Bjorn’s Corner: New aircraft technologies. Part 38. Flight tests

By Bjorn Fehrm

November 17, 2023, ©. Leeham News: We are discussing the different design phases of an airliner development program. After covering Conceptual, Preliminary, and Detailed design and the manufacturing of prototypes and their roles, we now look deeper at the flight test phase.

Figure 1. The flight test fleet for the certification of a new airliner. Source: Leeham Co.

The Flight tests

We looked at the overall flight test period, the needed flight test aircraft, and what area they covered last week, Figure 1.

Now we discuss the actual flight testing, its organization, and what it must cover to get the aircraft certified.

The flight tests and all the activities that go along with the tests on the ground are divided into two phases;

  1. The first phase is the OEM’s flight tests. Here, the OEM verifies the characteristics and performance of the aircraft and performs any corrections to the airframe, propulsion, or avionics to fulfill the agreed certification requirements from the regulator(s) and the promised performance to customers.
  2. Once the OEM has performed its tests, corrections and retests and can present a Certification compliant aircraft, it applies for the regulator to join for the certification flight tests. For FAA certification, this is called to get “Type Inspection Authorization, TIA.”
The OEM’s flight tests

The initial flight tests of a new aircraft are done to clear the flight envelope of the aircraft, as discussed in Part 36. It’s the most risky part of the flight test campaign, containing the described risk of flutter but also other risks.

Early jet airliner high-speed flight tests found dangerous transonic effects, such as Mach tuck (the aircraft dived deeper and deeper, ignoring Yoke back pulls), when testing maximum dive speed at Machs where supersonic flow moved the aerodynamic center back on the wing, and transonic effects rendered the horizontal tailplane less effective.

We also have the famous T-tail accidents with the BAC 1-11, where the wing wake shadowed the horizontal tailplane during stall tests, causing a deep stall where the crew and aircraft perished.

Another risky moment is the verification of One Engine Inoperative (OEI) characteristics, where the aircraft operates close to stall speeds with a highly loaded vertical tail, controlling the missing engine yaw moment. Straight stalls are less risky, but stalls with uneven flow on the airframe and highly loaded flight surfaces can result in dangerous discoveries.

The stall tests at various speeds, altitudes, and configurations used to be risky. Today, the detailed knowledge of flow separation behavior and flight control effectiveness from performed low-speed wind tunnel tests has made it less risky. But we have recent examples where the ever larger engine nacelles for wing-mounted engines create new pitch-up tendencies at stall that must be handled.

An essential part of the OEM flight tests is the performance flight tests. The projected field performance at normal, and hot-and-high conditions must be verified, and the fuel consumption of the aircraft must be documented at different flight conditions and speeds/altitudes. The tests are called NAMS flying (Nautical Air Miles (NAMS) fuel consumption test flights).

If there is a problem with the NAMS results, extra CFD runs and flight tests must investigate the root cause of any non-foreseen drag. It can be airframe problems but also a powerplant not performing to specification. It doesn’t have to be an engine or nacelle problem in the latter case. There are numerous examples of airframe/powerplant installations where interference drag between the airframe and nacelle/pylon has caused NAMS performance problems.

Any problems detected during OEM flight tests mean an extended test period, extra investigations and tests on the ground, and a delay in the certification flight tests.

Certification flight tests

When the OEM deems it’s ready for Certification flight tests and the Regulator agrees, the flying for Certification points can start.

The certification flights are done with a team of Regulator and OEM pilots and test engineers. All data is analyzed during and after the flights to verify that the test cases have covered the intended test points and that the agreed means of compliance have been fulfilled.

It takes a better part of a year to do these tests involving hundreds of carefully planned and conducted flights. All the flight test data must be documented and handed over to the regulator in a format that can support the validation of the applicable certification criteria.

Flight Test duration

An OEM flight test typically takes a year for a reasonably trouble-free project. When certain aspects of the aircraft need a redesign, this extends the time frame, and we have current examples of multiyear extensions.

The Certification flight tests then typically add another year, where the last months are dominated by report and documentation work to ensure all required documents are in the form and revision required so the Regulator can issue a Type Certificate.

