Originally Posted by
FCeng84
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 ...
Request for clarification:
Do I understand this correctly?
If MCAS detects an AOA that satisfies whatever criteria is needed, it activates 10 seconds (or less, depending on Mach number) of nose down trim for a maximum of 2.5 degrees nose down trim and stops.
If NO pilot trim input is commanded, MCAS remains disabled?
Thanks, GY