Originally Posted by
hec7or
"Unfortunately this MCAS intervention is not a classic "runaway stabiliser"
agreed, so leaving the AP/AT engaged will give you the wrong pitch attitude and thrust setting, disconnecting the AP will give you an intermittent stab trim runaway.
Both easily controllable once memory items have been actioned, but you have to recognise it first as a multiple failure.
That is exactly the point, the recognition of the actual problem is very subtle due to the intermit nature of the MCAS being disabled for 5 seconds after manual trim. The first step in the UA checklist is disconnect the autopilot...
Initially just setting the attitude & thrust will require lots of manual trimming, but once it is right,
only then does the MCAS starts to intervene. Now the PM gets out the QRH performance and starts giving you target attitudes and thrust for you weight while you are trying to fly level at 5000 (Boeing provide this exact altitude for Flight with UA), it takes quite some time to let the speed stabilize the speed Straight and Level(S&L) for the crosscheck and to work out the erroneous indication(s).
However just flying S&L for the crosscheck is complicated by the MCAS/ pilot trim couple. As I said in an earlier post, it is possible that all three IAS indications were actually valid and in agreement, further confusing the issue.