Re:Boeing MAX production cut signals long grounding
by Zak on Sunday April 07, 2019 @10:05PM
(#19601370)
The information I'm aware of so far is that the Lion Air flight crew did repeatedly attempt to adjust the trim, but it's not known if they used the cutout switches. A previous flight with the same aircraft did experience runaway trim,and the cutouts were used before the problem became unrecoverable.
I find the design of this system shockingly ill-considered. At the very least, it should require input from both sensors and not activate if the sensors disagree. It seems to me a better system would be to automatically add some nose-down trim if the pilot applies full forward pressure on the stick since the primary underlying concern is insufficient elevator authority in certain impending stall conditions for that standard pilot response to be effective.
That's what the Lion Air crew did. They died. The Ethiopian Airlines crew followed Boeing's safety bulletin. They also died.
But I do take your point, which may prove to be correct. It is hypothetically possible to electronically trim against MCAS then immediately cut out the electronic trim system before MCAS has a chance to re-initiate (5 secs after last trim command). Then you mechanically trim for the remainder of the flight, which should be possible without too much pressure on the horizontal stabilizer.
It is crazy to think that that may ultimately be the life saving procedure here, given its complexity/nuance. Particularly for a largely undocumented (for flight crew) system.