4 ms·

The older 737 could do that too - it's known as a stabilizer runaway, and could happen if (for example) the stabilizer trim switch failed closed in the nose-dow

The older 737 could do that too - it's known as a stabilizer runaway, and could happen if (for example) the stabilizer trim switch failed closed in the nose-down-trim position. It's something pilots train for, and there are trim-disable switches to handle this. Presumably Boeing believed that this was sufficient for pilots to cope with MCAS too. The problem is that MCAS doesn't run continuously - instead it runs in 10 second bursts, so it looks different from a runaway trim event that pilots had trained for. And MCAS doesn't just result in runaway trim - the failed AoA sensor is also used to correct airspeed, so this gave the pilots an unreliable airspeed indication, and it also triggered the stick shaker on the captain's side, indicating an aerodynamic stall. The combination of all these things together seems to have confused the pilots, and resulted in them not disabling electrical trim. Edit: here's the relevant data from the Lionair flight preliminary report: http://nrg.cs.ucl.ac.uk/mjh/lionair.png http://nrg.cs.ucl.ac.uk/mjh/lionair.png

pas 8y ago
How come the pilots weren't aggressively keeping up with/against the MCAS? They simply gave up? Does that require some very serious effort (pushing many buttons, fiddling with the control), or do they have to simply keep "the joystick" in one direction?
- mhandley 8y ago
  Originally the captain was flying, and he was keeping up with MCAS using manual electrical trim. But near the end, he handed control to the first officer, so that he could read the quick-reference handbook and troubleshoot. And the first officer, not knowing how much trim the captain had been using, failed to keep up with MCAS. Very sad - if the captain had kept flying, they may have survived.
- jki275 8y ago
  From what I've been reading on this, the yoke doesn't have enough control authority to override the MCAS system.
  - ams6110 8y ago
    There may not be enough elevator authority in a severely out of trim configuration. However the trim switches can be used to correct that, or failing that the electric trim system can be switched off and the trim wheels can be turned manually to bring the aircraft back into trim.
    - jki275 8y ago
      Of course, the system could be turned off. That's the issue -- they didn't flip the switches to turn it off. Had they done that, they wouldn't have crashed.
sokoloff 8y ago
> the failed AoA sensor is also used to correct airspeed, so this gave the pilots an unreliable airspeed indication Source? This is the first I’ve heard this claim.
- roelschroeven 8y ago
  From what I read it's a bit different. At high AoA the airspeed sensor doesn't work very good, therefore at high AoA the computer indicates that the airspeed is unreliable. The failed AoA sensor indicated high AoA, so the computer indicated unreliable airspeed.
  - FabHK 8y ago
    Just as GP said: > this gave the pilots an unreliable airspeed indication EDIT to clarify: Apparently this was misread as "unreliable (airspeed indication)" = "displaying an airspeed which was unreliable", rather than "(unreliable airspeed) indication" = "signal that airspeed should not be relied on".
- mhandley 8y ago
  Take a look at the graphs I linked to - they show this quite visibly. Both airspeed and altitude are taken from the pitot-static tubes, but these misread at extreme angles of attack, as the airflow isn't directly in the direction of the pitot tube. Thus the air data computer uses AoA information to correct the raw altitude and airspeed readings. Edit: can't find the reference I was looking for, but try this thread: https://www.airliners.net/forum/viewtopic.php?t=738067#p10639427 https://www.airliners.net/forum/viewtopic.php?t=738067#p1063...
  - sokoloff 8y ago
    Those appear to be a few percent off each other. I concede that’s “not ideal”, but I doubt the airspeed indication skew contributed to the accident.
    - mhandley 8y ago
ddalex 8y ago
> The problem is that MCAS doesn't run continuously - instead it runs in 10 second bursts, so it looks different from a runaway trim event that pilots had trained for What's even worse is that a runway trim could be disabled by pulling hard on the yoke which would disable the autopilot driving the trim wheel motors. Then the wheels could manually be brought into position. MCAS is not disabled by pulling on the yoke. Even if you set the trim wheels to the correct angle manually, MCAS would continue to move them, in increments that do not resemble a runway trim where you need to hold fast on the wheel. So a sequence of actions commonly used by pilots to fix the runway trim is not disabling MCAS, making the pilots very confused about what's actually pointing the aircraft nose down.
- userbinator 8y ago
  What's even worse is that a runway trim could be disabled by pulling hard on the yoke which would disable the autopilot driving the trim wheel motors. Then the wheels could manually be brought into position. Not if the runaway is caused by a stuck trim switch --- which is why there are the cutout switches and the procedure for runaway trim involves using them. Finally, if that doesn't work, manually grasp and hold the trim wheels: https://www.youtube.com/watch?v=cQirIH_DuAs https://www.youtube.com/watch?v=cQirIH_DuAs (...and if that doesn't work, there's a big mechanical problem which is unlikely to be recoverable, since the trim wheels are directly linked to the stabilizer.)
  - ddalex 8y ago
    > Finally, if that doesn't work, manually grasp and hold the trim wheels You might think of grabbing and holding the wheels is there is a continuous movement there looking like an automated system going haywire. But with MCAS, it's not. It appears that the trim wheels move in short bursts in random movements, which they ALWAYS do. There is simply no reason to suspect that the wheels move in a single direction (pointing the nose down), unless you can visually see them. Which you can't because they're beneath and behind the pilot seat.

