8 ms·

Indeed looks like they did disable MCAS using the stabilizer trim cutout switches, but the aircraft was trimmed nose down when they did so. Aerodynamic loading

Indeed looks like they did disable MCAS using the stabilizer trim cutout switches, but the aircraft was trimmed nose down when they did so. Aerodynamic loading prevented manual trimming using the trim wheels. They eventually turned electric trim back on, as they didn't really have any other options, but didn't trim back enough, and left it turned on. At this point the right airspeed indicator (which should be more reliable) reads 365 knots. VMO is 340, so they're already in big trouble. 5 seconds later, MCAS kicked back in, and then it was all over. Very sad.

fgonzag 8y ago
So MCAS put them in a hard to recover position which required electric trim to recover, and as soon as they turned on the electric trim MCAS turned on again and forced them into a crash? Then this is much worse than initially suggested, it means a pilot can be aware of MCAS and still be put in a extremely dangerous position due to the extreme control authority of the module.
- dsfyu404ed 8y ago
  Per the article a combination of things put them in that situation. Not just MCAS. Why is everyone so wedded to the narrative that this is the fault of MCAS and nothing else? Complex systems that have millions of man hours spent making them safer don't usually have back to back failures because of one faulty subsystem. We've incrementally refined commercial air travel to the point where single source accidents are basically nonexistent. I'm not saying that MCAS isn't mostly to blame but the fact that the maximum inputs from the pilot is still lesser than the control authority from the trim raises some serious questions MCAS or not. Had "extend flaps" been at the bottom of the checklist that would have disabled MCAS and likely prevented this crash.
  - quenstionsasked 8y ago
    MCAS issues nose-down, speed increases and nose pitches down. Nose never pitches back up. Plane crashes. Sure, maybe some complex combination of maneuvers could have helped them out of this but there's really everything pointing the blame at this one system. Pilots did what the boeing manual suggested (even included at the end of this report).
    - deleted 8y ago
      [deleted]
  - fgonzag 8y ago
    It's true there were aggravating factors, like the inability to manually rectify trim, which (would seem to me) would be a aerodynamic or design issue. The point still stands, the root cause was MCAS, without it this would never have happened. MCAS would not have pitched them into a dive, and even if they were pitched into a dive by some non MCAS system, they would have been able to turn on electric trim and recover. MCAS both caused the situation, and made fixing it impossible, so it was the root cause of the crash.
    - Rooster61 8y ago
      > The point still stands, the root cause was MCAS Agreed. One could argue that the AoA reading mismatch was the root cause, but the plane alerted the pilots to the discrepancy just as it was meant to (through stick shake and panel readouts). They were aware of the issue, took measures to mitigate them, and MCAS still drove the plane into the ground.
    - dsfyu404ed 8y ago
      Likewise had Boeing designed a control surfaces system such that the pilot always has more control authority than the trim this also would not have happened. I didn't say that MCAS wasn't the lions share root cause here. It's just not as simple as the "hurr durr, MCAS bad" that a lot of people keep trying to portray the more nuanced statements of various regulatory agencies as being equivalent to. MCAS might be bad but on an aircraft with a different trim system it wouldn't be "hurr durr" levels of bad.
      - deleted 8y ago
        [deleted]
      - rootusrootus 8y ago
        Are there any airliners from any manufacturer where the pilot always has enough authority to override full trim?
        VBprogrammer 8y ago
  - crooked-v 8y ago
    MCAS forced the trim to a point where (because of aerodynamics) it was impossible to manually change the trim even after disabling MCAS, and was designed in such a way that it was impossible to engage the electrical trim controls (which could have saved the plane in this case) without also reenabling MCAS.
    - dsfyu404ed 8y ago
      Flaps out supposedly would have kept MCAS off but the pilots weren't expected to know that.
      - ams6110 8y ago
        Post Lion Air, they should have known that.
        simion314 8y ago
        Why exactly? if Boeing did not updated the manuals and checklists how would pilots know that? from internet forums? Should a pilots ignore the checklists and try steps he read on the internet ? I could not find if Boeing updated their manuals/checklists after the first crash.
        cjbprime 8y ago
        Yes, Boeing issued this service bulletin after Lion Air: http://www.avioesemusicas.com/wp-content/uploads/2018/10/TBC-19-Uncommanded-Nose-Down-Stab-Trim-Due-to-AOA.pdf http://www.avioesemusicas.com/wp-content/uploads/2018/10/TBC... And the FAA made it an emergency airworthiness directive in response. The steps on it were broadly followed by the Ethiopian flight.
        cladari 8y ago
        Am I missing it because I don't see flaps mentioned anywhere in that link.
  - dao- 8y ago
