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The Boeing 737 NG has stabilizer trim cutout switches marked as MAIN ELECT (left) and AUTOPILOT (right). Turning off the right switch disables automatic stabilizer trim in autopilot mode and automatic stabilizer trim by the STS in manual mode. The stabilizer can still be trimmed with electric trim switches on the control wheel or manually with the hand crank. Turning off the left switch disables the electric trim on the control wheel.

The Boeing 737 MAX has stabilizer cutout switches marked as PRI (left) and B/U (right).

With the MAX, Boeing added MCAS to the existing STS functionality. If Boeing kept the cutoff switches functionality intact, then turning off the right switch would disable both STS and MCAS, keeping the electric trim operable. Instead, in case of misbehaving MCAS Boeing requires to cut off BOTH switches without explaining what each of the switches does. With both switches turned off the only option to change stabilizer trim is to use a hand crank.

  • On the NG, if only the left switch is cut out, does it disable electric trim on the control wheel only, or it completely disables electric assist no matter where the command comes from, so in effect when the left switch is turned off it does not matter what is the position of the right switch?
  • What is the meaning of the new MAX labels?
  • What is the exact functionality of these switches individually?
  • Why the algorithm of turning off auto trim system has been changed?
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  • $\begingroup$ What gets me is why you have to reach over to flip switches to disable a runaway stab. I would've expected a disconnect button on the control wheel. $\endgroup$
    – John K
    Commented May 17, 2019 at 17:42
  • $\begingroup$ @JohnK They already have a control wheel position based cutout which is more instinctive than the switch. It also has been the same design for 61 years across every Boeing jet. The current problems have nothing to do with the cutout, but rather the identification of the issue. $\endgroup$
    – user71659
    Commented May 17, 2019 at 17:57
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    $\begingroup$ @user71659 Not exactly. Even if the issue is correctly identified, MCAS cannot be cut out with the right switch only, AND it does not turn off when the yoke is pulled AND it re-engages five seconds after the electric trim switches are released. The beast can be tamed only by cutting out both switches, which leaves the pilot facing the task to manual crank the stab all the while while his mate is pulling the yoke like crazy. This is new. $\endgroup$
    – Rusty Core
    Commented May 17, 2019 at 18:15
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    $\begingroup$ @user71659 The Ethiopian Air pilots did cut both switches, but then found to be physically impossible to hand-crank the stabilizer, so they turned the system back on to use electric trim on the control wheel. No matter the procedure, what I am asking is the specific functionality of each of the switches. $\endgroup$
    – Rusty Core
    Commented May 17, 2019 at 18:40
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    $\begingroup$ Yeah you have to unload the tail to be able to move manually, which means diving more... I was amazed to learn the '37 has a Piper Cub trim system with electric motor add-ons, but it is an early 60s design. $\endgroup$
    – John K
    Commented May 17, 2019 at 19:06

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What do you know, I found the answer to my exact question in the Seattle Times:

First of all, even after two fatal crashes,

Boeing declined to detail the specific functionality of the two switches.

Nevertheless,

The Seattle Times found that the left switch on the 737 NG model is capable of deactivating the buttons on the yoke that pilots regularly press with their thumb to control the horizontal stabilizer. The right switch on the 737 NG is capable of deactivating just the automated controls of the stabilizer.

So far this corresponds to the information I found in the 737 NG FCOM myself.

As Boeing was transitioning from its 737 NG model to the 737 MAX, the company altered the labeling and the purpose of those two switches.

On the newer 737 MAX, according to documents reviewed by The Times, those two switches were changed to perform the same function – flipping either one of them would turn off all electric controls of the stabilizer. That means there is no longer an option to turn off automated functions – such as MCAS – without also turning off the thumb buttons the pilots would normally use to control the stabilizer.

If the above is correct, then not only MCAS, but STS/autopilot as well cannot be turned off on the MAX while keeping electric trim on the control wheel functional.

Peter Lemme, a former Boeing flight-controls engineer, said if the company had maintained the switch design from the 737 NG, Boeing could have instructed pilots after the Lion Air crash last year to simply flip the “AUTO PILOT” switch to deactivate MCAS and continue flying with the normal trim buttons on the control wheel.

(^ bold is mine). This is exactly what I was alluding to in my question. I really want to hear the reasons for the change.

Lemme said he’s surprised that Boeing made the change to take away the functionality that could have allowed the pilots to shut off MCAS without shutting off the electric switches at their thumbs.

Oops.

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    $\begingroup$ Avherald published an extract of the SSM for the stab cutout switches on the MAX. You can take a look $\endgroup$
    – sbabbi
    Commented May 22, 2019 at 22:21
  • $\begingroup$ So PRI means primary and B/U backup. They just kept both switches on the panel, and reduced/converted there meaning to be a logical AND switch? I’m not a pilot, but this different kinds of abbreviations - next to each other - are confusing? $\endgroup$
    – Peter
    Commented Sep 5, 2019 at 22:30

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