5 ms·

As I understand it the aircraft has a quite complex relationship with trim because of fundamental aerodynamic flaws (engines are too far forward). Am I right in

As I understand it the aircraft has a quite complex relationship with trim because of fundamental aerodynamic flaws (engines are too far forward). Am I right in saying other aircraft e.g. an A320 would handle this sort of failure without as much risk to the plane?

na85 6y ago
>As I understand it the aircraft has a quite complex relationship with trim because of fundamental aerodynamic flaws (engines are too far forward). That's not really correct. The engines being in a different position means the aircraft doesn't meet a very specific criterion of the FARs (positive stick force gradient). The 737 Max has the exact same relationship with trim as any other airliner. >Am I right in saying other aircraft e.g. an A320 would handle this sort of failure without as much risk to the plane? No. TFA explicitly states this issue was not related to MCAS. It's likely an analogous failure on an A320 or a 737NG would still have necessitated aborting the flight.
- slacktide 6y ago
  One thing that has always bothered me about how MCAS was implemented - if it truly was a stick force gradient issue, why not make the change in the Elevator Differential Feel Computer, which already manipulates the stick force gradient during approach to stall, rather than physically moving a control surface? Aside note, the elevator feel computer is a mechanical / non-electronic computer that is stuffed full of aneroids and solenoids and cams and followers and servovalves. Straight out of the 1950’s.
  - bonzini 6y ago
    Maybe that's why, I don't blame engineers who didn't want to touch it...
  - salawat 6y ago
    >Aside note, the elevator feel computer is a mechanical / non-electronic computer that is stuffed full of aneroids and solenoids and cams and followers and servovalves. Straight out of the 1950’s. That's likely why they didn't. That type of change is hard to conceal and far more difficult to keep out of the documentation. A software change could potentially be handwaved. Full on revamping of said system would probably have edged a regulator enough to have raised an objection or a deeper dive into the nature of the reconfiguration.
    - tremon 6y ago
      A software change could potentially be handwaved This to me seems a much greater process failure.
  - VBprogrammer 6y ago
    Where have you been all through the 737 Max debacle! It's refreshing to read someone who genuinely understands this issue. And for what it's worth I've pondered that before and my only conclusion was as others have said here, probably a lack of detailed organisational knowledge on the design of the pitch feel computer. Perhaps there are confounding factors, it has its own pitot tube but not a AoA vane for example. Or perhaps changing it may have triggered more regulatory oversight than MCAS did (unfortunate given events which followed).
geocrasher 6y ago
MCAS was only installed to cover up aerodynamic changes (not flaws) that would have required greater recertification and retraining of pilots. To save that expense, they covered it up with MCAS so they could say "See, it's just another 737!" which caused all the issues we know so much about.
- andy_ppp 6y ago
  So you’re saying without MCAS the plane flys just as safely but differently which would have required retraining. That’s interesting, thanks!
  - geocrasher 6y ago
    Yes @andy_ppp that's what I'm saying :)
  - cratermoon 6y ago
    No, the plane does not fly safely without MCAS. Without the MCAS there the aircraft has a tendency to pitch up, resulting in a stall situation.
    - redis_mlc 6y ago
      > Without the MCAS there the aircraft has a tendency to pitch up, resulting in a stall situation. That's not why MCAS was added. Airliners have certification requirements for "flying qualities" (smoothness) as a passenger and IFR platform. Without MCAS, the 787 MAX pitch-up in some cases exceeded the certification requirements, so MCAS was added to control the rate of pitch-up. Source: commercially-rated pilot.
    - geocrasher 6y ago
      LOTS of aircraft have a tendency to pitch up or down based on throttle. This isn't unique to the 737 MAX. A quick google on "thrust line pitch moment" gave me this: https://www.homebuiltairplanes.com/forums/threads/thrust-angle-for-pylon-mounted-pusher.26950/ https://www.homebuiltairplanes.com/forums/threads/thrust-ang... " I want to adjust the thrust angle from where it was (0° with respect to the chord line) to reduce the downward pitching moment when thrust is added (or more importantly the upward pitch moment when thrust is reduced)"

