Originally Posted by
FCeng84
MCAS does not use any equipment not already on the airplane, so nothing new to keep track of with regard to component failures. The manifestation of an MCAS failure would depend on the nature of that failure so hard to address that question without specifics. Lion Air discussion covers manifestation of MCAS response to an AOA sensor signal bias / failed high scenario.
As for the rationale behind basing MCAS on a single sensor, that is the default architecture for 737 with each FCC using only the sensors wired to that unit. MCAS seems to have been designed relying on two assumptions with regard to pilot response to erroneously high AOA: (1) If the pilot activates electric trim the column forces will be trimmed to (or near) zero; and (2) Repeated cycles of pilot trimming off column forces followed by automatic stabilizer control driving the stabilizer away from trim when flying at an otherwise steady condition would be recognized by the crew as errant auto stabilizer control action and treated in the same manner as runaway stab => pilot selection of stabilizer cutout via aislestand switches. I would imagine that the MCAS updates that are to be available soon design so as not to rely as much (if at all) on these two previous assumptions.
It is my understanding that MCAS can/will come active at AOA less than that for stick shaker in some configurations, but that MCAS will not be active at/below the AOA associated with any normal trim condition.
One of the objectives of MCAS is to recognize pilot activation of electric trim commands as indication that the crew is working to offload column forces. The pilot trim process is usually somewhat iterative with the flight crew activating trim for a period of time and then evaluating via continued manual flying through the column whether or not further refinement of the stabilizer position is needed. It seems reasonable to choose five consecutive seconds of no further pilot trim command input as indication that the pilot is satisfied with the trim and thus the column is at/near zero.
Can you provide a response to 1174 that would rule out that scenario?