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Test were done with MCAS regarding how soon you have to react before it becomes unrecoverable. This was 10 seconds and it was hard for boeing-trained pilots ins

by rocqua 5y ago

Test were done with MCAS regarding how soon you have to react before it becomes unrecoverable. This was 10 seconds and it was hard for boeing-trained pilots instructed in MCAS to recover from this in tests where they knew it would happen. The problem was that at some point the trim is so much that it becomes nigh impossible to hand crank the trim down, the trim flap applies so much pressure it cannot be overcome by hand. You had to disable mechanized trim because otherwise MCAS would kick in. So you would need to recognize the problem soon enough to prevent MCAS from moving the trim so far that it became unrecoverable. (Source: Netflix documentary)

mlyle 5y ago
> This was 10 seconds 10 seconds is a long time of the trim wheel being cranked nose up. The same situation would apply to a stuck trim switch on the yoke. (Indeed, a stuck trim switch is worse in this respect because it cranks constantly and doesn't "yield" to the pilot pushing the switch the other way... of course, the checklist didn't presume a pilot should be diagnosing stuck AOA vs. stuck trim switch and thus recommended immediate use of the trim cutout switches instead of trying to electrically trim the other way). There's a reason that trim runaway is a "recall" procedure in the QRH that pilots are expected to know-- it gets scary. http://www.b737.org.uk/images/runawaystab2020.jpg http://www.b737.org.uk/images/runawaystab2020.jpg But it was the assumption before the MCAS disaster that pilots could deal with trim runaway, given that there was a history of flight crews dealing with trim runaway incidents in flight successfully. But MCAS was worse because: A) It did it more often, so flight crews were exposed to this dangerous situation more than ever before B) It did it in a sneakier, strange way (no constant movement of the trim wheel). Still, some real-world flight crews did overcome MCAS doing the wrong thing without crashing, even if others failed the test.
- rocqua 5y ago
  I imagine point B is quite relevant here. MCAS going wrong is meaningfully different (and less immediately scary) from the recall procedure that the training doesn't kick in.
  - mlyle 5y ago
    Sure. The main points I have, I guess, are: - It's perhaps understandable why engineering teams-- pushed by management to deliver a 737 that doesn't require retraining-- decided that if flight crews could handle runaway trim, they could handle any errant MCAS activations and that the level of redundancy and fault tolerance necessary in the system would be low. - On the other hand, real flight crews obviously do not do great when exposed to this test. - If you give a flight crew a wacky situation with conflicting information (like AF447 or the 737 MAX)-- expect failures, even if the situation is technically 100% recoverable and flyable and some crews exposed to the situation succeed. It's great that we have humans capable of reason up in the front of the plane, but reason is often faulty and doesn't get you the solution you need on the first try.

by rocqua 5y ago

mlyle 5y ago

> This was 10 seconds 10 seconds is a long time of the trim wheel being cranked nose up. The same situation would apply to a stuck trim switch on the yoke. (Indeed, a stuck trim switch is worse in this respect because it cranks constantly and doesn't "yield" to the pilot pushing the switch the other way... of course, the checklist didn't presume a pilot should be diagnosing stuck AOA vs. stuck trim switch and thus recommended immediate use of the trim cutout switches instead of trying to electrically trim the other way). There's a reason that trim runaway is a "recall" procedure in the QRH that pilots are expected to know-- it gets scary. http://www.b737.org.uk/images/runawaystab2020.jpg http://www.b737.org.uk/images/runawaystab2020.jpg But it was the assumption before the MCAS disaster that pilots could deal with trim runaway, given that there was a history of flight crews dealing with trim runaway incidents in flight successfully. But MCAS was worse because: A) It did it more often, so flight crews were exposed to this dangerous situation more than ever before B) It did it in a sneakier, strange way (no constant movement of the trim wheel). Still, some real-world flight crews did overcome MCAS doing the wrong thing without crashing, even if others failed the test.

rocqua 5y ago
I imagine point B is quite relevant here. MCAS going wrong is meaningfully different (and less immediately scary) from the recall procedure that the training doesn't kick in.
- mlyle 5y ago
  Sure. The main points I have, I guess, are: - It's perhaps understandable why engineering teams-- pushed by management to deliver a 737 that doesn't require retraining-- decided that if flight crews could handle runaway trim, they could handle any errant MCAS activations and that the level of redundancy and fault tolerance necessary in the system would be low. - On the other hand, real flight crews obviously do not do great when exposed to this test. - If you give a flight crew a wacky situation with conflicting information (like AF447 or the 737 MAX)-- expect failures, even if the situation is technically 100% recoverable and flyable and some crews exposed to the situation succeed. It's great that we have humans capable of reason up in the front of the plane, but reason is often faulty and doesn't get you the solution you need on the first try.