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The Boeing 737 MAX MCAS system has been in the press a lot as it has been implicated as a possible cause of 2 fatal crashes. The MCAS system was put in to prevent a wing stall caused by excessive Angle of Attack. My question is, outside of testing, has the system ever operated as intended and prevented a stall or approach to a stall?

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    $\begingroup$ If the system works as expected, how would even the average pilot know? Does the MCAS create any trace? How would anybody know without evaluating the FDR? $\endgroup$ – bogl Mar 26 at 10:42
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    $\begingroup$ The MCAS system can make noticeable and pretty coarse control inputs as has been demonstrated in the 2 accidents @bogl. $\endgroup$ – GdD Mar 26 at 14:36
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    $\begingroup$ Agree, but noticing the control interference checks only one of the boxes. Will the pilot file a report afterwards, and will he state that MCAS was helpful and that he could not have handled the near stall without it? I think that would be exceptional. $\endgroup$ – bogl Mar 26 at 14:45
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We don't know - until we hear about such a case!

A positive answer would require

  • a trace of MCAS action
  • sufficient evidence that the pilot could not have handled the (near) stall without MCAS.

This information could come from

  • a pilot who
    • notices the trim interference, and
    • perceives the situation as dangerous, and
    • reports the event

or

  • a flight data recorder that is analyzed, and
    • corrective MCAS action is found, and
    • it is evident from the data the pilot(s) did not / could not control the situation.

[ Am I missing any other way how MCAS could leave a trace? ]

A well-trained vigilant pilot can handle (near) stall situations in most cases.

MCAS is designed not only to get out of a stall, but to prevent the airplane from stalling in the first place. That means that the plane should never get into a critical stall situation, if MCAS does its preventive work.

All that just to say, it would be possible to answer "yes" as soon as there is knowledge of one averted incident, but it is impossible to conclude "no" from the absence of information.

I am very well aware that I wrote this theoretical answer while sitting in an arm chair, and I will happily remove it as soon as anybody with hands-on knowledge can provide more satisfying information.

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Most airlines have a Flight Data Analysis section which uses the routine download of the FDR to automatically flag deviations from the normal flight profile for investigation. The kind of maneuvering that would activate MCAS would vastly exceed the typical limits and be subject to a detailed investigation. Most of the 737 Max FDR do have provision for discrete recording of automatic trim inputs from the FCC, which may or may not be MCAS, but the surrounding data will allow you tell MCAS apart from the normal automatic trim operation by the FCC.

The lines of communication between the operator and the NAA on information derived from this source are somewhat limited due to confidentiality provisions that pilot groups required in order to embrace this constant scrutiny of their daily work.

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MCAS is designed only for extreme abnormal flight attitudes, normal activation would be rare. Activation of this sort is also likely to show on data logs and it seems likely that it may even set some an alert flag for the operation even if the pilot fails to report such an abnormal flight condition. Engines have been remotely monitored while in flight for many years(basic temp, pressure, and RPM), with some lag as it must traverse several layers of electronics and sat links.

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  • $\begingroup$ 1-Boeing wanted the system to be transparent, so presumably never reported. 2- Analysis of recorded data gives the actual AOA, but not the stalling AOA . Note that the stall alarm occurs well before the stalling AOA $\endgroup$ – user40476 Jun 3 at 16:08

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