If some instruments on the 737 MAX 8 fail on the Captain's side and autopilot stops, can the First Officer engage autopilot (as the instruments on his side are fine)? In the ET 302 crash the Captain tried to engage the auto pilot but it kept disengaging because of bad AoA readings (which in turn also affected other readings). Could the First Officer have engaged the autopilot as instruments on his side were working fine?


2 Answers 2


The following applies to the Boeing 737 NG series, but probably there are no large differences for the MAX series.

The FCOMv2 (4.20.2 Automatic Flight - System Description) mentions only the following systems in the Autopilot Engagement Criteria and the Autopilot Disengagement sections:

The A/P automatically disengages when any of the following occurs:

  • [...]
  • either left or right IRS system failure or FAULT light illuminated
  • loss of electrical power or a sensor input which prevents proper operation of the engaged A/P and mode
  • loss of respective hydraulic system pressure.

Note: Loss of the system A engine-driven hydraulic pump, and a heavy demand on system A, may cause A/P A to disengage.

The sensor input is probably what you are asking about because this would affect the instruments as well. In general, if a system only affects one side, the other autopilot should still work. One notable exception would be the IRS, where any IRS failure causes the autopilot to disconnect.


The two autopilot systems on the 737 are called A and B, and they are not necessarily linked to Captain and First Officer. Both the Captain and the First Officer can press both engagement buttons on the MCP (marked with 1 in the image from the FCOM below).

737 MCP

  • $\begingroup$ But does channel A use left side data and channel B right side data, or both channel all three data? $\endgroup$
    – Jan Hudec
    Oct 25, 2019 at 19:30

It depends. There are some failures that inhibit both autopilots, and others that may affect only one. Also, policy may be more restrictive -- the QRH may direct not engaging the affected-side autopilot, or it may direct to not engage either autopilot, even though the system wouldn't inhibit it per se.

Please note that "instruments" on the Max (and NG) are almost all displayed on the screens. If a screen fails, it probably won't affect either autopilot, although if the failure is far enough back in the black boxes (say, a DU or something at that level of hardware), it might. The failures that can inhibit autopilot engagement are more typically within systems, not instruments.

  • $\begingroup$ Thanks Ralph. Have added to my question to make it clearer. $\endgroup$ May 16, 2019 at 7:53
  • $\begingroup$ @OutOfTheBox Your edit made the question into speculation about an accident, which is off-topic. It would be better to revert it to what you had before. I'm not going to speculate about hypotheticals in that crash. $\endgroup$
    – Ralph J
    May 16, 2019 at 13:18
  • $\begingroup$ My question was based on the data recovered from the Flight Data Recorder and Voice Recorder. The left side had stick shaker and different AoA, Airspeed and Altitude than the right side. I suspect the AP failed to engage because of the stick shaker and / or unacceptable values. If so, could the other AP have been engaged, as I assume the other AP uses values from the right side which were still good. That is the question. $\endgroup$ May 17, 2019 at 22:18

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