4 ms·

> 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 train

by 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.

by ddalex 8y ago

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.