PPRuNe Forums - View Single Post - MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
Old 2nd Dec 2019, 15:25
  #4222 (permalink)  
MechEngr
 
Join Date: Oct 2019
Location: USA
Posts: 116
Originally Posted by Maninthebar View Post
Which must be diagnosed as runaway trim after 3 seconds EXCEPT when it is operating in certain corners of the flight envelope

Which are secret
There is no definition for "runaway trim" so it is impossible to diagnose it.

What is clear is that if one determines "runaway" as continuously operating, then any such motion will stop at the full-stops, which stops the runaway; ergo, the trim can not, in fact, runaway. No matter what, it will actually stop.

Were I writing manuals I would not use that term, but instead use "undesired trim movement that adversely affects pitch control forces."

Three seconds is not a requirement, but an expectation. The full fatal dose of MCAS nose-down trim is up to 27 seconds of trim run time, 9 seconds at a time. At higher speed the fatal operating time appears to be closer to 12 to 15 seconds (ET302 was nearly 18 seconds in total; 9 + 6 + 3.)

What is missing from the documentation is the pitch control force required to hold level flight and what amount of force should be an indicator to re-trim the plane. It's clear for the ET302 flight that nearly -3 units off of nominal trim did not generate such large forces as to cause the PF to trim that out before cutting out the trim motors. The first manual trim input to counter MCAS did not happen until -5 units delta had been reached and that was after 15 seconds of MCAS AND trim, though only 2 units were offset. However, that remaining -3 units of trim offset would cause the AND force to increase as the speed increased.

I've seen no documentation of expected control forces at any point in either crash flight. .
MechEngr is offline