Boeing 737 Max Recertification Testing - Finally.
Lake, repetitive questions and arguments from endless tech threads. (MCAS could have worked many times, but the crew might not know in normal operation - no AoA failure.)
Take up the technical discussion after the proposed changes are published in the NPRM.
Then we might debate fact not fiction.
Take up the technical discussion after the proposed changes are published in the NPRM.
Then we might debate fact not fiction.
Paxing All Over The World
Lake1952
One of the key restrictions that Boeing was working to was that most carries, notably SouthWest and AA (I sit to be corrected) refused to accept a new 73- that required additional training. They wanted as small a conversion as possible to save money.
As is often the case, saving money in the short term ...
One of the key restrictions that Boeing was working to was that most carries, notably SouthWest and AA (I sit to be corrected) refused to accept a new 73- that required additional training. They wanted as small a conversion as possible to save money.
As is often the case, saving money in the short term ...
Nor is it "undistinguishable from autotrim" - the latter can be arrested by the control column microswitches, MCAS can't.
Also, I guess that MCAS was intentionally tested (and activated) at least in Boeing test flights, and perhaps in certification flights, which would make the statement demonstrably false.
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Also, where will we see the FAA response to JATR:
Recommendation R3.4: The FAA should review the natural (bare airframe) stalling
characteristics of the B737 MAX to determine if unsafe characteristics exist. If unsafe
characteristics exist, the design of the speed trim system (STS)/MCAS/elevator feel shift
(EFS) should be reviewed for acceptability.
And
Recommendation R3.5: The FAA should review 14 CFR 25.201 (Stall Demonstration)
compliance for the B737 MAX and determine if the flight control augmentation functions
provided by STS/MCAS/EFS constitute a stall identification system.
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FAA response to certain parts of JATR . . . .
A very pointedly relevant question! A few suggestions for possible answers:
1) The anticipated NPRM for the AD for modifications (as explained upthread by wrench1) would be an interesting place for FAA to address what it has done about JATR recommendations as well as what it has deferred, rejected or otherwise not acted on. And then the public comments on the NPRM could, at least as a general matter under administrative law, prompt the FAA to modify the AD in more than minor ways. But the JATR recommendations noted by Benjamin's post would have to have been designed into the recertification flight test program already, no? And is it known whether this was done?
2) Relatedly, and also as clarified by wrench1 earlier, the pending JOEB and FSB inputs will yield a report which in turn will be subject to public comment, for administrative purposes. Those groups' reports, and the comments on their work, also could press FAA to address the JATR's recs . . . although again, if the recertification flight test program hasn't already included what JATR recommended, would FAA go back and retest?
3) The FOIA (Freedom of Information Act) lawsuit by the Flyers' Rights group, seeking all the documents provided by Boeing to FAA with regard to recertification, has been plodding along for months. The point of the lawsuit is not mere receipt of documents, though - the explicitly stated purpose of the lawsuit is to obtain those documents so that Flyers' Rights and a kind of "shadow FAA certification experts cadre" it has assembled (which includes famed Capt Sullenberger IIRC) can assess whether FAA has acted properly or not. Which would imply a vehicle to try to intervene via further court action to arrest a flight test and recertification program which has failed to account properly for the JATR's recommendations, and halt the return to service. (I don't speak for the Flyers' Rights group, I'm not involved in their case at all, and this is just an SLF-att'y assessment of what ....*plausibly could* happen.)
A very pointedly relevant question! A few suggestions for possible answers:
1) The anticipated NPRM for the AD for modifications (as explained upthread by wrench1) would be an interesting place for FAA to address what it has done about JATR recommendations as well as what it has deferred, rejected or otherwise not acted on. And then the public comments on the NPRM could, at least as a general matter under administrative law, prompt the FAA to modify the AD in more than minor ways. But the JATR recommendations noted by Benjamin's post would have to have been designed into the recertification flight test program already, no? And is it known whether this was done?
2) Relatedly, and also as clarified by wrench1 earlier, the pending JOEB and FSB inputs will yield a report which in turn will be subject to public comment, for administrative purposes. Those groups' reports, and the comments on their work, also could press FAA to address the JATR's recs . . . although again, if the recertification flight test program hasn't already included what JATR recommended, would FAA go back and retest?
