Difference in autopilot + FD

[FRIDAY]

Could anyone please point out the key differences or particular circumstances for use, between the autopilot system and the flight director system or are they really just the same? Thank you in advance. :confused:

[HugMonster]

The obvious difference you would notice is that the Flight Director system moves the command bars on your display. If you have it in "Alt hold" and "Heading" modes, then turn the heading bug, the command bars will move - you follow them, and find you're turning onto the selected heading. Select a 1500 fpm descent on the FD, and the bars will pitch down. Follow them, and, lo and behold, you're dropping at 1500 fpm.

The autopilot switches in the servos on the relevant control surfaces, and it follows the FD commands instead of you.

That's a very simple approximation - hope it answers your question.

[dusk2dawn]

Ages ago I was involved in the first installation of a King KFC 150 (I think) on a PN68. The duch King engineer told me then that the A/P and F/D used the same computer. On the 727 I fly today there is definitely 2 different systems. The A/P requiring re-sensitizing of the GS in 1500' AGL, the F/D does not require re-sensitizing.

[Redback]

I suppose, put in the simplest terms, the FD shows you what to do (according to the FMC or MCP) and you then move the controls to follow the directions. If the autopilot's engaged it will move the controls for you.

As for where and when we use what. The FD is displayed all of the time and the autopilot is only engaged once we've had enough of hand flying or we're required to for an Autoland.