pas 8y ago

How come the pilots weren't aggressively keeping up with/against the MCAS? They simply gave up? Does that require some very serious effort (pushing many buttons, fiddling with the control), or do they have to simply keep "the joystick" in one direction?

mhandley 8y ago
Originally the captain was flying, and he was keeping up with MCAS using manual electrical trim. But near the end, he handed control to the first officer, so that he could read the quick-reference handbook and troubleshoot. And the first officer, not knowing how much trim the captain had been using, failed to keep up with MCAS. Very sad - if the captain had kept flying, they may have survived.
jki275 8y ago
From what I've been reading on this, the yoke doesn't have enough control authority to override the MCAS system.
- ams6110 8y ago
  There may not be enough elevator authority in a severely out of trim configuration. However the trim switches can be used to correct that, or failing that the electric trim system can be switched off and the trim wheels can be turned manually to bring the aircraft back into trim.
  - jki275 8y ago
    Of course, the system could be turned off. That's the issue -- they didn't flip the switches to turn it off. Had they done that, they wouldn't have crashed.

sokoloff 8y ago

> the failed AoA sensor is also used to correct airspeed, so this gave the pilots an unreliable airspeed indication Source? This is the first I’ve heard this claim.

roelschroeven 8y ago
From what I read it's a bit different. At high AoA the airspeed sensor doesn't work very good, therefore at high AoA the computer indicates that the airspeed is unreliable. The failed AoA sensor indicated high AoA, so the computer indicated unreliable airspeed.
- FabHK 8y ago
  Just as GP said: > this gave the pilots an unreliable airspeed indication EDIT to clarify: Apparently this was misread as "unreliable (airspeed indication)" = "displaying an airspeed which was unreliable", rather than "(unreliable airspeed) indication" = "signal that airspeed should not be relied on".
mhandley 8y ago
Take a look at the graphs I linked to - they show this quite visibly. Both airspeed and altitude are taken from the pitot-static tubes, but these misread at extreme angles of attack, as the airflow isn't directly in the direction of the pitot tube. Thus the air data computer uses AoA information to correct the raw altitude and airspeed readings. Edit: can't find the reference I was looking for, but try this thread: https://www.airliners.net/forum/viewtopic.php?t=738067#p10639427 https://www.airliners.net/forum/viewtopic.php?t=738067#p1063...
- sokoloff 8y ago
  Those appear to be a few percent off each other. I concede that’s “not ideal”, but I doubt the airspeed indication skew contributed to the accident.
  - mhandley 8y ago

ddalex 8y ago

> The problem is that MCAS doesn't run continuously - instead it runs in 10 second bursts, so it looks different from a runaway trim event that pilots had trained for What's even worse is that a runway trim could be disabled by pulling hard on the yoke which would disable the autopilot driving the trim wheel motors. Then the wheels could manually be brought into position. MCAS is not disabled by pulling on the yoke. Even if you set the trim wheels to the correct angle manually, MCAS would continue to move them, in increments that do not resemble a runway trim where you need to hold fast on the wheel. So a sequence of actions commonly used by pilots to fix the runway trim is not disabling MCAS, making the pilots very confused about what's actually pointing the aircraft nose down.

userbinator 8y ago
What's even worse is that a runway trim could be disabled by pulling hard on the yoke which would disable the autopilot driving the trim wheel motors. Then the wheels could manually be brought into position. Not if the runaway is caused by a stuck trim switch --- which is why there are the cutout switches and the procedure for runaway trim involves using them. Finally, if that doesn't work, manually grasp and hold the trim wheels: https://www.youtube.com/watch?v=cQirIH_DuAs https://www.youtube.com/watch?v=cQirIH_DuAs (...and if that doesn't work, there's a big mechanical problem which is unlikely to be recoverable, since the trim wheels are directly linked to the stabilizer.)
- ddalex 8y ago
  > Finally, if that doesn't work, manually grasp and hold the trim wheels You might think of grabbing and holding the wheels is there is a continuous movement there looking like an automated system going haywire. But with MCAS, it's not. It appears that the trim wheels move in short bursts in random movements, which they ALWAYS do. There is simply no reason to suspect that the wheels move in a single direction (pointing the nose down), unless you can visually see them. Which you can't because they're beneath and behind the pilot seat.