na85 6y ago

>As I understand it the aircraft has a quite complex relationship with trim because of fundamental aerodynamic flaws (engines are too far forward). That's not really correct. The engines being in a different position means the aircraft doesn't meet a very specific criterion of the FARs (positive stick force gradient). The 737 Max has the exact same relationship with trim as any other airliner. >Am I right in saying other aircraft e.g. an A320 would handle this sort of failure without as much risk to the plane? No. TFA explicitly states this issue was not related to MCAS. It's likely an analogous failure on an A320 or a 737NG would still have necessitated aborting the flight.

slacktide 6y ago
One thing that has always bothered me about how MCAS was implemented - if it truly was a stick force gradient issue, why not make the change in the Elevator Differential Feel Computer, which already manipulates the stick force gradient during approach to stall, rather than physically moving a control surface? Aside note, the elevator feel computer is a mechanical / non-electronic computer that is stuffed full of aneroids and solenoids and cams and followers and servovalves. Straight out of the 1950’s.
- bonzini 6y ago
  Maybe that's why, I don't blame engineers who didn't want to touch it...
- salawat 6y ago
  >Aside note, the elevator feel computer is a mechanical / non-electronic computer that is stuffed full of aneroids and solenoids and cams and followers and servovalves. Straight out of the 1950’s. That's likely why they didn't. That type of change is hard to conceal and far more difficult to keep out of the documentation. A software change could potentially be handwaved. Full on revamping of said system would probably have edged a regulator enough to have raised an objection or a deeper dive into the nature of the reconfiguration.
  - tremon 6y ago
    A software change could potentially be handwaved This to me seems a much greater process failure.
- VBprogrammer 6y ago
  Where have you been all through the 737 Max debacle! It's refreshing to read someone who genuinely understands this issue. And for what it's worth I've pondered that before and my only conclusion was as others have said here, probably a lack of detailed organisational knowledge on the design of the pitch feel computer. Perhaps there are confounding factors, it has its own pitot tube but not a AoA vane for example. Or perhaps changing it may have triggered more regulatory oversight than MCAS did (unfortunate given events which followed).

geocrasher 6y ago

MCAS was only installed to cover up aerodynamic changes (not flaws) that would have required greater recertification and retraining of pilots. To save that expense, they covered it up with MCAS so they could say "See, it's just another 737!" which caused all the issues we know so much about.

andy_ppp 6y ago
So you’re saying without MCAS the plane flys just as safely but differently which would have required retraining. That’s interesting, thanks!
- geocrasher 6y ago
  Yes @andy_ppp that's what I'm saying :)
- cratermoon 6y ago
  No, the plane does not fly safely without MCAS. Without the MCAS there the aircraft has a tendency to pitch up, resulting in a stall situation.
  - redis_mlc 6y ago
    > Without the MCAS there the aircraft has a tendency to pitch up, resulting in a stall situation. That's not why MCAS was added. Airliners have certification requirements for "flying qualities" (smoothness) as a passenger and IFR platform. Without MCAS, the 787 MAX pitch-up in some cases exceeded the certification requirements, so MCAS was added to control the rate of pitch-up. Source: commercially-rated pilot.
  - geocrasher 6y ago
    LOTS of aircraft have a tendency to pitch up or down based on throttle. This isn't unique to the 737 MAX. A quick google on "thrust line pitch moment" gave me this: https://www.homebuiltairplanes.com/forums/threads/thrust-angle-for-pylon-mounted-pusher.26950/ https://www.homebuiltairplanes.com/forums/threads/thrust-ang... " I want to adjust the thrust angle from where it was (0° with respect to the chord line) to reduce the downward pitching moment when thrust is added (or more importantly the upward pitch moment when thrust is reduced)"