Originally Posted by
MechEngr
Three questions have gone unaddressed:
1) What is the exact definition of trim runaway that was trained as trim runaway? It's 100% certain that trim will always stop when the stab reaches the trim stops and that is indistinguishable from every time MCAS stopped. It cannot go further and therefore it cannot runaway if it always stops. Was the expectation to let it run until it hit the upper or lower stop and then do something?
2) What amount of force are pilots trained to use trim to offset? In the case of the Ethiopian crash the plane was 2 units out of trim before the trim motors were disabled. They felt confident with that amount, having trimmed back from nearly 4 units, and waited to disable the motors. From that point until they re-enabled the trim motors did the control forces increase and to what level?
3) Why are there cutout switches? There must have been an unavoidable potential defect in order for there to be an interrupt put in place. It seems the general feeling is that because MCAS was unknown to the pilots prior to the Lion Air crash that no pilot could do anything to avoid it because it wasn't an already known defect or set of defects that the pilot would diagnose. What defect were pilots told the switches were for?
1) What is the exact definition of trim runaway that was trained as trim runaway? It's 100% certain that trim will always stop when the stab reaches the trim stops and that is indistinguishable from every time MCAS stopped. It cannot go further and therefore it cannot runaway if it always stops. Was the expectation to let it run until it hit the upper or lower stop and then do something?
It was something like "a continuous un-commanded trim operation" the background was a faulty locked switch or other component/electrical failure that put power to the motor. But the shorted switch was the main idea (fault) - put in an input then take thumb off and it keeps going and does not stop until the fixed stop (hopefully) but the motor could still run on a clutch eventually burning out.
100% is not certain it could shear the mechanical stops.
A runaway trim was originally believed to occur after the pilot inputted a up or down command and that command continued after the pilot stopped the input. As an example put your foot down on the car accelerator, when you lift your foot you expect the car acceleration to decrease. If speed continues to increases you have a runaway throttle.
2) What amount of force are pilots trained to use trim to offset? In the case of the Ethiopian crash the plane was 2 units out of trim before the trim motors were disabled. They felt confident with that amount, having trimmed back from nearly 4 units, and waited to disable the motors. From that point until they re-enabled the trim motors did the control forces increase and to what level?
There will not be a fixed amount of force trained for and I expect it varies a lot during changes of flight phase. I doubt very much the Ethiopian crew felt "confident" at any stage of that flight soon after lift off.
After the trim motors were deactivated the aircraft speed increased, so the forces would have increased and I expect a lot.
3) Why are there cutout switches? There must have been an unavoidable potential defect in order for there to be an interrupt put in place. It seems the general feeling is that because MCAS was unknown to the pilots prior to the Lion Air crash that no pilot could do anything to avoid it because it wasn't an already known defect or set of defects that the pilot would diagnose. What defect were pilots told the switches were for?
The cutout switches were originally there to isolate the undesired conditions including the runway trim, the switches were there long before the MAX so MCAS is not relevant to the question.
Originally the pilots were told what both of the switches isolated independently including the manual trim switch, that changed on the NG and after.
Soon after the Lion Air crash runaway trim was no longer required to be "continuous" and I doubt that to this day there is any definition of runaway trim that is taught to be correct as a standard (run time or stick force).
I would also question the simulators control forces accuracy in many conditions.