3) The FOIA (Freedom of Information Act) lawsuit by the Flyers' Rights group, seeking all the documents provided by Boeing to FAA with regard to recertification, has been plodding along for months. The point of the lawsuit is not mere receipt of documents, though - the explicitly stated purpose of the lawsuit is to obtain those documents so that Flyers' Rights and a kind of "shadow FAA certification experts cadre" it has assembled (which includes famed Capt Sullenberger IIRC) can assess whether FAA has acted properly or not. Which would imply a vehicle to try to intervene via further court action to arrest a flight test and recertification program which has failed to account properly for the JATR's recommendations, and halt the return to service. (I don't speak for the Flyers' Rights group, I'm not involved in their case at all, and this is just an SLF-att'y assessment of what ....*plausibly could* happen.)
While FAA can use JATR recommendations as just recommendations - EASA seem clear on what is required for EASE to certify the aircraft in EASA land.
“Aircraft longitudinal stability is subject to airworthiness requirements. Boeing has to demonstrate compliance of the 737 MAX airframe with these requirements. Consequences of failures of systems affecting potentially the aircraft stability need to be assessed using acceptable safety analysis methodology also subject to airworthiness requirements,” EASA said in its published statement.
https://www.aviationtoday.com/2019/0...eturn-service/
“Aircraft longitudinal stability is subject to airworthiness requirements. Boeing has to demonstrate compliance of the 737 MAX airframe with these requirements. Consequences of failures of systems affecting potentially the aircraft stability need to be assessed using acceptable safety analysis methodology also subject to airworthiness requirements,” EASA said in its published statement.
https://www.aviationtoday.com/2019/0...eturn-service/
I find it strange that the re-certification test flights carried out were in the shorter variant -7, that currently zero units have been delivered and small orders.
I find it hard to believe that they did not have a -8 laying around to do these tests with, as the crashed aircraft were both -8 variants.
The original MAX test flights were in a -8 and reported to have an unusually forward C of G.
I find it hard to believe that they did not have a -8 laying around to do these tests with, as the crashed aircraft were both -8 variants.
The original MAX test flights were in a -8 and reported to have an unusually forward C of G.
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If the horizontal stabilizer is the same, and the nacelle aerodynamics and wing are the same, the airplane with the shortest moment arm between the cg and the stabilizer would likely be the worst case airframe configuration for the evaluation of control forces in the approach to stall for compliance with the requirement for the control force to increase in order to increase angle of attack (14 CFR 25.203(a)).
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Bend alot
This does not seem correct. The change in pitch moment is regardless of the length of the aft fuselage. The shorter the aft fuselage the greater the required force to offset the pitch moment change, so the greater the stabilizer movement to generate that force. Consider if the tail was at the rear of the wing but had to compensate for the same pitch moment. The loads would be far higher and require far more deflection to attain.
I don't foresee a large difference in pitch moment vs AoA for the nacelles based on aft-fuselage length so just going with the shortest aft fuselage should represent the worst case for that parameter.
This does not seem correct. The change in pitch moment is regardless of the length of the aft fuselage. The shorter the aft fuselage the greater the required force to offset the pitch moment change, so the greater the stabilizer movement to generate that force. Consider if the tail was at the rear of the wing but had to compensate for the same pitch moment. The loads would be far higher and require far more deflection to attain.
I don't foresee a large difference in pitch moment vs AoA for the nacelles based on aft-fuselage length so just going with the shortest aft fuselage should represent the worst case for that parameter.
Thread Starter
Agreed - shorter fuselage is always the worst case for engine pitch-up effects due to the reduced moment arm of the tail surfaces.
737 maximum engine ratings are limited on the shorter models for the same reason. I didn't work the NG, but on the 'classic' 737-3/4/500 series, the -300 and -500 models had a physical throttle block to prevent full throttle operation (the engines were common with a common rating). Otherwise under certain conditions, if you firewalled the throttle the engine pitch-up moment could be greater than what the tail surfaces could overcome.
