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During Lion Air Flight 610, a Boeing 737-MAX, it seems that the angle of attack sensors caused the software to believe the aircraft was stalling, thus initiating a nose-down pitch to recover from the (non-existent) stall.

I was under the impression that the difference between Airbus' fly-by-wire system and the Boeing types, is that Boeing aircraft can have their computer imposed inputs overridden (as discussed in my previous question).

So in the case of this flight, would pulling up have not recovered the flight, and if so, why not?

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  • $\begingroup$ Also, there are many variants of the Boeing 737, presumably with different behaviors in specific situations. It would be better if you specify which variant you have in mind. $\endgroup$ – a CVn Dec 6 '18 at 11:55
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    $\begingroup$ @aCVn The 737 does not have a stick pusher, only a stick shaker. Usually, only T-tail aircraft use stick pushers because they are more susceptible to deep stalls. $\endgroup$ – Bianfable Dec 6 '18 at 12:07
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    $\begingroup$ @Bianfable I didn't know that. Thanks for the correction. $\endgroup$ – a CVn Dec 6 '18 at 12:27
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    $\begingroup$ @Cloud We shouldn't need to refer to external material just to know what you're asking. Sure, the information is out there, but it's better to include pertinent information in the question itself. Basically, by making it as easy as possible for people to answer, you're more likely to get (good) answers. $\endgroup$ – a CVn Dec 6 '18 at 13:02
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    $\begingroup$ An independent analysis of published flight data plots for that accident suggested that the column force plots allegedly reveal that on this flight the pitch disconnect (intended to retain controllability for one crew member in case the other control column jammed) was triggered, rendering the pilot flying in control of one elevator panel only (the other elevator could have been controlled by the other crew member) which then was insufficient to counter stabilizer nose down moment at full aft deflection of a single control column. $\endgroup$ – Cpt Reynolds Dec 6 '18 at 21:44
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The new 737 MAX has a more advanced stall protection system called maneuvering characteristics augmentation system (MCAS), which automatically adjusts the stabilator trim in case it detects a stall. This makes it very hard to overcome the down force with elevator input alone (pulling up).

In case of a wrong trim adjustment, the pilots have to adjust the trim manually and maybe even turn off the stab trim via the cutout switches in the center pedestal (shown below). This was apparently not conveyed properly in the MAX differences training and the FAA has since issued an Emergency Airworthiness Directive.

737 Stab cutout switches (image source: flaps2approach.com)

So compared to an Airbus, the Boeing pilots can still have all manual control, but they have to use manual pitch trim.

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  • $\begingroup$ "The new 737 MAX has a more advanced stall protection system called maneuvering characteristics augmentation system (MCAS)" — from what I've read, MCAS has nothing to do with stall protection. It is meant to add more counterforce to the column at high AOA to comply with certification requirements. "So compared to an Airbus, the Boeing pilots can still have all manual control" — compared to the NG, the MAX pilots lose capability to turn off STS/MCAS but to continue using electric trim. The image you attached does not apply to the MAX. $\endgroup$ – Rusty Core 5 hours ago
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So in the case of this flight, would pulling up have not recovered the flight?

"Pulling up" (pulling back on the yoke) would NOT have recovered the flight. In fact, the Lion Air pilots were pulling back on the yoke for most of the flight (see the flight data recorder's Control Column Force graph).

Can computer imposed inputs be overridden on the Boeing 737-MAX?

Yes. Pulling back on the yoke controls the elevator and would override any computer control of the elevator. The problem with the Lion Air flight is that the MCAS (which kicked in due to erroneous AoA sensor data) controls the stabilizer, not the elevator. That's why pulling back on the yoke didn't work.

The stabilizer is the entire "tail wing", whereas the elevator is just the flap on the trailing edge of the stabilizer. When the stabilizer is at full deflection, its effect on pitch will be greater than the elevator (yoke).

The stabilizer trim can easily be overridden as well (but not with the yoke). You can either:

  1. Use the thumb switch to control the stabilizer trim electrically.
  2. Hit the Stabilizer Trim Cutout switches (that would disable the automatic Stabilizer Trim, including MCAS)
  3. Manually/physically turn the Stabilizer Trim Wheel.

The problem with the Lion Air flight is that they probably did not recognize that it was the trim they needed to override, so pulling back on the yoke was futile.

