Originally Posted by
hans brinker
I can confidently say I have read every single MAX post on these (and a few other) forums, and I think it is time to repeat a question that has not been answered by Boeing.
If you have a real runaway trim B expects you to take action in 3 seconds, and save the day. When they designed the MAX, they found the stick force to light in certain regions of the envelope, so they introduced a 10 second runaway trim. If the pilot responds the way B expects they will cut-off the trim so the solution to the stick force problem doesn't work. How was this ever certified?????
I too was vexed by this but after reading the excellent analysis linked to in post 3536 (the one at Satcom Guru; sorry no permission to link), I think I understand the reasoning.
The expectation was that, faced with sudden pitch changes due to runaway trim, pilots would quickly (the 3-4 seconds) react by correcting with elevator (I can buy that). At that point, the column trim switches that prevent electrical trim in the opposite direction would activate. Thing is, these are precisely the ones that MCAS bypasses to operate against pilot elevator command.
So the next step in my interpretation is that B counted on pilots not reacting to the non-continuous trim inputs of MCAS, which not only are time-limited, but have a 5-second "grace" period between activations. Unlike regular runaways. Hence relying on the runaway trim checklist to downgrade MCAS risk (if that happened) is questionable.
So I'm still thinking that the rationale for simultaneous MCAS operation and safety is a case of having the cake and eating it too.
BTW, the article in Satcom has lots of juicy bits that I guess aren't entering the radar of this thread due to its length.