Originally Posted by
capngrog
I must have missed something in the latest FAA findings regarding the Boeing 737MAX. The reports of these findings seem to be unclear on the precise cause of a recently discovered problem with the MAX, but reports range from the trim switches not moving the stabilizer fast enough to the FAA test Pilots finding that the Boeing procedures did no allow sufficiently rapid recovery from the runaway trim condition. In none of these reports have I found any reference to the trim switches themselves failing to operate. On the other hand, the fact that the electric trim SYSTEM will not work in alleviating severe out of trim conditions under high aerodynamic loads has been well documented.
Only guessing here.
But when details will be available, I wouldn't be surprised if the test scenario is something like:
- create a set of events prone to overload the CPU1 of the active FCC
- decrease thrust so that STS activates with a trim down command
- check that using the trim rocker buttons or the control column cutout switches doesn't stop the STS
- possibly, check that when the conditions that should stop STS motion are reached (course limitation), STS continues because the CPU is not ready to evaluate the closing parameters.
And concerning the set of events that overwhelms the CPU, I wouldn't be surprised if it was simply a continuous activation of MCAS (high AOA on one side) which is continuously inhibited by the new condition checks aimed at preventing a mistaken activation. It is certainly more processing time demanding than the original MCAS software.