17 Comments on “Bjorn’s Corner: New aircraft technologies. Part 38. Flight tests

  1. It has happened that the test aircrafts and early production aircrafts are a tad overweight and its engines does not meet the SFC promised (787 Terrible teens). Hence you need several test aircrafts and spare engines to run new tests for each improvement. As engines have their own certification with its requirements that might not give the life on wing that aircraft builder nor Emirates Sir Tim Clark expects, hence the aircraft manufacturer should specify extra tests besides what EASA/FAA requires for engine certification like a 3000 cycles “Max power desert run” on the engine manufacturers own test aircraft before shipping certified powerplants for installation on the production aircrafts.

    • Thats not part of certification is it. There is no way that 3000 max power takeoff runs are going to happen in desert conditions before an airline buys a new engined plane . Let them wait for 10 years after EIS, is the simple answer but of course that means launch order pricing is off the table ….so sad.

      Aero engine durability is a journey not a destination but Im sure more is known now than 10 years ago for the big fans

      • A normal well designed narrowbody engine shall make it. A well designed widebody engine has problems making it but can make it if not pushed too hot.

        • Claes:

          The Authorities are only interested in safety (and emissions/noise). As long as the safety is not impacted and the noise and emissions are met, the market determines what the limits are for durability and maint times.

          At some point we may see mini um seat pitch requirements.

          • Trans,
            The seat pitch requirements would not be an obstacle for the manufacturer. LOPAs and seat selections are the purview of the customer. Seats galleys and bins are all SFE and the requirement is that they must mount on the interface tombstones in the OE floor grid. Good thought though……

          • I know the FAA/EASA cert is focused on safety and inspections, but I wrote that aircraft manufacurers and airline engine customers are very interested in time/cycles on wing even in harsh conditions like in the Middle East. That is why they should demand more testing in those environments than just a FAA/EASA certifed engine. This is precisely what Sir Tim Clark is doing on the B777-9 and its GE9X engines and I think many more will follow his example in these areas.

          • ‘At some point we may see mini um seat pitch requirements.’

            Is this not what the Exit Test is all about?

            My ex-girlfriends sister was an engineer at BBD and was over-seeing the testing for an aircraft there. Showed me some footage that they had – it was quite interesting.

            Flight attendants screaming at the top of their lungs, helping (pushing?) people through the doors and once it was all said and done – cheering and beverages for all.

            Suffice to say – nobody stopped to open a bin or reach under a seat, for luggage…

          • Frank P:

            Seat Pitch is only tangentially linked to evacuation.

            Evacuation is simply getting the max cabin loading out of the aircraft. If you have short seat pitch and the numbers are too high or they can’t move, then yea, indirectly it matters but not in and of itself.

            The Aircraft mfgs test at maximum cabin fill they can get so that any airline can use it (I don’t know that anyone actually does it but there could be some, I recall Philippine Airlines with 440 on an A330 (which Airbus claimed made the A330 a VLA so they could include it in their forecast – reality was the 747 and the A380 were but that is off track)

            There are also number of exits required and cabin crew per pax.

            Nothing says AHJ can’t impose a minimum pitch. If it was possible that the minimum pitch was higher pax than the evac max, then evac max wins. Pretty standard in regs is the higher standard rules.

            We can add in that on takeoff or landing (or declared emergency) the overhead bins lock. Some people try to or do grab their stuff in an evac and that needs to be controlled.

            I am also a proponent of making seat belt use full time, it would save a lot of mayhem (yes I keep my self belted, not tight but belted in).

          • Scott C:

            “The seat pitch requirements would not be an obstacle for the manufacturer. LOPAs and seat selections are the purview of the customer. Seats galleys and bins are all SFE and the requirement is that they must mount on the interface tombstones in the OE floor grid. Good thought though……”

            I like simple solutions and setting a minimum pitch standard would be just that, then each airlines determines what the mix they use from there. I am very wide shouldered and width is not an issue. Kind of a plug shape in fact. Granted some do not like the widths but on single aisle, you are stuck with it.

            AHJ could also issue a minimum width spec but that would only work with future aircraft (or stop dense packing on wide body). Nothing to be done on MAX or A320/220/E175 etc.

            As an aside my all time favorite Federal Regulation was how to deal with Lead Batteries.

            Rather then tell the battery mfgs what, where, when and how they simply said, by X date you will recover 90% of the lead you have put out into the world (there may have been a phase in on %).

            What the battery mfgs did was, OMG, and started talking to each other. What they came up with was any battery outlet has to take back batteries (they acualy used to pay you a bit). They have several centers that process batteries that they all pay into a percentage equal to what they build.

            Like magic, there was no longer a lead battery problem.

