And it's even worse than that: pulling back on the control yoke only disengages the MCAS autopilot for five seconds: after that, the system re-engages and will again point the nose down if it thinks the aircraft is stalling (including if it's not actually stalling but just getting bad data from a faulty sensor). Quoting from The Seattle Times regarding last October's 737 MAX Lion Air crash (emphasis added):
seattletimes.com said:
Scrutiny of Lion Air crash turns to automated systems that command Boeing 737 pitch
...
FAA mandates action
The FAA said its directive addresses the potential effects of false information coming from the sensor on the plane’s exterior that reports the plane’s “angle of attack” (AOA), which is the angle between the wing and the flow of air the jet is moving through.
This key data point is fed into the flight computer along with the temperature and air speed. These three metrics affect one another and are used by various systems that control the airplane’s flight.
The FAA said false AOA readings “can potentially make the horizontal (tail) repeatedly pitch the nose of the airplane downward, making the aircraft difficult to control.”
The danger, the FAA warns, is that this could “lead to excessive nose-down attitude, significant altitude loss, and possible impact with terrain.”
The Boeing service bulletin to airlines notes that though pilots may pull back on the yoke and adjust the horizontal tail to get the nose pointed back up,the condition pulling the nose down may then restart five seconds later.
The flight data of the Lion Air jet in the 12 minutes before it crashed shows a pattern of the plane repeatedly losing altitude and regaining it before it finally dived steeply into the sea.
Full article is here:
https://www.seattletimes.com/busine...737-safety-alert-with-an-emergency-directive/
Although it's possible to disable the MCAS manually, it requires manually cranking the trim wheel or pressing the MCAS cutout switch on the trim wheel, and 737 pilots are not trained on that because the MAX was certified to not require retraining for existing 737 pilots (!!). Regarding the MCAS system on the MAX:
leehamnews.com said:
It can be stopped by the Pilot counter-trimming on the Yoke or by him hitting the CUTOUT switches on the center pedestal. It’s not stopped by the Pilot pulling the Yoke, which for normal trim from the autopilot or runaway manual trim triggers trim hold sensors. This would negate why MCAS was implemented, the Pilot pulling so hard on the Yoke that the aircraft is flying close to stall.
It’s probably this counterintuitive characteristic, which goes against what has been trained many times in the simulator for unwanted autopilot trim or manual trim runaway, which has confused the pilots of JT610. They learned that holding against the trim stopped the nose down, and then they could take action, like counter-trimming or outright CUTOUT the trim servo. But it didn’t. After a 10 second trim to a 2.5° nose down stabilizer position, the trimming started again despite the Pilots pulling against it. The faulty high AOA signal was still present.
How should they know that pulling on the Yoke didn’t stop the trim? It was described nowhere; neither in the aircraft’s manual, the AFM, nor in the Pilot’s manual, the FCOM. This has created strong reactions from airlines with the 737 MAX on the flight line and their Pilots. They have learned the NG and the MAX flies the same. They fly them interchangeably during the week.
Boeing has since added information about the MCAS system to the manual, but few 737 MAX simulators exist for pilots to actually train on it. More information on the MCAS is here:
https://leehamnews.com/2018/11/14/b...-the-737-max-was-not-disclosed-to-the-pilots/
And on a side note, when the aircraft starts nosing-down when it shouldn't, it's inherently a stressful situation for the pilots, particularly at low altitudes: their 737 training kicks in and they pull back on the stick to disengage the autopilot and level the plane -- except in the case of the 737 MAX with a faulty AoA or mach speed sensor, when the MCAS kicks in again five seconds later and again points the nose down. The only way to disengage the MCAS is to remember to crank the trim wheel manually or press the MCAS cutout switch on the trim wheel -- and 737 pilots haven't trained for that since "the MCAS system is supposed to be fully automatic and not disengage".
The root of all this MCAS behavior is that it's a hack that Boeing did to fit the larger engines on the MAX and so had to move the engines forward to still have enough clearance, thus changing the aircraft's center-of-gravity as compared to normal 737s:
leehamnews.com said:
Until the MAX, there was no need for artificial aids in pitch. Once the aircraft entered a stall, there were several actions described last week which assisted the pilot to exit the stall. But not in normal flight.
The larger nacelles, called for by the higher bypass LEAP-1B engines, changed this. When flying at normal angles of attack (3° at cruise and say 5° in a turn) the destabilizing effect of the larger engines are not felt.
The nacelles are designed to not generate lift in normal flight. It would generate unnecessary drag as the aspect ratio of an engine nacelle is lousy. The aircraft designer focuses the lift to the high aspect ratio wings.
But if the pilot for whatever reason manoeuvres the aircraft hard, generating an angle of attack close to the stall angle of around 14°, the previously neutral engine nacelle generates lift. A lift which is felt by the aircraft as a pitch up moment (as its ahead of the CG line), now stronger than on the 737NG. This destabilizes the MAX in pitch at higher Angles Of Attack (AOA). The most difficult situation is when the manoeuvre has a high pitch ratio. The aircraft’s inertia can then provoke an over-swing into stall AOA.
To counter the MAX’s lower stability margins at high AOA, Boeing introduced MCAS. Dependent on AOA value and rate, altitude (air density) and Mach (changed flow conditions) the MCAS, which is a software loop in the Flight Control computer, initiates a nose down trim above a threshold AOA.
https://leehamnews.com/2018/11/14/b...-the-737-max-was-not-disclosed-to-the-pilots/
So yes, people can argue that "the pilots should know to disengage the MCAS", but 737 pilots aren't
trained to interact with the MCAS at all -- the MAX was marketed as not requiring retraining for existing 737 pilots, and retraining is not (currently) required.