Originally Posted by
Centaurus
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I first flew the Boeing 737-200 in 1977 and the Pilot Training Manual for that aircraft explained quite clearly how to use the manual trim in event of a runaway electrically operated stabilizer trim. However, following the introduction of the Boeing 737-300 and subsequent models, Boeing, for some unknown reason, withdrew those directions and instead inserted an amended version which included the words “in extreme cases it may be necessary to aerodynamically relieve the air loads to allow manual trimming.” The amended directions did not say precisely how this was to be accomplished and it was left to pilots to interpret what Boeing meant. Pilots with English as second language would have little idea what Boeing meant.
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?