and finally refuses to fly over “Alpha Max”.

Figure 2. Airbus pitch stall protection with AoA versus lift left and the speed tape color changes. Source: Airbus.

In the next Corner, we will look at alternate ways to implement a feedback FBW system, with examples like the Boeing FBW for the 777/787.

22 Comments on “Bjorn’s Corner: Fly by steel or electrical wire, Part 6

  1. It is also of interest how the different systems handels failure modes in the sensors and components coupled with the different autopilot/FMS and autothrottle/Engine thrust logic modes and how pilots can be tricked to stay on automation instead of disconnecting all automation and fly manually.

  2. Thank you Bjorn.

    As the Airbus side sticks are passive, and with their FBW system has envelope protection, I presume that they do not have a stick shaker ? Same question on the A220.

    Do the 777/787 aircraft still have stick shakers ?

    • Hi JaDak,

      the Airbus classic FBW and the A220 FBW (which I have both flown) do not have stick shakers. They have aural backing speed warning (SPEED, SPEED, SPEED) when passing Alpha Prot and the speed is still decaying. Re 777/787 dunno, have to check (haven’t flown these).

    • Hello JakDak and Bjorn,

      Regarding: “Do the 777/787 aircraft still have stick shakers ?”

      The following is from the audio of the NTSB animation of the July 6th 2013 Asiana Airlines 777-200ER accident at SFO at the link below, at around 3 minutes 10 seconds into the animation, or accident day time 11:27:40 AM PDT.

      “Eleven seconds before impact an audible alert sounded because the airspeed was too low. Four seconds later the pilot monitoring advanced the thrust levers, followed by stick shaker activation and a verbal call to around.”

      https://www.youtube.com/watch?v=8MFPSfGoT1U

      • Regarding: “Do the 777/787 aircraft still have stick shakers ?”

        From page 17 of section 6.15 of the Continental Airlines Boeing 777-224 at the link below.

        “Warning of an impending stall is provided by left and right stick shakers, which independently vibrate the left and right control columns.”

        http://www.ameacademy.com/pdf/boeing/Boeing-777-FCOM.pdf

        • Bjorn, Robert thank you.

          I am very interested in human factors, and their role in aiding safety. I’d like to see Airbus move to active, linked side sticks, and back driven throttles.

          I do like the idea of FBW ensuring that a passenger aircraft never gets to the point where a stick shaker is necessary.

          I really don’t like the idea of a one person cockpit with a second pilot on the ground monitoring multiple aircraft.

          Just because it’s possible does not mean it should be done.

          • And there is a lot of truth in that.

            Kind of like drones. Now they are like an infestation of lice.

          • JD:

            My view of what is missing is what is the most effecive way to get someones attention.

            The Bitching Betty and alarms or something tactile like a stick shaker.

            There has been all too little human factors research into this and all to much techy input that we can do it, so we should.

            My take is that some of Boeing ops (feedback thrto0ttles) is good and some is bad. Airbus has the auto throttle floor but no feedback.

            The problem is they just do it and don’t find out what really is best or works the way you want it, opinion vs science based.

            Sure hand stick controllers are cool and the latest but do they really work better than a yoke in front of you for your input and feedback?

            What should be done is find out and implement the best of it, not just throw it in on the latest whiz bank its cool.

          • Bjorn described the then Cseries controls
            “…… is a Fly By Wire (FBW) aircraft with a passive side stick controller, very much like an Airbus. As discovered during my A350 flight, this is OK. Active feedback sidesticks would have been better, but they were not available to the C Series project.

            The throttles are like Boeing throttles; they are back-driven, i.e., one can see and feel when the auto throttle is working, which is good. This is typical of the C Series. It takes the best ideas from Airbus and Boeing and puts them together”
            https://leehamnews.com/2016/04/29/bjorns-corner-c-series-flight-controls/

          • I suspect that as people become used to autonomous drone delivery and as they become accustomed to eVTOL style urban taxi operations such as lilium or vanhana that will initially have one pilot but then evolve into zero pilots the likelihood of pilotless airliners increases. I think you are right to focus on psychological factors. Apart from ergonomics ultimately frequent simulator drills of complex scenarios probably the best protection.

          • Hello William,

            Regarding: ” Apart from ergonomics ultimately frequent simulator drills of complex scenarios probably the best protection.”

            In the case of the Asiana 214 crash what was missing was not “simulator drills of complex scenarios” but rather a lack of basic airmanship and hand flying skills so severe that the crew was unable to establish a stabilized approach and complete a safe landing on a clear day with light winds when the instrument landing system was out of service. A crew competent in hand flying their airplane should have been easily able to accomplish a safe landing on a clear day with light winds by the method of looking out the window, scanning your instruments (this includes looking at the airspeed indicator often enough to not get a low speed warning or stick shaker activation) and working the flight controls with your hands, instead of adjusting course by punching buttons on the autopilot while either not looking out the window or not having the basic airmanship skills to recognize that you are way too high or way too low when you do so. The remedy for this, in my opinion, is not running complex scenarios in a fancy computerized simulator, but rather remedial instruction in what in the US are Private Pilot level skills in a simple Cessna or Piper trainer with no autopilot and no fancy computer displays, or better yet in a glider with no electrical system at all. In the case of the glider, either you learn to judge your approach by the method of looking out the window, or you crash short of the airport, there is no adding power to bail yourself out from a crappy approach.

