Originally Posted by
robocoder
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.
I have read the satguru. If MCAS activates, pilots pull back on the column within 3 seconds, but that doesn't stop the trim due to MCAS disabling the switch. next step is a memory item, switch the trim off, so at second 5 the trim is switched off, but the aircraft needed the full 10 seconds of MCAS to get back to an acceptable elevator control force level. Still don't understand how that was deemed acceptable. Definitely cake+eating!!