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When FD switch is on, FD is on. When FD switch is off, FD is off.

But when FD switches are recycled, FD switch on, but FD bars are not shown during approach. Is this due to approach FD logic? If so, what is the approach logic and when does it arm?

I understand everything about availability of FD bars during TOGA and Alt Acq.

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At the moment when both switches (and the autopilot) are off, there is no active guidance mode (like HDG, V/S, LNAV, LVL CHG, APCH, etc). (In fact that's typically the reason to turn them both off, to disengage the active mode/s.)

Then after they're back on, they only display when both a lateral (heading, VOR, LNAV) and a vertical (level change, vertical speed, altitude hold, VNAV) mode are selected. Until one each has been selected, a single-cue FD remains out of view; a dual cue FD will show a cue for whatever mode is active.

For completeness, both Approach and TOGA are lateral and vertical modes.

If you're on an approach & switch both FD switches off then back on, you'll only get the bars back after you also press APCH to arm the approach mode. The LOC will capture, and then the GS, and at that point you'll have the FD bars again. But you get those only after you arm the mode.

One of the reasons we might cycle the FD's is if a TR unit has failed, which causes problems when the bus separation relay opens at GS capture. We get the relay closed again by "removing" GS capture by clearing the GS FD mode by cycling the FD switches. Now your electrical system looks normal, but if you re-engage APCH mode, you'd get the same problem back again. So, don't do that! Either fly the approach visually, or in LNAV/VNAV, or go around & get a new plan. But the idea is to NOT have APCH mode active, in this case. (Or, at most, you'll have one pilot with an FD in APCH mode, but the other pilot won't have an FD, due to the bus separation.)

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  • $\begingroup$ Why no FD bars when you recycle them? $\endgroup$ Nov 5, 2023 at 15:49
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    $\begingroup$ No active mode = no bars. Both FD's + AP off means all modes are cleared, and turning an FD on doesn't in itself activate any modes. $\endgroup$
    – Ralph J
    Nov 5, 2023 at 18:18
  • $\begingroup$ Thank you so much for the correct answer Ralph J. I get it now. $\endgroup$ Nov 5, 2023 at 19:54

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