Originally Posted by
L39 Guy
The Runaway Stabilizer checklist states "Condition: Uncommanded stabilizer trim movement occurs continuously". I think we can agree that an MCAS event is "uncommanded".
What does continuous mean? 5 seconds, 10 seconds, 1 minute, one hour? And let's say that the uncommanded stabilizer trim movement is caused by an intermittent short circuit somewhere (trim switch, wiring harness, etc) that produces a 5 second, 10 second, 1 minute uncommanded stabilizer trim movement? How can you tell the difference? Do you really care what the source is? Does the source affect the outcome? Do you really think that someone flying along, fat, dumb and happy and suddenly has the nose pitch down will have the presence of mind to count how long the trim is moving? I would suggest the shock value would preclude that.
As well, one does not want to get into the game of diagnosing the source of the failure (MCAS or otherwise) while the control of the aircraft is at stake. One checklist to cover all scenarios is more than adequate. Secure the malfunction, fly the airplane and land asap and save the diagnosis and troubleshooting once you're back on terra firma.
Totally agree that no one is going to time the length of the runaway, at least I would hope not - act as soon as it is seen.
My point was more about the nature of the emergency AD that could have been much clearer about uncommanded trim that stops with any pilot trim then restarts about 5 seconds later.
In that case stress importance of first fully trimming then hitting cutout.
This is hinted at in a note at the end of the procedure, I say it should have been highlighted.
Unlike 'stuck switch" uncommanded trim the MCAS case does allow for pilot electrical trim first then followed by cutout, a quick blip of the switch is all that would be needed to test if this was possible. That could be a step in the runaway trim procedure, I agree do not need a seperate MCAS checklist if that was included.
As I said before I am sure Boeing pilots/engineer could come up with a clear procedure if given the mandate to do so.