Originally Posted by
L39 Guy
An MCAS failure, owing to the sensor failure (birdstrike or whatever other cause), causing the nose to pitch down in manual flight with the flaps up, presents identical characteristics to the pilot as a stab trim runaway (nose down) which has been taught on this aircraft forever. I can see why Boeing did not want to complicate things by creating a separate MCAS procedure as this would cause pilots to try to differentiate between a classic stab trim runaway and an MCAS failure while the aircraft is out-of-control. And since the emergency response is identical there is nothing to be gained either by trying to diagnose the problem in the air.
Not exactly the same signs as stab trim runaway which would be trained/expected to be continuous whereas MCAS (unpatched) worked in up to 10 second bursts then stops until the next pilot trim input.
A MCAS checklist could stress both the need for and safety of -fully- trimming before using the cutout switches to avoid possibility of manual trim not working due to aero loads.
Had this been included in the AD in response to Lion Air it may have helped.
In roughly the time it takes to read this post MCAS provided 2 nose down inputs to ET.