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?

  • $\begingroup$ 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

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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