            The following are excerpts from a FAA handbook for US Private Pilots (i.e. pilots two levels below Airline Transport Pilot who are not allowed to fly any airplane of any passenger capacity for hire – see link after excerpts for full text). In the US, Private Pilots are expected to know that you should not try to salvage a low approach by pulling back on the elevator without adding power. As far as I am concerned, no matter what mode the autopilot and autothrottles were in, should have been in, or what confusion existed as to how they worked, as a matter of fundamental basic airmanship, someone should have been scanning the basic flight instruments often enough to detect that the airpseed was decaying as the nose was pulled up in the attempt to extend the touch down point (never blindly trust the autopilot to do what you think it should be doing), and someone should have had their hands on the throttles ready to firewall them instantly or confirm that the TOGA button had done so in the event of a go-around order, low speed warning, or wind shear alert, in which case they would have realized the throttles were still at flight idle as the nose was being raised. Someone should have realized that something was wrong if they did not hear the engines revving or see the engine instruments revving up as the nose was raised.

            “For example, if the pitch attitude is raised too high without an increase of power, the airplane settles very rapidly and touches down short of the desired spot. For this reason, never try to stretch a glide by applying back-elevator pressure alone to reach the desired landing spot. This shortens the gliding distance if power is not added simultaneously. The proper angle of descent and airspeed is maintained by coordinating pitch attitude changes
            and power changes.”

            ” When it is realized the runway cannot be reached unless appropriate action is taken, power must be applied immediately to maintain the airspeed while the pitch attitude is raised to increase lift and stop the descent. When the proper approach path has been intercepted, the correct approach attitude is reestablished and the power reduced and a stabilized approach maintained. [Figure 8-31] Do not increase the pitch attitude without increasing the power because the airplane decelerates rapidly and may approach the critical AOA and stall.”

            ” During instruction in landings, one of the most important skills a pilot must acquire is how to use visual cues to accurately determine the true aiming point from any distance out on final approach. From this, the pilot is not only able to determine if the glide path results in either an under or overshoot but, taking into
            account float during round out, the pilot is able to predict the
            touchdown point to within a few feet.”

            https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/airplane_handbook/media/10_afh_ch8.pdf

          • In my opinion, the Asian 214 crew was more in need of more experience in landings like this one, than they were in need of more complex scenarios in simulators. No autopilot, no computer displays, no autothrottle, no throttle, no engine, no TOGA button, no go arounds. I agree that complex simulator scenarios are a very important part of airline crew training in general; however, they cannot make up for glaring deficiencies in basic airmanship.

            https://www.youtube.com/watch?v=_fdY347FWOE

          • Excerpts from the NTSB report on the Asiana 214 (see pages xii and xiii).

            “The safety issues discussed in the report relate to the need for the following:”

            “More manual flight for Asiana pilots. Asiana’s automation policy emphasized the full use of all automation and did not encourage manual flight during line operations. If the PF had been provided with more opportunity to manually fly the 777 during training, he would most likely have better used pitch trim, recognized that the airspeed was decaying, and taken the appropriate corrective action of adding power. Federal Aviation Administration (FAA) guidance and a recent US regulatory change support the need for pilots to regularly perform manual flight so that their airplane handling skills do not degrade.”

            But also, among the 14 other bullet points on pages xii to xv, the following regarding the automation on the 777.

            “Reduced design complexity and enhanced training on the airplane’s autoflight system. The PF had an inaccurate understanding of how the Boeing 777 A/P and A/T systems interact to control airspeed in FLCH SPD mode, what happens when the A/T is overridden and the throttles transition to HOLD in a FLCH SPD descent, and how the A/T automatic engagement feature operates. The PF’s faulty mental model of the airplane’s automation logic led to his inadvertent deactivation of automatic airspeed control. Both reduced design complexity and improved systems training can help reduce the type of error that the PF made.”

            Complex simulator scenarios with fancy computer displays and simpler design with less fancy displays would be exactly what is needed to address the later of the above two bullet points; however, on a clear day, a pilot skilled in 777 visual approaches, as recommended by the prior bullet point, would be able to tell by looking out the window if the automation was doing a crappy approach for some reason, whether it be the fault of the operator or the automation, or the fault of some combination of the operator and the automation, and be able to confidently turn off the automation and complete a safe landing, thus functioning as an independent backup to the automation, rather than as a captive slave to it.

            Here is the full statement of probable cause (see page 129).