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    $\begingroup$ I am confused again. This picture shows ailerons on the main wing and elevators on the tailplane: dutchops.com/Portfolio_Marcel/Articles/Flight%20Controls/… So, (1) does the whole stabilizer moves on the 737, (2) or only rearmost parts, elevators? (3) Or both? (4) Or elevators are installed on the main wing, along with ailerons? Ah, seems (3) is correct: "The elevator is used for pitching the aircraft and is attached to the horizontal stabilizer, which can also be moved by systems like stabilizer trim and autopilot trim." $\endgroup$ – Rusty Core May 10 at 7:28
  • $\begingroup$ So, when Boeing says that operation of MCAS will "likely go unnoticed by the pilot," they are lying? Because when MCAS is operating, the trim wheels are rotating? This cannot be "unnoticed". $\endgroup$ – Rusty Core May 10 at 7:32
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    $\begingroup$ @RustyCore, Yes, the trim wheels are moving during MCAS activity, but the trim wheels are around the knees. So if a pilot is looking out the window or at some other instrument, they may not notice the trim wheel moving. Also, STS moves the trim wheel, so the pilot may assume STS is causing the trim wheel activity and not MCAS. $\endgroup$ – mike May 10 at 22:53
  • $\begingroup$ Looking at the videos, the stabilizer trim wheels make such a loud noise, that one has to be completely out to not notice their rotation (I haven't been in a pilot's cabin so cannot judge). Both STS and MCAS causing stabilizer adjustment, so if a pilot notices something funky, it is not really important which subsystem caused the funkiness, because the operations to turn on automatics are the same (stab trim switch + manual crank), and it is not that specific subsystem is turned off. It seems that STS uses the same sensors as MCAS, although I don't know whether it uses both AOA sensors or one. $\endgroup$ – Rusty Core May 10 at 23:06
  • $\begingroup$ @RustyCore The stickshaker (which was going off the entire time) is much louder than the trim wheels. STS also uses only one AOA sensor. $\endgroup$ – mike May 10 at 23:11
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Mike's answer is right. The yoke controls the elevator not the stabilizer.

MCAS trims the stabilizer, which can be overriden through the thumb switch, trim cutout switches (disables MCAS and the thumb switch), or by manually turning the Trim Wheel. (To see what that looks like, refer to the end of this video.)

To expand on Mike's answer, the preliminary report for the Ethiopian Airlines flight has been published.

According to the report, after MCAS activated, the pilots first executed option 2 (hit the Stabilizer Trim Cutout switches). They then attempted to trim manually via the wheel (option 3), which was apparently unsuccessful - this can be due to the aerodynamic forces on the stabilizer making the wheel very hard to turn. At last, they reengaged electric trim, and trimmed in the Aircraft Nose Up (ANU) direction with the thumb switch (option 1).

A few seconds later, MCAS reengaged in the Aircraft Nose Down (AND) direction for 5 seconds. 15 seconds later, the recording ends.

According to the report, they were pulling on the yoke continuously to keep level flight during most of the recording.

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    $\begingroup$ So basically, it's very hard to get rid of what MCAS has done especially when low on altitude. And not so simple as has been suggested. $\endgroup$ – ymb1 Apr 5 at 13:49
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up till now we don’t know fully what MCAS does, of course one feature concerns the automatic THS but we should not forget the MCAS object is to compensate the longitudinal instability introduced by the engines position and angle.Therefore something might be needed to correct this ahead of the automatic trim because the automatic trim based on AOA is not usually needed in normal flight but longitudinal stability is mandatory in all phases of flight; in some press articles it is referred to a pusher, a column pusher that modifies the forces on the column, a kind of automatic continuous correction to bring longitudinal stability. Blocking the trim runaway by pulling the column is usually the first instinctive action to override computer input, switching off the trim cutaway switches is the next action to override computers input, however in this case you will need to crank manually the trim wheel.Kind Regards. Jo

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    $\begingroup$ If you don't understand why pulling on the control column doesn't disable MCAS inputs, then you haven't fully understood the condition MCAS was implemented to address. Which might be the basis for a new question, but not for an "answer". This post doesn't provide an answer to the stated question. VTD. $\endgroup$ – Ralph J 11 hours ago
  • $\begingroup$ If you have a new question, please ask it by clicking the Ask Question button. Include a link to this question if it helps provide context. - From Review $\endgroup$ – bogl 10 hours ago

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