FD

A flight director is an instrument that displays lateral and vertical guidance cues to help a pilot fly a precise path. It takes raw data from the aircraft's navigation systems, flight management computer, or ground-based radio aids, and converts those into visual commands shown on the pilot's attitude indicator or primary flight display. The two most common cues are a command bar (a pair of crossed needles or a V-bar) that tells the pilot which way to bank and pitch, and a course deviation indicator that shows left-right alignment. Unlike an autopilot, the FD does not move the control surfaces; the pilot does, following its guidance.

Flight directors come in two main types. The conventional FD overlays command cues directly onto the attitude indicator (artificial horizon), with one needle showing pitch commands and another showing roll commands. Modern glass cockpits display the FD as a V-bar or magenta flight path vector integrated into the primary flight display, sometimes with a separately displayed localizer scale. Most large transport aircraft have dual flight directors, one on each pilot's display, and they can be coupled to an autopilot so the autopilot follows the FD commands automatically.

How it gets used in the real world

The FD is most valuable during instrument approaches, especially non-precision approaches where the pilot must navigate via VOR, NDB, or GPS without a precision glideslope. During a precision ILS approach, the FD will capture and track both the localizer (lateral) and glideslope (vertical) signals, giving the pilot clear visual reference if the autopilot is disconnected or unavailable. Many operators train pilots to hand-fly approaches using only the FD and altitude indicator, without relying on the horizontal situation indicator, as a safety skill.

The FD depends entirely on its input source. If the navigation source is wrong, the FD will guide the pilot wrong. A misconfigured flight management computer, a VOR station with a known compass rose offset, or degraded GPS can all cause the FD to give false commands. Pilots must cross-check the FD against other instruments, especially during critical phases like approach and landing. The FD is also sensitive to pitch and roll trim errors in the aircraft; if the aircraft is not trimmed correctly, the FD may command pitch or roll that the autopilot struggles to maintain.

Maintenance technicians troubleshoot FD problems by verifying the input source integrity (VOR accuracy, IRS alignment, GPS status), checking that the flight director mode selector is functioning, and ensuring the display is synced with the attitude reference system. Many glass cockpit platforms have built-in test functions that allow ground testing of the FD logic without flying. Degraded or inoperative flight directors must be reported in the maintenance log; operations may continue on many aircraft, but precision approaches are restricted until both FDs are serviceable.