            “The National Transportation Safety Board determines that the probable cause of this accident was the flight crew’s mismanagement of the airplane’s descent during the visual approach, the pilot flying’s unintended deactivation of automatic airspeed control, the flight crew’s inadequate monitoring of airspeed, and the flight crew’s delayed execution of a go-around after they became aware that the airplane was below acceptable glidepath and airspeed tolerances. Contributing to the accident were (1) the complexities of the autothrottle and autopilot flight director systems that were inadequately described in Boeing’s documentation and Asiana’s pilot training, which increased the likelihood of mode error; (2) the flight crew’s nonstandard communication and coordination regarding the use of the autothrottle and autopilot flight director systems; (3) the pilot flying’s inadequate training on the planning and executing of visual approaches; (4) the pilot monitoring/instructor pilot’s inadequate supervision of the pilot flying; and (5) flight crew fatigue, which likely degraded their performance.”

            The full NTSB accident report may be found at the following link.

            https://www.ntsb.gov/investigations/AccidentReports/Reports/AAR1401.pdf

          • The crew of Asiana 214 crashed their 777 into a seawall (actually a baywall) short of runway 28L at SFO while attempting, shortly before crossing the runway threshold, a way too late go -round while way too low and way too slow on final approach. In the video at the link below, a professional Delta 757 crew, trained to Delta and US standards, flying a stabilized approach at proper glideslope and airspeed, successfully completes a go-around AFTER passing the threshold on the very same runway that Asiana 214 crashed into while attempting a go-around before reaching the threshold.

            See around 3:30 in the video for the go-around. The yacht harbor and wooded peninsula passed at about 2:30 in the video (Coyote Point Park), is about 2 miles away from my childhood home, and is where I often rode my bike to watch airplanes landing at SFO in my childhood a long. long, time ago.

            If the last seconds go-around was ordered by ATC, it is a good thing that the crew had not switched from the approach to ground frequency several miles before landing, as the crew of Air Canada Flight 781 did on 10-22-17.

            https://www.youtube.com/watch?v=swuJ15lwlBU

          • Asiana werent the only airline to have an automation first policy
            https://en.wikipedia.org/wiki/Air_Canada_Flight_624
            The A320 crashed just short of the runway at Halifax

            “The final report was released in May 2017, finding Air Canada crew procedures to be the primary cause of the accident. It found that according to the airlines standard operating procedures when crew selects Flight Path Angle Guidance Mode, and once the aircraft passes through the final approach fix, the pilots are no longer required to monitor the aircraft’s altitude or make any adjustments to the flight path angle. ”

            The plane may be able to make a lot of ‘decisions’ its self, but when the airlines air crew SOP writers seem to take that a step further and say ‘we know best’

        • As you know fly by wire Airbus does not need stick shakers to warn of impending stall since the flight control system prevents stall by limiting angle of attack, controlling pitch rate and maintaining minimum air speed. (Alpha floor). This type of flight envelop protection has become known as “hard limited”. Boeing’s first FBW aircraft was the B777 and it and the B787 has steadfastly maintained the use of the control yoke but use “soft limited” laws for flight envelop protection. As I understand it with this philosophy if the aircraft approached a stall inappropriate pilot control inputs would be progressively reduced or countered combined with progressively increasing stick force and engine thrust but a partial stall might ultimately be allowed to develop. The Asiana Airlines Crash revealed a hole in the stall prevention software, it might have been in flare mode by the time TOGA procedure was started. The tendency of a fatigued, startled or disoriented airline pilot to stall an aircraft has lead to the occasional tragedy and putting in limits has been a good idea. The challenge is to make these life saving systems ultra reliable so that their rare failure itself does not lead to a situation. I think there is little between the two systems, those arguing for the control yoke soft limit approach have argued that the inappropriate behaviour of the first officer (pilot flying) who held AF447 into a stall immediately on autopilot disconnect might have been revealed to the other first officer sooner with a yoke. I’m not so sure as both crew were disoriented. I think a better approach to instrument failure is needed such as inferring speed from inertial or GPS data and greater redundancy from more diverse sensors. A third pilot on the flight deck without flying duties seems surprisingly effective at resolving problems.

          • And for a takeoff where the co pilot ( pilot flying) “let go of the control column just after rotation and resulting in the plane settling back hard on the runway causing a crash ( it was at max takeoff weight).
            TWA Tristar at JFK airport New York in July 1992
            https://en.wikipedia.org/wiki/TWA_Flight_843

            Even computer controlled ships rudders now can have a vestigal wheel or even a stick, so why continue with a tall control column – whos original purpose was to provide a lever force.

    • That is the idea, but a good portion of humans react differently in stressful and confusing situationd and try to debug what happend.

      • Yea, I come from the school of if we are stable don’t screw with it. That is what test pilots are for with empty airframes.

  3. In light of the reports about the Norwegian 787 engine issues over Rome:

    Bjorn do airframers or their customers ever install engines with different hours on their aircraft ?

    It would seem a prudent thing to do from the point of view of safety. If one engine is hundreds of cycles newer than the other, the chances of having two engines fail is slightly reduced is it not ?

    Also what do you make of a 737-800 engine shutdown near Athens that flies on to Prague a further 2 hours 20 minutes away instead of an immediate divert ?

    https://www.aviation24.be/airlines/smartwings/engine-of-a-smartwings-boeing-737-800-shuts-down-in-flight-crew-continues-to-prague-for-another-2-hours-and-20-minutes-on-remaining-engine/

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