Originally Posted by
Cows getting bigger
FCeng84, thanks for the very useful aerodynamics lessons. Joining others, we may be jumping ahead somewhat in our assumptions about the Ethiopian and indeed Lion crash but I personally believe that having now turned-over the MCAS stone the issue must be addressed, regardless of it's (non) role in recent accidents. Some (random) questions/observations:
Before the pilot world knew about MCAS, how would an MCAS failure have been manifested? Should it have been an 'MEL' Item? In other words, if it was not visible/known to pilots, how would anyone have known the system was working? Consequently, was it just looked upon as a sticking plaster necessary to get over a certification requirement and something that didn't need to be monitored or even in place for real world operations? I'm reminded of VW diesel engines and emission tests.
What was the rationale behind only using a single data source to the MCAS?
I think it has already been addressed but, does the flight regime mean that MCAS will activate at/before/about the same time as the stick shaker?
How did Boeing come to the conclusion that a 5 second pause after pilot trim inputs was the MCAS reset period?
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.