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Old 4th Nov 2018, 22:03
  #591 (permalink)  
henra
 
Join Date: May 2010
Location: PLanet Earth
Posts: 820
Originally Posted by gums View Post
Had to go back to the Alaska Airline crash with the " jackscrew nut. Wanted to see if there had been a squak on a preceding flight. Couldn't tell, but it would seem to me with all the damage that jackscrew had and the metal trapped in the big nut, that there would have been some slight indication to the PF. OTOH, with the use of the AP, the jammed stab was overcome by basic elevator trim. Finally, aero forces and commands by the PF caused the frst failure.
Salute gums!
That was indeed a really harrowing accident. And one which was completely unrecoverable by the crew. I can't imagine a similar mechanical problem in an only 6 months old (read: brand new) 737 MAX, though, despite the fact that the final dive reminds indeed somewhat of Alaska 261.

Looking for a silver lining, this crash is a wakeup call to examine not only procedures but the design of the stab and pitch feel system, then all the air data inputs.

I tend to go with 'bird ( as in Machin') about possible PIO due to bad data coupled with a flaky pitch trim. You do not need a massive structural failure to reach extreme vertical pitch and speeds. We saw that at Rostov and that whole maneuver was done by the crew ( still have chills seeing the video). And one of our 73X drivers here claims that losing some or all of that trim/feel system can be challenging, especially at higher speeds.
A mis- behaviour of the STS / Elevator feel system as a result of an Air Data problem ist indeed probably quite high up the probability scale as a main culprit for this one. That said there should have been ways of stopping the interference of STS, e.g. by pulling the AP stab trim CB.
Still it might be worth having a second look if the systems cannot be made to act more benignly. UAS in itself is probably a challenge to a Crew and topping this with automatic mis- trimming due to bad air data is obviously not a terribly good idea and has led to accidents in the past. That is one area of flight safety where there might be a little room for improvement from an engineering perspective (although how to do it exactly will have to be very cautiously thought through - it is easy to create unwanted side effects in other cases when not done properly).

Regards,
Henra
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