PPRuNe Forums - View Single Post - NYT: How Boeing’s Responsibility in a Deadly Crash ‘Got Buried’
Old 24th Jan 2020, 19:37
  #106 (permalink)  
MechEngr
 
Join Date: Oct 2019
Location: USA
Posts: 134
Originally Posted by Semreh View Post
So you agree that MCAS is not continuous, and therefore it is entirely reasonable not to characterise its operation as continuous/runaway, so not applying the runaway stabiliser QRH procedure is a reasonable (non) response?

The Runaway Stabiliser QRH characterises 'Runaway Stabiliser' as:

"Condition: Uncommanded stabilizer trim movement occurs continuously."

MCAS can be 'one-shot' or continual, interrupted by Stab Trim operations, and operating again if the MCAS trigger conditions still apply 5 seconds later. It is not, as you point out, continuous, which is enough to put doubt into the mind of a cognitively overloaded pilot whether the Runaway Stab QRH is appropriate.

It doesn't help that STS operates by executing uncommanded stabiliser movements, so pilots regard that as normal operations. Seeing and hearing the trim wheels move is normal flight deck background, which is part of the reason why MCAS was so insidious.
Does STS require 60 pounds on the control wheel to maintain the desired pitch? I'm confused why you think that STS performance has anything to do with this. Some pilots have commented that they have to unwind some STS inputs via the trim switches; why would they not do the same for MCAS?

At what point should a pilot refuse to make trim inputs while pulling 60 pounds on the wheel when trim is available to them? After how many uncommanded adverse trim inputs should a pilot assume that the trim system has failed?

OTOH MCAS operated continuously long enough that when it was not offset the plane crashed. You would not close your eyes for a full 9 seconds driving on a crowded highway, so pick the amount of time the pilots should ignore an adverse trim increasing the control forces.

I have previously recommended that it change from trim blah-blah-blah to adverse trim forces, but I would have assumed that most pilots would know to trim the plane against high control loads. It turns out that it is a bad assumption and that training allows for pilots who try to overcome such forces with their own musculature.

As you want to be a language pedant you are welcome to it.
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