According to this article, my assumptions about the Boeing 737 MAX flight control system are (please correct me if I am wrong):

  • The aircraft has two flight control computers (FCC)
  • The aircraft has two angle of attack (AoA) probes
  • The Maneuvering Characteristics Augmentation System (MCAS) is implemented as a function of each FCC
  • Data from the port AoA probe is supplied only to FCC 1
  • Data from the starboard AoA probe is supplied only to FCC 2
  • Only one FCC (FCC 1 or FCC 2) is active during each flight, with the other serving as a backup system

Assuming a situation where only the AoA probe used by the active FCC produced incorrect AoA information and thereby causes the MCAS to intervene while the other AoA probe is functioning correctly, would it be possible to either

  • switch to the other FCC (which uses input data from the other AoA probe)


  • switch the active FCC to use input data from the other AoA probe

and thereby prevent the MCAS from continuing to use incorrect AoA data?

  • 3
    $\begingroup$ speculation about ongoing investigation are explicitly off topic $\endgroup$
    – Manu H
    May 10, 2020 at 14:27
  • 2
    $\begingroup$ @ManuH It's no longer speculation. The final report for Lion Air has been available. $\endgroup$
    – JZYL
    May 10, 2020 at 14:29
  • 3
    $\begingroup$ @JZYL you mean "it is no longer ongoing investigation". The question is still speculative, trying to know what would have happened if we rewrite history, event trying to know what would be reasonable to assume if we rewrite history. Less subjective formulation would be welcome. $\endgroup$
    – Manu H
    May 10, 2020 at 14:50
  • 4
    $\begingroup$ "What if the system wasn't what it was but had the option (where?) to manually switch between FCC inputs to the Speed Trim, how would that system have performed?" Hard to know, since that depends on how well this band-aid to a band-aid got implemented. Totally opinion-based. $\endgroup$
    – Ralph J
    May 10, 2020 at 15:55
  • 2
    $\begingroup$ I changed the question to remove references and speculation about the two flights that crashed. Note that my question was about whether or not the actual system design has the capability to let pilots switch between FCCs or ADRs, and not about how the system would have performed if its design were different. $\endgroup$
    – graphbobby
    May 10, 2020 at 16:27

1 Answer 1


As far as I understand from reading the B737 Operations Manual and the Lion Air Accident Investigation report,

  1. Each Flight Control Computer (FCC) receives AOA input only from the onside ADIRU and vane.

    The AOA and Mach inputs are provided to each FCC by the associated Air Data Inertial Reference Unit (ADIRU). Each ADIRU receives AOA information from one of the two resolvers contained within the associated AOA sensor (i.e. the Left ADIRU uses left AOA vane and the Right ADIRU uses the right AOA vane).

    Source: FINAL KNKT. Aircraft Accident Investigation Report

  2. The Maneuvering Characteristics Augmentation System (MCAS) and the Speed Trim System are controlled by the master FCC, which alternates between A and B per flight.

    Similar to the Speed Trim Function, the MCAS function is also a flight control law contained within each of the two FCCs. MCAS is only active in the master FCC for that flight. At aircraft power-up, the master FCC defaults to the left side FCC; and will then alternate between the left and right FCC by flight. The master FCC is not affected by the position of the Flight Director switches.

    Source: FINAL KNKT. Aircraft Accident Investigation Report

  3. This master FCC is different from the master FCC used to compute for the Flight Director (F/D) and the autopilot (A/P). For the purpose of AFCS, the master FCC can be cycled via the F/D switches on the Mode Control Panel (MCP), and the master FCC is illuminated.

    The master FCC is indicated by illumination of the respective master (MA) F/D indicator light. The master FCC is determined as follows:

    • with neither A/P engaged in CMD, the FCC for the first F/D turned on is the master
    • with one or both A/Ps engaged in CMD, the FCC for the first A/P in CMD is the master FCC, regardless of which F/D is turned on first.

    Source: Boeing 737-600/-700/-800/-900 Operations Manual

There does not appear to be a way for the crew to know which FCC is actively controlling the MCAS, nor a switch out the controlling computer, apart from pulling the circuit breakers.


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