737 maximum engine ratings are limited on the shorter models for the same reason. I didn't work the NG, but on the 'classic' 737-3/4/500 series, the -300 and -500 models had a physical throttle block to prevent full throttle operation (the engines were common with a common rating). Otherwise under certain conditions, if you firewalled the throttle the engine pitch-up moment could be greater than what the tail surfaces could overcome.
AFAIK it's limited to one MCAS-cycle per event. So it won't start to trim over and over again when jammed somehow.
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Er, no - or at least we can't say it won't do that without much more info.
The previous version didn't in fact trim "over and over again" unless it was "reset" - and then the next trim is for a new event, because it's been reset (if a system has no memory of previous state due to a reset, anything is a new event). If the pilots had known that "all" they had to do was hold the column against one MCAS-cycle of trim and not reset it (not touch the trim), MCAS might not have been as lethal.
Thing is, there has to be a reset, there has to be an end-of-event (even if it's WOW or end-of-flight), and after reset MCAS can trim again. I think I understand why the original reset was done the way it was, working from what has been revealed about the way MCAS was implemented I've tried to put together plausible implementation logic / pseudocode, and if you reset it the way they did (going from published info) you end up with something that is very definitely in the "simple, elegant and wrong" class of solutions. Really simple, really neat, deceptively enticing. I can even see how the post-flight-test changes that made it more lethal may have eluded some reviews, because they could have been done with no actual code changes at all (no logic, just values of constants - data initialization).
What I can't see is how you "fix" the reset problem without introducing a lot more state into the logic, way more aircraft-state inputs, way more complex calculations, and probably orders of magnitude more lines of code. Even then, somewhere, somewhen,there still needs to be a reset, and hiding behind that implementation is the risk of MCAS trimming over and over again. Now I haven't done real time control stuff for decades, nor flying code, there may be a solution that is simple and elegant and right, they may even have found it... but I have a suspicion (especially given the reported problems with it) that MCAS is now a solution that is complex, ugly, and maybe right - but very hard to prove it isn't wrong in some circumstances.
The previous version didn't in fact trim "over and over again" unless it was "reset" - and then the next trim is for a new event, because it's been reset (if a system has no memory of previous state due to a reset, anything is a new event). If the pilots had known that "all" they had to do was hold the column against one MCAS-cycle of trim and not reset it (not touch the trim), MCAS might not have been as lethal.
Thing is, there has to be a reset, there has to be an end-of-event (even if it's WOW or end-of-flight), and after reset MCAS can trim again. I think I understand why the original reset was done the way it was, working from what has been revealed about the way MCAS was implemented I've tried to put together plausible implementation logic / pseudocode, and if you reset it the way they did (going from published info) you end up with something that is very definitely in the "simple, elegant and wrong" class of solutions. Really simple, really neat, deceptively enticing. I can even see how the post-flight-test changes that made it more lethal may have eluded some reviews, because they could have been done with no actual code changes at all (no logic, just values of constants - data initialization).
What I can't see is how you "fix" the reset problem without introducing a lot more state into the logic, way more aircraft-state inputs, way more complex calculations, and probably orders of magnitude more lines of code. Even then, somewhere, somewhen,there still needs to be a reset, and hiding behind that implementation is the risk of MCAS trimming over and over again. Now I haven't done real time control stuff for decades, nor flying code, there may be a solution that is simple and elegant and right, they may even have found it... but I have a suspicion (especially given the reported problems with it) that MCAS is now a solution that is complex, ugly, and maybe right - but very hard to prove it isn't wrong in some circumstances.
Third hand rumor I heard is the only way to get the Max back in the air is a let on some hard stability certification requirements. The FAA is OK going there but other regulators not so much. I am guessing a lot of behind the scenes negotiations happening right now as to the exact wording of the AD......
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Boeing are laying off 19 (out of a total of 24) of their most experienced 737 MAX licensed engineers in Dublin with RTS on the horizon. It strikes me as an incredibly callous commercially driven decision bearing in mind all the regulatory eyes that will be on the project.
On an empathetic level it will be a bitter pill to swallow for the engineers who have no prospects of similar employment in the near future particularly in Ireland. There isn't a job to be had in the sector.
On an empathetic level it will be a bitter pill to swallow for the engineers who have no prospects of similar employment in the near future particularly in Ireland. There isn't a job to be had in the sector.