This is how the MCAS works:
•either FCC commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and at airspeeds approaching stall
•activated without pilot input and only operates in manual, flaps up flight
-the yoke trim switch or STAB TRIM cutout switches will override MCAS input
-the control column cutout switches do no stop electric trimming
•stabilizer incremental commands are limited to 2.5 °; uses A/P trim (0.27 units/sec)
-the function is reset once AOA falls below the AOA threshold or if a yoke trim switch is activated
•if the elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command
•in the event of erroneous AOA data, the pitch trim sys. can trim the stabilizer nose down in increments lasting up to 10 sec, but are limited to a 2.5° commanded change per run
Now, the 737 also has a speed trim system, which works automatically during manual flight. For those of us that fly the 737, it looks a little unusual to see the trim spinning after takeoff (without pilot input), but it’s there to make our job a little easier. We see this most during takeoff, climb, or go-around. Typically with high thrust.
I have never seen this in any aeroplane I’ve flown. Usually the pilot actuates the stab trim. Having said that, hearing and seeing the trim wheels spinning after takeoff and during climb to a 737 pilot is normal.
However, the MCAS operates in a similar manner and during periods of high workload (takeoff), perhaps the pilots didn’t recognise it was the MCAS and not the speed tim? Maybe they recognised and trimmed it back out, without realising that if the condition still exists the MCAS will activate again after 5 seconds, and they’re back to square one? This doesn’t assume they were getting erroneous indications on their PFD’s wit one or more faulty alpha vanes.
Both actions can be stopped (momentarily) by operating the stab trim switches on the control yoke. Both can be stopped permanently by operating the stab trim cutout switches (as per the Runaway Stabilizer checklist in the QRH.
Even if the crew disabled the system as described above, I do not believe they could have stalled the aeroplane just because of that. The Max still has the same stall warning system as the other NG’s, that being an aural low speed warning, followed by a stick shaker.
I was chatting to one of our engineers today. He’s rated on the Max.
He showed me data from that aircraft, and for the majority of the time it was below 1000’ AGL. The MCAS does not function with flap extended. And, as far as I know, the Max always uses some flap for takeoff. At 1000’ AGL, the aircraft should still have takeoff flap extended and, assuming there are no obstacles, it would have commenced acceleration around this altitude THEN commenced the flap retraction schedule.
So another question is - was the active FCC getting invalid flap position data, causing the MCAS activate at such a low level?
All I can say is I’m glad I wasn’t there...
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