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by WalterBright 49 days ago
The backup for MCAS was simple:

1. restore normal trim using the thumb switches (which override MCAS inputs)

2. turn the trim system off.

The proof of that is that's what the crew did in the first MCAS incident, and they landed safely.
1 comments
bumby 49 days ago
Check the previous note I left above with the * on why that is considered a poor mitigation.

Administrative procedures are bad mitigations in general but especially bad when a) it’s a safety critical issue and b) the hardware for an engineering mitigation is already installed. That’s like saying death could have been avoided if people would have just packed parachutes (PPE). Maybe true, but bad hazard mitigation.

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WalterBright 48 days ago
I do understand your point, and the MCAS system needed improvement.

But still, dealing with runaway stabilizer trim is a basic thing every pilot needs to know. 1 crew did it, and proceeded normally and safely. Two other crews did not follow emergency procedures, and paid the ultimate price. After the first crash, Boeing sent around an Emergency Airworthiness Directive reiterating the procedure. The Egypt Air crew did not follow the procedure.

The reason the stab trim cutoff switch is prominent on the center console is because it is a very important switch.

I've also talked to 737 pilots and another who emailed me about it and confirmed that they considered those crashes as pilot error.

Nevertheless, I agree that the MCAS system was deficient.

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bumby 47 days ago
The main reason I don’t consider it pilot error is because the pilots did not get training on the system. So the proper mitigation required quick understanding of a system they did not know about, which is incredibly difficult with an intermittent failure like MCAS. If it was identified too late, the force required for trim was too great to be applied manually. Expecting that knowledge and timing to be in place is why it’s not a reliable mitigation when there is no training.

There are lots of proximate causes, but the lack of training to avoid a new airframe certification is a huge one.

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