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I have noticed that the requirement to turn on the Flight Directors in a commercial aircraft is in one of the very first checklists. Why does this have to be the case when the Flight Directors are really just an indication of where the aircraft needs to fly to?

Why, for instance, is it not in the "Before Take-off" checklist?

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  • $\begingroup$ Well for one example, you need to program in the flight plan into the FD, might as well do it while sitting at the gate than when running through before take-off items. $\endgroup$
    – Ron Beyer
    Commented May 1, 2017 at 14:07
  • $\begingroup$ Depending on what you mean by "a commercial aircraft", they don't. Some companies might have that in their checklist or SOP, but not all do. The air carrier I fly for does not. We usually do turn on the flight director shortly before takeoff via the TOGA button, but it is not a requirement or part of any checklist. $\endgroup$
    – J W
    Commented May 1, 2017 at 14:11
  • $\begingroup$ @RonBeyer You don't program a flight plan into the F/D. Flight plans go into the FMS. The F/D gets path data from the FMS when in LNAV (lateral) or VNAV (vertical) guidance modes. The F/D can also track a single path set via several mode options on the Mode Control Panel (MCP). $\endgroup$
    – Gerry
    Commented May 2, 2017 at 0:39
  • $\begingroup$ @Gerry You are right, I mixed up FD and FMS in my mind. $\endgroup$
    – Ron Beyer
    Commented May 2, 2017 at 1:39
  • $\begingroup$ Well, why not...? :) $\endgroup$
    – Waked
    Commented May 2, 2017 at 18:26

2 Answers 2

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For a jetliner, the 'why early' is to test the auto flight system. Turning on the F/D also turns on the Flight Mode Annunciator (FMA) and displays the armed modes.

So it's an early check that all systems are go.

enter image description here
From a 777 manual, any fault will be indicated on the FMA as it reads above.

Here are the flow checks pertaining to the F/D and FMA during the cockpit setup:

enter image description here

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The FD displays the proper pitch and bank for an aircraft to achieve desired performance. Having the FD up and ready in advance of takeoff is important to reduce the risk of it not being ready for use at takeoff. The FD normally receives input from the primary flight gyros, and uses that input to calculate control commands. Those control commands can be directly routed to an autopilot, or displayed for pilot to utilize.

Gyros need to be spun-up and ready, and the FD computer needs to be initialized and calculating solutions at takeoff time, so delaying the startup to takeoff could cause delay due to non-readiness.

Additionally, FD and FMS work together, and typically FMS is powered up when the avionics are initially powered up. This permits routing, and other pre-flight tasks to be performed on the FMS. Most larger aircraft, and particularly long haul aircraft will have dual FMS, usually independently programmed by different crew members.

In general, powering up avionics tends to reduce corrosion, assure operation, permit self-tests to be performed in advance of leaving the gate, and has little cost compared to operational benefit (such as a return to the ramp for OTS equipment).

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  • $\begingroup$ This doesn't answer the question which is about the Flight Director, which together with the autopilot and autothrust systems comprise the autoflight systems. It is totally independent of the navigation systems you're discussing. $\endgroup$
    – Gerry
    Commented May 2, 2017 at 11:57
  • $\begingroup$ @Gerry, you are absolutely correct, and I have modified my answer accordingly. Thank you. $\endgroup$
    – mongo
    Commented May 2, 2017 at 15:07

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