    • Claes:

      In the case of the 787, the engine themselves were not performing at SFC the mfgs said they would.

      I believe in the Boeing to Airlines contract documents, the aircraft met that part of the contract though as I recall barely.

      GE and RR began immediate PIPs to correct their contract failures.

      Boeing had their own overweight issues ( 5 tons as I recall) but that can be tested around.

      GE did two pips and then apparently continued to PIP but did not advertise it.

      RR did two or three, then gave up and did a pretty much all new engine in the Trent 1000 TEN (I just call it the Ten). They never did meet spec though it may have been close enough in a range to deal with Boeing.

      RR then cascaded the TEN into the Trent 7000 full bleed air so that was maybe a split convenience thing that got them the 7000 even though the TEN has been a commercial and tech failure (it was only after the 7000 was out that RR figured out they had a Harmonic that translated into the TEN via the 25% that was not new.

      The A330NEO of course is not a success either though we keep hearing it will be a few years down the road. Stay tuned on that one.

      • Think GE pretty quickly improved the GEnX and RR had some issues but fixed all except one on the early Trent 1000TEN’s and by now should have fixed that one as well. The T7000 is running a tad cooler than the T1000 and has a good chance to improve when RR and Delta Tech ops works together.

        • Claes:

          I believe that is correct or will be as the RR engines affected hit the max cycles that require them to be fixed (that was scary as RR kept revising that downward as more engines failed in flight)

          What RR has not fixed on the Trent 10 is the SFC, it meets the spec but GE has improved theirs and are over spec now and ahead of RR.

          • It is hard to fix SFC, you can redo the fan to pump more air in the bypass stream and increase bypass ratio and tune the LPT to give the extra power required with nacelle exit area modifications. GE redid the Fiat Avio designed high lift airfoil LPT and compensated the extra mass by going to 3D printed TiAl parts in the aft stages of the GEnX LPT pretty early. Getting the HPC to be more effective and increase the Turbine inlet temp is normally major surgery.

      • Rolls Royce didnt ‘give up’ on improving the T1000 at all.
        What they had, which GE didnt, was a new widebody engine come out of development for the A350 ( GEs GE9X was still years ago)

        Thus the T10 had a new variant name because …”It features a scaled version of the Airbus A350’s Trent XWB-84 compressor and Advance3 core technology”

        • Duke:

          In the equipment world if you change 50% of a machinem, its considered new.

          RR replaced 75% of the Trent 1000. That more than meets the definition as new.

          RR is no longer selling the Trent 1000. They (at least in theory) have fixes for all its issues, those would be fixes not improvements.

          RR is not going to try to improve the Trent 1000, there is no gain and all loss. If someone wants an RR engine they order a Trent 10 (not that anyone seems to be doing that, its all GE selection from what I see).

          When two major carriers switch engines after having built up a fleet of RR engine and been RR uses forever, you know your goose is carbon.

          The 3 spool at one time had better SFC which paid for its higher maint costs as well as overhaul (and longer on the wing to boot). No longer true.

          But when your competition is getting better SFC then you may well want to switch (or as new airlines buy a 787 they just choose the GE).

          • Look at the CFM-56 , the final version bears little resemblance to the early versions The technology doesnt advance in Pips, far more step changes than that
            Such as the CFM56-7 series for the 737 new generation
            ‘Improvements are due to its 61-inch titanium wide chord fan, 3D aerodynamics designed new core and low-pressure turbine with single crystal high-pressure turbine and Full Authority Digital Engine Control (FADEC). Fan blades are reduced from 36 (CFM56-5) to 24″ Wikipedia
            Same for the CF34 family – the last version for the Embraer E series is unrelated to the earlier ones , when its actually a mini CFM-56

            Anyway the Trent 1000 Ten is still part of the T1000 family, which started nearly 20 years back for the then proposed Dreamliner , so of course its very different for the planes coming the assembly lines

          • Duke:

            There is a complete difference between engines evolving and what happened with the Trent 1000.

            The Trent 1000 is no longer made.

            Clearly there is a heritage aspect to the Trent line, dating back to the RB211 and 3 Spool engines. That is true of all engines other than the early centrifugal s that were a dead end.

            The Trent 10 was intended to deal with all the Trent 1000 issues as well as be the base for the Trent 7000.

            While I see the Jumo 004 as the ancestor to the modern jet engines, its not the same line as a GenX or a PW GTF.

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