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Various websites have stated that during the 737-MAX crash in Indonesia, the pilots could not pitch up because of a faulty AOA sensor pitching the plane down.

The stabilizer has a rear fulcrum and front jack screw. The elevator, positioned by yoke, is hinged to the stabilizer. If the yoke is pulled back, the elevator moves up. Then the stabilizer must move down, either automatically or manually, to remove the yoke loading. However, if the yoke is fully pulled back, why doesn’t this force the stabilizer down directly so that the pilot always has total control of pitch without checking the flight manual or pushing switches?

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The yoke does move the elevators, whereas the trim moves the horizontal stabilizer. The stabilizer is much larger than the elevators, and therefore is more effective than the elevator at extreme angles. Even with full backpressure, the nose would continue to pitch down.

enter image description here

Nonetheless, all 737NGs/MAXs have trim cutout switches, 1 for the autopilot and 1 for the normal electric trim. Using these would immediately disable the electric trim. These are generally part of a standard trim runaway situation, but the crew did not recognize the situation and failed to use them.

enter image description here

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    $\begingroup$ +1 Good answer but IMO does not explain "why doesn't the yoke also move the elevator at yoke extremes (full forward and full back) so that pilots always have pitch control?". That's a good question IMO. $\endgroup$
    – summerrain
    Jan 7, 2019 at 1:54
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    $\begingroup$ Because the yoke DOES control the elevator. The basis of the question is incorrect, the yoke does NOT control the Trim. In fact, the 737 does NOT have Trim Tabs, it has Servo Tabs for when the Flight Controls are in Mechanical Mode. $\endgroup$
    – RAC
    Jan 7, 2019 at 9:51
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    $\begingroup$ Note that the previous crew did use the switches. The condition already existed on PK-LQP during the flight (JT-43) preceding the accident one and the crew cut out the trim (shortly after take-off) and completed the flight. They reported all the various symptoms they had to maintenance, but the troubleshooting instructions failed to mention the possibility of them being caused by bad AoA probes, so maintenance didn't check it. $\endgroup$
    – Jan Hudec
    Jan 8, 2019 at 20:47
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    $\begingroup$ @RAC I think he is asking why isn't it designed with logic that if you have ran out of elevator, the stab starts moving in the same direction. I don't deal with autoflight much, but I think it uses a similar concept of control with the elevator but if it is holding an input for long it'll adjust the trim to unload the elevator. $\endgroup$
    – OSUZorba
    Mar 14, 2019 at 4:04
  • $\begingroup$ The effectiveness of the stabilizer in comparison to the elevator is not simply a matter of size (surface area). The stab deflection range is much less than the elevator, which tempers its effectiveness. $\endgroup$
    – Pete P.
    Mar 2 at 3:09
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On airplanes without flight envelope protection, the pilot needs to feel how much force they are applying to the flight control so they get a feel for how close to the edge they are getting. That is the reason for the regulation that requires the control force to increase linearly for each additional increase in pitch attitude — and the need for MCAS, which restored that linearity at very high pitch angles on the MAX (by de-trimming the elevator).

Accordingly, in manual flight, there is no auto trim. The pilot puts the airplane into the desired pitch attitude using the elevator, which deflects enough to provide ample pitch authority, and then must manually trim out the yoke force using the trim switch or trim wheel.

The autopilot auto trims soon after changing pitch with the elevator. This minimizes the chance that there will be a residual yoke force which the pilot does not (cannot) anticipate when the A/P is disconnected. Such residual force, suddenly released when relinquishing control to the pilot, is essentially an abrupt control input, which at best can be upsetting to passengers, at worst can cause injury, structural damage or loss of control. That is also why the A/P will not engage if it detects force is being manually applied on any of the controls. It is also why the first step in the stab runaway procedure is: Control wheel — grasp firmly; before turning off the A/P, as it likely had already begun using the elevator to counter the runaway.

If the stab were to work as suggested, holding the yoke at its stops would effectively start a stab trim runaway!

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  • $\begingroup$ reconsider "pitch attitude"? $\endgroup$
    – quiet flyer
    Mar 2 at 11:41
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The stabiliser moves the elevator, not the other way around.

  • The elevator is for direct, high frequency and limited authority control of the aircraft attitude.
  • The stabiliser is for trimmed aeroplane position: release the stick and it will return to the zero force position, which is the stick trim, which should correspond to steady state flight.

With a released stick, if the trim switches are flicked the whole stabiliser - including the elevator hinge - moves and brings the elevator to a new neutral position. Also, the trim of the aeroplane changes. The elevator acts as a fast addition to aircraft trim, by design: trim is very powerful and should be slow and gradual, pilot pitch control must be able to provide fast pilot action.

The pilots may have good reason to trim the aeroplane fully nose down if for instance freight has shifted backwards. The B737 has Q-feel as mentioned in this answer, if they don't fly too fast they still have full elevator control with full nose-down trim.

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