That would be the autopilot disconnect button or, in this case, the trim switch, the pressing of which is the correct immediate response to this occurrence, followed by the disabling of the system using switches on the center pedestal. While it's certainly hazardous for Boeing to knowingly expose pilots to this occurrence, I'm also conflicted by the fact that they failed to respond properly. The procedure is similar on Boeing, Airbus, Embraer and Bombardier aircraft. Standardized, in a way.

MCAS is designed to work only with autopilot disengaged. With autopilot and autothrottle engaged there would be similar logic at play because the general goal would be to not have the automation induce a stall. With everything disabled there is a risk that applying full throttle will result in the airplane pitching up dramatically beyond the ability of the elevator to pitch the airplane back down and potentially inducing a stall. This is an inherent problem with putting big engines under the wing (and is a problem in the 737 Classic[0], NG, and MAX).

You could almost certainly disable just about anything by pulling the appropriate circuit breaker. You don't want to do that because the plane may be left in a state where you can't control it manually and you only have a few thousand feet of altitude to recover.

There are, in fact, two switches to turn off the electronic control of the horizontal stabilizer. The previous Lion Air crew used them after the pilot riding in the jumpseat went back and grabbed his copy of what I'd assume was the flight manual and had an ah-hah moment[1]. The flight 610 crew were pouring over something (quick reference handbook?) but didn't express any awareness of the stabilizer being trimmed[2].

Depending on how bad things got there are a few problems with this. First, electrically moving the stabilizer is much faster than moving it manually. MCAS can operate at two speeds, IIRC both are slower than the speed dictated by the buttons on the yoke (plus the buttons on the yoke will pause MCAS). But even MCAS will move the stabilizer much faster than a pilot could by cranking the manual wheels. I believe that moving the stabilizer from one extreme at the fastest speed to the other takes about a minute. You can find videos of the stabilizer mechanism on a 777 on youtube, it's just not something that's designed to move quickly.

The next problem is that if the stabilizer is pitched down sufficiently (perhaps by MCAS), your first instinct may be to pull back on the yoke (to move the elevator) to regain pitch. Well, once you've done that the aerodynamic forces may be such that you can't move the stabilizer into a nose up position until you let go of the yoke and let the plane pitch down further (Boeing references needing to unload the stabilizer in some documents). That's something you've gotta have cat-like reflexes for when you're already flying so low.

The really sad part is that the pilots could have disabled MCAS and retained electric control over the stabilizer by extending the flaps. But how are the pilots going to know this when Boeing refused to document anything?

0: http://avherald.com/h?article=419f2f9e

1: https://www.grid.id/read/04966850/deretan-kejanggalan-yang-d...

2: https://www.straitstimes.com/asia/se-asia/cockpit-voice-reco...

Well, there is no such button. Perhaps there should be though a better solution might be to fix the automation. Airbus planes seem to work ok without a button but don't have a crash the plane if one sensor fails system.

That's an excellent question, and will likely be one of the conclusions of the accident investigations.

Could be related to the issue that MAX is unstable in flight with that system off. Unlike previous 737s.