    > Why is everyone so wedded to the narrative that this is the fault of MCAS and nothing else? Because the other factors don't single this flight out, and the plane wouldn't have crashed without MCAS misbehaving, or more fundamentally, without the flawed plane design that required such a system in the first place.
  - rob74 8y ago
    Sadly, it looks like those millions of man hours of incremental safety refinement also created a false sense of security. That (and management overriding engineering concerns) are the only explanations I have for how MCAS was implemented: because it had to be able to correct the pitch-up tendency of the 737 MAX and stay "invisible", so additional training could be avoided, Boeing apparently chose to use only one AOA sensor as "single source of truth" - because using both would have increased the chance of the system having to deactivate itself, which would probably have made the FAA mandate the additional training that Boeing wanted to avoid. This is just my personal theory however...
wereHamster 8y ago
> Aerodynamic loading prevented manual trimming using the trim wheels. Are the trim wheels mechanically connected to the trim mechanism in the back? Is there no way to trim the horizontal stabilizer electrically without also enabling MCAS? What would happen if the pilots continually pressed the trim up switch on the yoke? That should've prevented MCAS from activating, right? I thought Boeing airplanes were built such that "the pilot is always in control". That doesn't seem to be the case anymore, if by cutting the power to certain subsystems you can't control the airplane anymore.
- fgonzag 8y ago
  I thought all jets had to be capable of performing non powered glides (as in no electrical power, not just engine power). I guess that's no longer a requirement.
  - weaksauce 8y ago
    The planes when setup correctly have a pretty good glide slope actually. When there is no power the pilots can deploy the apu to restore basic power for flight controls.
    - rob74 8y ago
      I assume you mean the Ram Air Turbine (https://en.wikipedia.org/wiki/Ram_air_turbine https://en.wikipedia.org/wiki/Ram_air_turbine)?
      - weaksauce 8y ago
        ah yeah. got my initialisms mixed up. the apu is not an emergency source of power.
    - cladari 8y ago
      The 737 has a better glide ratio than a Cessna 172 when both set up correctly.
  - 8y ago
hencq 8y ago
Yeah, from the report: > At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and First Officer confirmed stab trim cut-out. [...] > At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. > At 05:41:54, the First-Officer replied that it is not working. [...] > At 05:43:04, the Captain asked the First Officer to pitch up together and said that pitch is not enough. > At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU direction from 2.1 units to 2.3 units. > At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic trim command occurred and the stabilizer moved in the AND direction from 2.3 to 1.0 unit in approximately 5 seconds. The aircraft began pitching nose down. Additional simultaneous aft column force was applied, but the nose down pitch continues, eventually reaching 40° nose down. The stabilizer position varied between 1.1 and 0.8 units for the remainder of the recording. Seems this debunks the initial implicit commentary how this was due to 'third world' pilots and couldn't happen in the US.
- ams6110 8y ago
  Interesting, that when they turned the electric trim back on, they only momentarily blipped the switches to trim up, and the stabilizer did move. Then five seconds later MCAS pitched down again and it was over. Wonder why didn't they trim up more when they turned the electric trim back on. On the surface it appears that was their last chance to salvage the situation.
  - rootusrootus 8y ago
    That's what I think is most interesting. Given the position they found themselves in, going against the directive from Boeing to leave the trim cutout for the duration of the flight, means they were desperate to get the trim back up. So why did they not immediately make use of the reenabled electric trim control to counteract MCAS for as long as it took? They had kept the plane flying up until reenabling the electric trim, but that one action pushed it over the edge and could not be reversed? Confusing.
brucemoose 8y ago
Layman question -- Does the MCAS system take current height from terrain into account when forcing the nose down?
- dsfyu404ed 8y ago
  It would appear that it does not.
- cjbprime 8y ago
  Nope. (And arguably it shouldn't: a stall at low altitude is much worse than a stall at high altitude, because you lack recovery time. Most fatal stalls begin below 1000ft AGL, which is where this plane was. So your stall avoidance system shouldn't shutdown just because you're at low altitude: that's when you want to avoid stalls.)
  - robryan 8y ago
    Would there be different change in altitude characteristics in a nose up vs nose down situation?
  - chopin 8y ago
    MCAS doesn't work in most low altitude conditions as it is switched off when flaps are extended (the case both for departure and landing).
unionemployee 8y ago
Had they paid closer attention to their airspeed. The aircraft could have been saved. By reactivating the system at such high speed, adding further nose down trim, there was no way to keep the aircraft level. Tunnel vision.

