Flight Director Systems

1. Introduction — Purpose of the FDS

The Flight Director System (FDS) was originally developed as an aid to the pilot during landing. It provides steering and attitude signals on one instrument, reducing workload. As autopilots became more advanced, FDS signals could be coupled to the autopilot for more complex tasks.

With an FDS, information about attitude, heading and flight path can be integrated with navigation information to produce:

• Easy-to-interpret visual instructions for the pilot (via flight director bars), and/or
• Direct input to the autopilot, or both simultaneously

2. FDS Information Sources

3. Flight Director System Components

Electronic Attitude Director Indicator (EADI)

A standard artificial horizon providing pitch and roll. The Director part comes from its ability to display demand information from the FDS using Flight Director Command Bars. The pilot "flies to" either the intersection of the cross-bars or the point between wedge-shaped pointers. Both displays are intuitive and functionally identical.

Primary Flight Display (PFD)

Part of EFIS — brings all flying information onto one display. Contains EADI surrounded by speed, altitude and VSI tapes plus a compass display. Includes the Flight Mode Annunciator (FMA) area.

Electronic Horizontal Situation Indicator (EHSI)

A gyro-magnetic compass display with:

• Course Deviation Indicator (CDI) bar
• Deviation dots (scale varies)
• From/to pointer
• Selected course window
• DME display of range
• Heading bug

Modern aircraft use a Navigational Display (ND) and a centralized Autoflight Mode Control Panel (AMCP / MCP) for course selection rather than a knob on the HSI.

Flight Director Computer (FDC)

Gathers and processes all inputs. Older aircraft use analogue ADC/VG outputs; modern systems are purely digital. Outputs go to symbol generators for EADI/EHSI and/or the autopilot.

Optional Components

FMA (Flight Mode Annunciator) on EFIS Aircraft

On older annunciator panels: LOC light = amber (armed) → green (captured). GS light = amber (armed) → green (captured).

4. Raw vs Computed Information

5. FD Fail Indications

6. Flight Director Modes

Command Attitude Changes

The FD command bars can be offset vertically to provide an aiming point for maintaining a constant pitch angle (level flight, climb, or descent). Achieved by moving the Pitch Trim knob/wheel. Pitch trim is inhibited whenever any other pitch mode is active.

Altitude Acquire / Altitude Hold

Signals from ADC through FDS maintain selected level. If aircraft deviates above desired altitude, FD commands pitch down; if below, it commands pitch up. Command bars indicate neutral position at selected level.

7. Navigation Modes

Heading Mode

Pilot selects desired heading via knob on HSI, MCP, or central panel. FDS indicates a fly-to command to bring aircraft onto desired heading.

LOC / VOR (LNAV)

Allows VOR, Localizer, or INS/IRS/FMS/GPS nav information to be fed and displayed on the FDS. After tuning/identifying the VOR, selecting desired track and VOR/LOC on the mode selector, the FDS gives steering commands to intercept and maintain track.

Flight Director Approach (FDA)

ILS frequency is tuned and identified; QDM set in the course window; mode selector set to AUTO/APP. Sequence:

flowchart TD
  A["APP mode selected\nAnnunciator shows LOC armed"] --> B["Localizer captured\nAnnunciator shows LOC (green)"]
  B --> C["Approaching GS\nAnnunciator shows GS armed (amber)"]
  C --> D["GS captured\nAnnunciator shows GS (green)\nFDS commands nose-down pitch"]
  D --> E["Continue ILS approach\nGain scheduling applied as\naircraft nears threshold"]

8. FD Take-off Mode

Both FD systems should be switched on prior to starting the take-off roll. Take-off mode is engaged by pressing a TO/GA switch on the throttles.

Take-off FD Commands

Terminating TO Mode

• Below 400 ft RA: both FD switches must be turned OFF
• Above 400 ft RA: select other FD pitch modes or engage autopilot

Engine Failure During Take-off

9. FD Go-around Mode

For the 737-400, two criteria must be met:

1. In-flight, below 2000 ft RA, and NOT in the TO mode
2. TO/GA switch pressed

After GA engagement: command bars appear for both pilots; TO/GA annunciated; MCP IAS/MACH display blanks; airspeed cursors display manoeuvring speed for existing flap.

GA FD Commands

Single-Engine GA Target Speed

10. FD Manoeuvre Protection & Gain Scheduling

FD Manoeuvre Protection

Modern FDC has aircraft performance parameters stored in memory. With inputs from ADC and other systems, it ensures it never commands a manoeuvre that would overstress the aircraft. This is the precursor to fly-by-wire envelope protection.

Flight Director Gain Scheduling

The FDC varies ("schedules") the gain of pitch and roll demands relative to the task. Most significant during the FDA (Flight Director Approach).

Why it's needed: The ILS glide slope beam diverges from the runway. At different distances, the same 1° of beam error represents very different vertical distances:

As the aircraft nears the transmitter, the required correction magnitude decreases. Gain scheduling reduces commanded corrections accordingly — from nearly full authority early in the approach to perhaps ½ or ⅓ of original value near the threshold.

Gain Scheduling Methods (in order of sophistication)

11. Dual FDS & Comparator

Large aircraft with 2 FDS (one per pilot) can use the FD Comparator to monitor command bar positions of both systems.

Comparator Active Conditions

• Both FD switches ON and neither autopilot engaged
• Only active in TOGA or APP mode below 800 ft RA

Comparator Inhibited When:

• On the ground
• Either FD affected by electrical bus transfer
• Failure of either FD sensor or FD itself

Master Reference Tables

Key Numerical Values — Chapter 25