Originally Posted by
gums
Sorry, but my understanding of bad AoA inputs or a combination of sensor inputs to the ADRIU's is that would trigger the a/p disconnect. Would you trust mach to govern elevator trim if it showed 0.3M when at 35,000 feet?
I understand that the MCAS is not supposed to work if A/P engaged. Or is it only that I cannot connect A/P if MCAS trying to kill me?? I also got from the FCOM stuff that A/P would disconnect if we had unreliable/disagree IAS/alt/etcetera. SHeesh. The fault tree I have to run thru when I have all the warnings and bells and whistles and stick shaker in the cockpit is beginning to bug me.
My scenario is based upon the FCC and other boxes disconnecting a/p. Not MCAS, which ain't supposed to work unless flying pseudo-manual with flaps up.
Thanks gums
The idea of "pseudo-manual" mode raises an interesting point about how airliner automation has evolved somewhat ahead of certification philosophy. Besides the particular nacelle issue with the MAX addressed by MCAS, it makes sense to relax pitch stability (in all airliners) by pushing the rear CG limit further back, thus reducing tail download, and reaping the fuel efficiency benefit from the reduced drag. A system like STS can then supply certification-required speed stability (for example) that has been somewhat degraded. The aircraft is still stable and flyable minus augmentation but not to certification standards.
But in the situation that sensor failures lead to the autopilot deactivating, it likely drops you into a pseudo-manual flying mode probably dependent on the same suite of sensors. My understanding is that certification dictates that the system cannot automatically deactivate the manual flying aids (e.g. STS, EFS, MCAS) and annunciate that it has done so, even if based on known sensor input failures they likely will be degraded.