Originally Posted by
ontheklacker
MCAS to me is not a "system" per se, but simply an additional subroutine in the Flight Control Computer. Albeit, a poorly coded one. Most computers attached to flight controls have multiple channels that compare their inputs before making an output. A single faulty AoA sensor activated the MCAS subroutine of the FCC. Flaps up, high AoA (stalling), MCAS commands the existing trim system to trim the nose down until the high AoA indication goes away. A single failure should not have activated this logic. That's why commercial aircraft have at least 2 of everything.
This being said, anyone who has flown a 737 classic / NG / Max knows that commanded or uncommanded trim wheel movement is very noticeable - both thru sight and sound. The crew the day before the mishap flight readily identified the uncommanded movement and completed the QRH. The mishap crew did not. As to why they didn't, hopefully we will find out.
Why is there MCAS software on the Max, maybe the FAA was the driving force. It's interesting that this year the FAA has put high emphasis on full (not stick shaker) stall recovery during AQP. Maybe this is yet another result of the Colgan Air 3407 crash in Buffalo, NY, 2010?
Similar to my thoughts.
The MCAS seems to be a stick nudger with a complicated Boeing name.
Why would the 737 MAX need a stick nudger?
My guess is that during stall testing, and particularly turning flight, flaps up, power on, aft cg, and probably 3kts/second deceleration. The aircraft could not be shown to comply with the longitudinal static stability requirements implied in FAR25.203(a).
This is not common, but not unheard of. A stick nudger seems to be the quick fix.
Nowhere has it been implied that the actual stall characteristics are in question.
The questions about longitudinal control authority are asked in other sections of FAR25.
But stick nudgers are old hat, why not just install a nudger actuator?
The non-compliance with static stability requirements in other areas of the envelope meant the speed trim system was fitted.
It would be almost trivial engineering to add a nudger function to the STS system.
The failure mode and effects analysis was almost already done.
The switchery was in place for the runaway case.
There was already a crew procedure in place for the runaway case.
How could it runaway with just a single AOA signal indicating it was approaching the area of longitudinal instability?
Of course that is the real question.
For the aircraft to "know" it was in a configuration and flight conditions for the MCAS to operate it would seem to require a number of sensor input that were within the trigger range of the MCAS. If any of these signals was missing the false AOA signal should not have caused the MCAS to operate.
That Boeing, FAA, Pilots unions have not called for immediate changes to the MCAS, makes me think there is already more known than is being released publicly.