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I have seen the following occur during simulator events. With a left Source Off light illuminated and the captain's autopilot and autothrottles engaged (or re-engaged):

  • Captain's flight director lateral guidance cue will bias out of view when the autopilot is disengaged PRIOR TO glideslope capture.
  • Captain's flight director vertical guidance cue will bias out of view when the autopilot is disengaged AT glideslope capture.
  • Flight director will bias into view when either autopilot is then engaged.
  • Flight director will bias into view when either pilot selects TOGA.

I do not know why the above occurs. If anyone can explain this, I would be grateful.

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  • $\begingroup$ I do not have access to any 737-800 prints. Can you please explain the "Source Off" light. Are we talking the FCC itself or primary inputs like the IRS, ADC, ILS, or any other required valids. $\endgroup$
    – Mark Francis
    Commented Jan 21, 2022 at 3:28

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Interesting question. I believe the reason has to do with your cross bus tie relay (check out an image of the 737 electrical system).

When the 737 intercepts a G/S the Cross buss tie relay opens to protect the electrical system from a single source failure on an approach.

Now with that open I believe that might be affected your DC bus 1 which is connected to the CA autopilot.

Not 100% sure if that answers your questions.enter image description here

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