Crthuky

With regard to the Ethiopian crash of the 737 Max 8 aircraft, where according to preliminary reports, it appears to be related to the MCAS which is a safety system that pushes the nose down in the even of an excessively high angle of attack (pitch attitude), how difficult is it for the pilots to simply disable the MCAS and manually fly the plane?

To clarify my question, what I'm asking is can the pilot disengage the MCAS and if so, how do they do that? Is it just flipping a switch or popping a breaker, or is it a complicated set of procedures?

MCAS doesn't have its own on/off switch

It is a fly by wire feature designed to account for a particular flight regime that would not (or was not expected to) be encountered very often in normal operations. The system was adapted to account for some of the aerodynamics of the engine installation for this model. Its activation requires a number of preconditions, but we'll get to that in a moment.

A few things that should disable it (with caveats)

1. Lower the flaps. It is intended to work only if the flaps are up.



2. 
   

   Turn the Stab Trim switches to OFF. This disables the horizontal stabilizer's trim completely, and revert to manual trim (there are two guarded stabilizer trim switches in the aisle stand, see Windshear's answer). This means that the pilots will manipulate
   the trim wheels in order to apply nose/pitch trim during flight.
   

   
   
   

   This approach appears to be what a few crews did prior to the LionAir crash in October 2018 when the system gave them problems. (It is unclear how many of the crews knew that is was MCAS, versus any other trim anomaly). The previous LionAir crews on the accident aircraft ended up having to fly to their destination manually. Original Source: the Preliminary Report from that accident.

   
   
   

   
   

   The report indicates that during several previous flights of the 737 involved in the Lion Air crash, pilots encountered problems involving the AoA as well as the pitot tube used to measure airspeed. In a flight in the same plane the day before, to Jakarta, the pilot experienced many of the same symptoms as the pilots on flight JT610: the stick shaker activated during rotation, an indicated airspeed warning alert appeared, and the aircraft began automatically pushing the aircraft nose down.

   
   
   

   The pilot, after determining that his flight display system was malfunctioning, ran a runaway stabilizer non-normal checklist which led to the MCAS being disconnected when the stabilizer trim switches were turned off. The copilot flew the rest of the flight using manual controls and without autopilot.

   
   
   

   That Jakarta flight was using an angle of attack sensor that had been replaced after the previous Lion Air flight to Denpasar experienced problems. The problems with the Jakarta flight were recorded in the aircraft’s maintenance log. However, it is not clear whether the pilot communicated that he ran a runaway stabilizer non-normal checklist during the flight, which might have alerted the airline’s engineering staff that there was still a problem. (source)

   
   

   
   


3. 
   

   Enable autopilot. It is supposed to only work if the autopilot is off. But...CAVEAT

   
   
   

   That last part is a little bit complicated: the autopilot may not stay on if it - the autopilot system which is run by the FCCs - keeps getting spurious signals from the AoA sensors. (Or if the pitot static system is providing bad data to the FCCs).

   
   

How do you know that you need to disable MCAS?

The problem for a given crew: how do you know that MCAS is the sub-system that is giving you trouble? The AD released in December of 2018 updated the Runaway Stabilizer procedure; since there are other malfunctions possible in the trim system for pitch, such as a trim runaway, the same symptom may manifest during differently caused malfunctions. (See the example above). If additional symptoms are present, such as a stick shaker activating from an erroneous AoA indication, which problem do you solve first?

Which problem are you having?

Are you having both problems, or even something else?

That concern, the relationship between different potential failure modes, is an issue that gets to the heart of

1. What is in the pilots manuals




2. What training the crews do, or do not, get on this sub system.

The sub system is only supposed to activate under certain conditions (in theory): high angle-of-attack, flaps-up, flight with autopilot off. This includes high angle of bank flight in such conditions.

CAVEAT

Take a look at that last bullet in the lower right of the figure. It's only partially correct. When the pilots override with manual trim, MCAS will try again five seconds later (after the pilot has stopped) unless the triggering condition goes away.

"In the event of erroneous AOA data, the pitch trim system can trim
the stabilizer nose down in increments lasting up to 10 seconds,"
Boeing explained to operators in a bulletin issued last November,
following the JT610 accident. "The nose down stabilizer trim movement
can be stopped and reversed with the use of the electric stabilizer
trim switches but may restart 5 sec. after the electric stabilizer
trim switches are released. Repetitive cycles of uncommanded nose down
stabilizer continue to occur unless the stabilizer trim system is
deactivated." (source)

It is this behavior that is believed to have ultimately overcome the best efforts of the pilots in the LionAir crash. They were airborne for about six minutes before they were no longer able to overcome the horizontal stab position with their trim controls. (Caveat: current as of early 2019. When the final report comes out this may become a lot clearer).

Why is this activating during takeoff/departure?

That MCAS appears to activate during routine flight regimes is what is believed to be catching some crews by surprise - and not just the two accident crews. There is a recent article in Atlantic that excerpts some NASA reports by aircrews in the US about odd nose/pitch behavior in this model aircraft.

You can disable the stabaliser trim which will prevent MCAS from making inputs to the aircraft.