fgonzag 8y ago

So MCAS put them in a hard to recover position which required electric trim to recover, and as soon as they turned on the electric trim MCAS turned on again and forced them into a crash? Then this is much worse than initially suggested, it means a pilot can be aware of MCAS and still be put in a extremely dangerous position due to the extreme control authority of the module.

dsfyu404ed 8y ago
Per the article a combination of things put them in that situation. Not just MCAS. Why is everyone so wedded to the narrative that this is the fault of MCAS and nothing else? Complex systems that have millions of man hours spent making them safer don't usually have back to back failures because of one faulty subsystem. We've incrementally refined commercial air travel to the point where single source accidents are basically nonexistent. I'm not saying that MCAS isn't mostly to blame but the fact that the maximum inputs from the pilot is still lesser than the control authority from the trim raises some serious questions MCAS or not. Had "extend flaps" been at the bottom of the checklist that would have disabled MCAS and likely prevented this crash.
- quenstionsasked 8y ago
  MCAS issues nose-down, speed increases and nose pitches down. Nose never pitches back up. Plane crashes. Sure, maybe some complex combination of maneuvers could have helped them out of this but there's really everything pointing the blame at this one system. Pilots did what the boeing manual suggested (even included at the end of this report).
  - deleted 8y ago
    [deleted]
- fgonzag 8y ago
  It's true there were aggravating factors, like the inability to manually rectify trim, which (would seem to me) would be a aerodynamic or design issue. The point still stands, the root cause was MCAS, without it this would never have happened. MCAS would not have pitched them into a dive, and even if they were pitched into a dive by some non MCAS system, they would have been able to turn on electric trim and recover. MCAS both caused the situation, and made fixing it impossible, so it was the root cause of the crash.
  - Rooster61 8y ago
    > The point still stands, the root cause was MCAS Agreed. One could argue that the AoA reading mismatch was the root cause, but the plane alerted the pilots to the discrepancy just as it was meant to (through stick shake and panel readouts). They were aware of the issue, took measures to mitigate them, and MCAS still drove the plane into the ground.
  - dsfyu404ed 8y ago
    Likewise had Boeing designed a control surfaces system such that the pilot always has more control authority than the trim this also would not have happened. I didn't say that MCAS wasn't the lions share root cause here. It's just not as simple as the "hurr durr, MCAS bad" that a lot of people keep trying to portray the more nuanced statements of various regulatory agencies as being equivalent to. MCAS might be bad but on an aircraft with a different trim system it wouldn't be "hurr durr" levels of bad.
    - deleted 8y ago
      [deleted]
    - rootusrootus 8y ago
      Are there any airliners from any manufacturer where the pilot always has enough authority to override full trim?
      - VBprogrammer 8y ago
- crooked-v 8y ago
  MCAS forced the trim to a point where (because of aerodynamics) it was impossible to manually change the trim even after disabling MCAS, and was designed in such a way that it was impossible to engage the electrical trim controls (which could have saved the plane in this case) without also reenabling MCAS.
  - dsfyu404ed 8y ago
    Flaps out supposedly would have kept MCAS off but the pilots weren't expected to know that.
    - ams6110 8y ago
      Post Lion Air, they should have known that.
      - simion314 8y ago
        Why exactly? if Boeing did not updated the manuals and checklists how would pilots know that? from internet forums? Should a pilots ignore the checklists and try steps he read on the internet ? I could not find if Boeing updated their manuals/checklists after the first crash.
        cjbprime 8y ago
        Yes, Boeing issued this service bulletin after Lion Air: http://www.avioesemusicas.com/wp-content/uploads/2018/10/TBC-19-Uncommanded-Nose-Down-Stab-Trim-Due-to-AOA.pdf http://www.avioesemusicas.com/wp-content/uploads/2018/10/TBC... And the FAA made it an emergency airworthiness directive in response. The steps on it were broadly followed by the Ethiopian flight.
        cladari 8y ago
        Am I missing it because I don't see flaps mentioned anywhere in that link.
- dao- 8y ago
  > Why is everyone so wedded to the narrative that this is the fault of MCAS and nothing else? Because the other factors don't single this flight out, and the plane wouldn't have crashed without MCAS misbehaving, or more fundamentally, without the flawed plane design that required such a system in the first place.
- rob74 8y ago
  Sadly, it looks like those millions of man hours of incremental safety refinement also created a false sense of security. That (and management overriding engineering concerns) are the only explanations I have for how MCAS was implemented: because it had to be able to correct the pitch-up tendency of the 737 MAX and stay "invisible", so additional training could be avoided, Boeing apparently chose to use only one AOA sensor as "single source of truth" - because using both would have increased the chance of the system having to deactivate itself, which would probably have made the FAA mandate the additional training that Boeing wanted to avoid. This is just my personal theory however...

