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Comment by tus87 | Hacker News Reader

> 1. Use inputs from both AOA sensors, if they disagree by 5.5 degrees, disable MCAS.

Great, and now you have the other problem the MCAS was supposed to mitigate (the tendency of the MAX to do backflips).

Shrugs, if the MCAS disables with a clear notification to the pilots I think at this point most 737-Max pilots would know the procedure to deal with that.

Better than the plane flying itself into the ground.

Which raises a rather obvious question ...

> Shrugs, if the MCAS disables with a clear notification to the pilots I think at this point most 737-Max pilots would know the procedure to deal with that.

The plane is only certified to be 'identical to the 737' when MCAS is on, and operating correctly.

If MCAS has to be disabled in response to a common mechanical problem, then the plane must be re-certified.

Although at least now you're trying to solve a more logical problem. Increasing angle of attack => counter with elevator, if not sufficient reduce thrust

Reducing thrust will increase angle of attack. The angle of attack is the angle of the wing relative to the airflow, not the angle of the plane relative to the horizon.

Yes conceptually, no because of signage error.

If you were in a position MCAS would have to kick in, you were at positive AoA, reduced thrust would impart a negative AoA moment.

Exactly, I'm only talking about in the MCAS-scenario.

> you were at positive AoA, reduced thrust would impart a negative AoA moment

I don't think that is correct. I think you're still thinking of the angle of the plane with respect to the horizon, not the AoA.

If reducing thrust reduced AoA, then you could recover from a stall by reducing thrust. In reality, the opposite is the case. The only way to recover from a stall by manipulating engine thrust is to increase it (though this won't always be sufficient by itself).

Okay, I get what your saying. I just wasn't reading right, and while what I've said may be theoretically correct, under most circumstances, the control surfaces are the primary contributors to the aircraft's orientation.

Fiddling with engines in an MCAS activation situation would just offset the flight envelope into a more dangerous zone, that could be even more difficult to recover from.

Took me a bit, but I got there.

We're not talking about the aircraft's orientation, we're talking about the Angle of Attack. The AoA can increase even if the nose of the aircraft is dropping relative to the horizon.

AFAIAK the procedure is to not increase thrust, quite the opposite (disable the autothrust thingie), get the nose below the "horizon" (which is of course wind dependent), gain airspeed, and then start to increase thrust (which will automatically start to up the nose a bit), then climb back to whatever altitude you should be at.

I don't mean to say that increasing thrust is the standard stall recovery procedure in most aircraft, or that all stalls could be recovered from merely by increasing thrust. But if one could only manipulate thrust, one would increase thrust to recover from a stall, not decrease it.

Ah, yes, of course, theoretically if you can just increase thrust arbitrarily, that always solves the issue. (Again, of course, this might mean the whole plane becomes a rocket because the wings already fell off, so you might need some thrust vectoring too.)

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