PPRuNe Forums - View Single Post - MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
Old 2nd Dec 2019, 23:28
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Notanatp
 
Join Date: Jul 2019
Location: Mass
Posts: 23
Originally Posted by MechEngr View Post
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. .
It did not take 18 seconds for the ET302 crew to recognize the erroneous nose-down trim and respond. Nor was the time to recognize and begin to respond "fatal."

The first MCAS nose-down trim started at 05:40:00 and lasted (uninterrupted) for 9 seconds. It moved trim down 2.5 units from 4.6 to 2.1. The crew responded with nose-up trim at 05:40:12, three seconds after the AND stopped and 12 seconds after it first activated. They trimmed up, but only from 2.1 to 2.4.

The second MCAS nose-down trim started at 05:40:20 (5 seconds after the crew released the MET switch). It trimmed down for about 8 seconds from 2.4 to 0.4 before it was interrupted. The crew trimmed up for close to 9 seconds to 2.3. They then shut off electric trim.

They recognized that erroneous automatic trim had placed the plane in an out-of-trim condition. What was fatal was that they didn't trim back up closer to 4.6 before shutting off electric trim and then made the same mistake after turning it back on.

The time for either Lion Air Captain to recognize and respond to erroneous AND trim was not "fatal" either. The Lion Air report indicates that, on the flight before the accident flight, the crew allowed MCAS to run for the full 9 seconds (or nearly so) 3 times. None of those were "fatal" because the crew trimmed back up to 5-6 units each time. On the accident flight, the first MCAS activation (at 23:25:17) only lasted for 2 seconds before it was recognized and interrupted by the Captain trimming up (the Captain interrupted MCAS with nose-up MET 21 times and never allowed the AND command to run the full 9 seconds). The FO, on the other hand, allowed MCAS to run for the full 9 seconds (or nearly so) twice. He recognized there was a trim problem after the first MCAS activation but only trimmed up by an insignificant amount.

Last edited by Notanatp; 3rd Dec 2019 at 00:04.
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