Originally Posted by
FCeng84
- MCAS has an authority limit of at most 2.5 degrees of stabilizer (less as speed/Mach increase).
- MCAS will not move the stabilizer an increment more than its authority unless it has been reset by either pilot manual stabilizer trim command or engagement followed by disengagement of the autopilot.
- MCAS would not be reset as a result of mechanical manual stabilizer trim (i.e., pilot physically turning the trim wheel by hand)
The way I (and others, see e.g.
https://www.satcom.guru/2018/11/737-...is-option.html ) have interpreted the published/leaked MCAS descriptions is that it will limit movement to 2.5degrees
in one increment, the assumption is that multiple increments are possible.
In the description:
The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.
the reset conditions are mentioned in one sentence and then the next sentence covers if "AOA condition persists" - it isn't clear that one is conditional on the other, or how long the condition has to persist for. Consensus seems to be that STS, from which MCAS appears to derive, has a restart timer.
If it is in fact the way that you describe, then all the concerns expressed publicly (from various pilots) that MCAS doesn't honour the column cutouts would be wrong - if you are correct, then if you fight MCAS with stick back you'll face at most 2.5deg adverse trim, whereas if you fight it with trim it'll fight you all the way to the stops
[ The DFDR trace doesn't help tell us which way it is, because AFAICS every down trim is countered by manual up, just not enough to keep them flying ]
And of course if you are right then Boeing thought it wasn't important to tell the pilots which way was best to counter incorrect MCAS...