[crooked-v]

Part of the problem is that the MCAS system has a 20-second delay after the last time the pilot touched controls for the flaps, and it adjusts itself to a new position and then stops. There are no other indicators that it's happening, and since it's noncontinuous and has a delay, it would be easy to miss the only visual sign of the trim wheel readjusting itself once, especially since the wheel itself is by the pilot's thigh outside of forward line of sight.

In a normal runaway trim situation, by comparison, the trim wheel continues obviously spinning as long as the problem is going on, so it only takes a single glance at any time to notice it.

[oasisbob]

I'm not sure that's accurate. Other descriptions of the MCAS system state that it will continually modify the trim on the horizontal stabilizer for up to 10 seconds at a time.

http://www.b737.org.uk/mcas.htm

[usrusr]

Well, the point is that the experience from MCAS trying to be clever and subtle is very different from what one would expect in an old-school runaway trim situation caused by, say, a stuck relay.

Easy to chase down a wrong line of inquiry when the symptoms only appear sporadically and can be temporarily stopped by a whole range of actions that won't permanently fix it.

[inferiorhuman]

To summarise; MCAS will trim the Stabilizer down for 10 seconds (2.5 deg nose down) and pause for 5 seconds and repeat if the conditions (high angle of attack, flaps up and autopilot disengaged) continue to be met. Using electric pitch trim will only pause MCAS, to deactivate it you need to switch off the STAB TRIM SUTOUT switches.