Originally Posted by
gums
Salute!
Great article, pat.
The 40 secind scenario appears to be simply allowing the MCAS to act without the crew using the trim switches. Eventually, unless power is reduced, the aero forces on the elevator and the awesome column forces required will not permit a recovery. And who wants to pull power back just after gear up? ( although I did for my LEF episode as I was light and could easily fly at 200 knots - it was the old " doing O.K. now, so don't change anything" procedure that lets you live to be old and grey) I also feel most of the 737 folks here would have used the trim switches for a time before treating the problem as a FUBAR trim system and then turning off the power as the previous flight crew did.
The MCAS mod should be interesting and provide a feast of fodder here on pPrune, huh?
Gums sends...
One of the key elements to the baseline MCAS logic is that it will only put in a single increment of stabilizer motion as long as no pilot trim command is given. This goes in over no more than 10 seconds (less if operating at Mach number greater than 0.4 where MCAS single increment authority is less than 2.5 degrees). The amount of elevator needed to balance one MCAS increment of stabilizer motion will be approximately 5 degrees due to the 2:1 ratio of stabilizer to elevator pitch control power. That amount of elevator can readily be commanded via the column with plenty of additional pitch control authority available to perform any maneuvers needed to maintain desired flight path and speed (if, for instance, climbing with a fixed throttle while varying path to maintain speed). Even in the presence of errant AOA data causing MCAS to activate when actually at AOA well below the intended MCAS activation point, MCAS will not move the stabilizer more than one increment without the crew making a pitch trim input. The only path to compromised pitch control authority via the column is for the crew to make pitch trim inputs but not make sufficient pitch trim inputs to return the stabilizer to the proper trimmed position.
Where this notion of "40 seconds" comes from is a total mystery - particularly when coupled with the statement "... without the crew using the trim switches". The scenario that led to problems after 40 seconds must have included short, ineffective periods of crew pitch trim switch commands that did not establish column force free pitch trim but did enable MCAS to insert another increment of airplane nose down stabilizer.
FCeng84 provides clarity ...