Originally Posted by
GyroF-16
There have been several reports and links to reports earlier in this thread. From reading those, it looks like it went generally like this:
Somewhere between takeoff and about 2000 ft AGL, an AOA probe failed. This caused the stick shaker on the side fed by the bad sensor to activate. When the flaps were retracted, MCAS activated, running some nose-down trim. The pilots struggled to hand-fly the jet, as MCAS activated several more times. The Lion Air crew extended flaps again, briefly (which stopped MCAS inputs), then retracted them, got more erroneous MCAS, and nosed into the ground.
The Ethiopian Air crew eventually activated the stab trim cutout switch, (which stopped MCAS inputs) but then returned it to the “normal” position. MCAS then continued to trim nose down. Because the Ethiopian Air pilots also left the thrust levers at takeoff power, the airspeed build to over 300 kts. That led to aerodynamic forces that make manual adjustments to the trim wheel very difficult. Eventually the nose-down trim became too difficult to overcome, and the Ethiopian aircraft flew into the ground.
I do have access to the AFM and emergency checklists, but don’t think it’s wise to post on a public forum, as they are proprietary. I’d be happy to discuss them over beers, though.
Just know that the checklist for runaway stab trim directs a disconnection of autothrottles, and activation of the stab trim cutout switches. If these were both done promptly, and the pilots properly adjusted the thrust setting and hand-flew the airplane competently, all would have been well. I’ll also reiterate that the pilots can always override the MCAS inputs with the trim switch (prior to stab trim cutout). When the aircraft starts trimming itself in a way the pilot doesn’t like, he can always put the trim back where it belongs. Erroneous MCAS activation presents like a simple runaway trim. And the response (and checklist) is the same.
There’s more to it (both in the accident timelines and the crew response), but that’s the essence of it. There was a malfunction of the aircraft (AOA probe). The crew didn’t correctly recognize and respond to it.
Training in runaway trim is part of the 737 type rating, and is familiar to every professional pilot. When the trim runs in a Boeing aircraft, the pilots can see, hear, and feel it happening. And the stab trim cutout switch is checked as part of a challenge-response checklist prior to every flight. So the pilots certainly know where it is, and have been taught what its function is.
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