wereHamster 8y ago

> Aerodynamic loading prevented manual trimming using the trim wheels. Are the trim wheels mechanically connected to the trim mechanism in the back? Is there no way to trim the horizontal stabilizer electrically without also enabling MCAS? What would happen if the pilots continually pressed the trim up switch on the yoke? That should've prevented MCAS from activating, right? I thought Boeing airplanes were built such that "the pilot is always in control". That doesn't seem to be the case anymore, if by cutting the power to certain subsystems you can't control the airplane anymore.

fgonzag 8y ago
I thought all jets had to be capable of performing non powered glides (as in no electrical power, not just engine power). I guess that's no longer a requirement.
- weaksauce 8y ago
  The planes when setup correctly have a pretty good glide slope actually. When there is no power the pilots can deploy the apu to restore basic power for flight controls.
  - rob74 8y ago
    I assume you mean the Ram Air Turbine (https://en.wikipedia.org/wiki/Ram_air_turbine https://en.wikipedia.org/wiki/Ram_air_turbine)?
    - weaksauce 8y ago
      ah yeah. got my initialisms mixed up. the apu is not an emergency source of power.
  - cladari 8y ago
    The 737 has a better glide ratio than a Cessna 172 when both set up correctly.
- 8y ago

hencq 8y ago

Yeah, from the report: > At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and First Officer confirmed stab trim cut-out. [...] > At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. > At 05:41:54, the First-Officer replied that it is not working. [...] > At 05:43:04, the Captain asked the First Officer to pitch up together and said that pitch is not enough. > At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU direction from 2.1 units to 2.3 units. > At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic trim command occurred and the stabilizer moved in the AND direction from 2.3 to 1.0 unit in approximately 5 seconds. The aircraft began pitching nose down. Additional simultaneous aft column force was applied, but the nose down pitch continues, eventually reaching 40° nose down. The stabilizer position varied between 1.1 and 0.8 units for the remainder of the recording. Seems this debunks the initial implicit commentary how this was due to 'third world' pilots and couldn't happen in the US.

ams6110 8y ago
Interesting, that when they turned the electric trim back on, they only momentarily blipped the switches to trim up, and the stabilizer did move. Then five seconds later MCAS pitched down again and it was over. Wonder why didn't they trim up more when they turned the electric trim back on. On the surface it appears that was their last chance to salvage the situation.
- rootusrootus 8y ago
  That's what I think is most interesting. Given the position they found themselves in, going against the directive from Boeing to leave the trim cutout for the duration of the flight, means they were desperate to get the trim back up. So why did they not immediately make use of the reenabled electric trim control to counteract MCAS for as long as it took? They had kept the plane flying up until reenabling the electric trim, but that one action pushed it over the edge and could not be reversed? Confusing.

brucemoose 8y ago

Layman question -- Does the MCAS system take current height from terrain into account when forcing the nose down?

dsfyu404ed 8y ago
It would appear that it does not.
cjbprime 8y ago
Nope. (And arguably it shouldn't: a stall at low altitude is much worse than a stall at high altitude, because you lack recovery time. Most fatal stalls begin below 1000ft AGL, which is where this plane was. So your stall avoidance system shouldn't shutdown just because you're at low altitude: that's when you want to avoid stalls.)
- robryan 8y ago
  Would there be different change in altitude characteristics in a nose up vs nose down situation?
- chopin 8y ago
  MCAS doesn't work in most low altitude conditions as it is switched off when flaps are extended (the case both for departure and landing).

unionemployee 8y ago

Had they paid closer attention to their airspeed. The aircraft could have been saved. By reactivating the system at such high speed, adding further nose down trim, there was no way to keep the aircraft level. Tunnel vision.