What Is Flight Director System: 5 Important Facts


Flight Director System | Flight Director Simulator

What is Flight Director?

There are several instruments in an aircraft that aids the pilot in the decision-making process while flying. One of them is the Flight Director that is particularly responsible for trajectory maintenance.

A Flight Director functions as a guiding device that is surfaced on an aircraft’s Attitude Indicator and is responsible for displaying the pilot’s required attitude to follow a particular trajectory. In addition, it computes and shows the necessary pitch and bank angles for the aircraft to follow a predetermined course. The precise shape of the its display varies depending on the instrument. It is the autopilot system’s central processing unit and hence also referred to as the brain of the aircraft’s autopilot.

what is Flight director system
Flight Director embedded in Attitude Indicator; Image Source: Sheldor 2000Attitude indicator with FDCC BY-SA 4.0

Higher level computations, other than straight and level flight, require the autopilot to perform extra duties such as finding a specified course (intercepting), changing altitudes, and following navigation sources with crosswinds. The Flight Director is built with the computing ability to do these duties and typically shows the pilot’s indicators for guidance.

It can be employed in both manual and autopilot flight modes. The pilot’s workload is increased as a result of the Flight Director programming. The overall effort is reduced if the additional workload is compensated by enabling the autopilot to steer the aircraft. However, if you choose to use the FD display yet fly the aircraft manually, your burden will be significantly enhanced.

How does a Flight Director work?

A Flight Director is a beneficial tool that provides signals to assist pilot or autopilot control inputs along a predetermined and computed flight path. It includes the role of many other necessary elements for cumulative functioning.

ADC and a flight data computer responsibly transmit data the information to the Flight Director in the usual scenario. The ADC accounts for altitude, airspeed, and temperature data, heading data from magnetic flux valves, Horizontal Situation Indicator readings (or PFLIGHT DIRECTOR/multi-function display (MFLIGHT DIRECTOR)/electronic horizontal situation indicator (EHSI)), and navigation data obtained from FMS, VOR (very high frequency omnidirectional range) / DME (distance measuring equipment), and RNAV sources. The flight data computer combines all of the data into a command signal, including speed, location, closure, rift, track, intended path, and altitude.

The command bars on the attitude indicator indicate the command signal, which symbolize the required pitch and roll inputs, that vary in regards with specified objectives. The can exhibit two shapes-either inverted chevrons (or V-shaped symbols) or crossed bars, that facilitates guiding the aircraft to the desired location. The aircraft symbol is aligned on the attitude indicator command bars in both the aforementioned cases. Autopilot must be enabled otherwise, in order to execute the actual control motions to fly the chosen track and altitude.

Flight Director System Components

The major components of a Flight Director system are-

  1. Flight Director Indicator (FDI)
  2. Horizontal Situation Indicator (HIS)
  3. Mode Selector
  4. Flight Director Computer

Flight Director Indicator (FDI)

This is the first major component in the FD System and aids the pilot in maintaining the pre-decided pitch values.

The pitch command control fixes the intended pitch angle, necessary for the aircraft’s ascent or descent. The calculated attitude is then shown on the command bars in order to preserve the pre-selected pitch angle through a variety of modes, including HDG (heading), VOR/LOC (localizer tracking), and AUTO APP or G/S (automatic collection and tracking of ILS localizers and glide path).

The ILS glide slope enables the auto mode to activate a completely automated pitch selection algorithm in accordance with the aircraft performance and wind conditions. A climb is initiated by turning the control clockwise, while a fall is started by turning the control counter-clockwise. The manual glide slope or the GS option allows manual attainment and maintenance of the glide slope using pitch command signals. The GA (go around) mode displays climb command data combined with automated throttle/speed control.

In the HDG and VOR/LOC modes, the ALT HOLD (altitude hold) option can be used. The pilot can also toggle the switch in the AUTO APP mode before the aircraft enters the glide path. When activated, pitch commands are based on the altimeter’s current barometric altitude.

Horizontal Situation Indicator (HSI)

The horizontal situation indicator (HSI) accounts for the pilot’s understanding towards utilizing aircraft navigational aids.

The HIS is produced through an amalgamation of heading indication, radio magnetic indicator (RMI), track indicator, and range indicator. It may also show VOR, DME, ILS, or ADF data. Under the heading lubber line, a spinning compass card contains the aircraft heading value. This compass card requires calibration in 5° increments. The heading pointer transmits magnetic bearing data from the aircraft to the chosen ground station (VOR or ADF). The fixed aircraft icon and floating track bar visualizes the aircraft’s position about the pre-decided track (VOR or ILS localizer).

  1. Interception of inbound track- The pilot ensures that the TO-FROM indication to ensure it points to the track arrow’s head throughout the appropriate route. The pilot flies at the intercept angle, usually 30° to 45°, ensuring that the track arrow’s head is in the upper half of the HSI. The angle from the chosen track should not be greater than 90°.
  2. Outbound tracking- The selection window contains the tracks for the pilot to be chosen from, and ensures the TO-FROM indication points are in line with the tail of the track arrow for outbound tracking. The pilot then rotates the aircraft in the shortest possible path to an intercepting track, with the track arrow’s head in the top half of the HSI and an appropriate interception angle (usually 45°).

Flight Director Computer

The primary Flight Director computer receives data from the following sources:

  1. Attitude gyroscope
  2. Radar altimeter
  3. Compass system
  4. Barometer sensors
  5. VOR/localizer glideslope receiver

The computer uses this data to send steering instruction information to the pilot, allowing him to:

  1. Fly in a specified direction.
  2. Maintain a predetermined pitch attitude during flight.
  3. Maintain altitude.
  4. Intercept and maintain a predetermined VOR or localizer track.
  5. Use an ILS glide slope to fly.

How to use a Flight Director?

Flight Director without Autopilot

The Flight Director and the Autopilot system are meant to function in tandem. However, engagement or disengagement of the autopilot with the former depends on the installation.

If the pilot visualizes all processed information in the form of FD command bar cues, it can be safely assumed that the autopilot is not activated. Still, the pilot must manually maneuver the airplane to follow these cues in order to fly the specified flight path. This increases your burden since you must program the Flight Director to perform each procedure or maneuver while flying the airplane.

Flight Director with Autopilot

Presence of both the Flight Director and autopilot doesn’t mandate the pilot to use both of them simultaneously.

The Flight Director cues can be utilized without using the autopilot at all. It is also possible to operate the autopilot without additionally engaging the Flight Director. Activation of the autopilot allows easy adherence to the FD’s instructions and fly the airplane along the specified lateral and longitudinal routes.

It is often a decisive situation for a pilot as to whether carry out the FD signals manually or let the autopilot do it automatically. Therefore, verifying the autopilot mode and engagement state is essential for keeping awareness of who is piloting the airplane.

Difference between Autopilot and Flight Director

There are few differences between the Flight Director and the Autopilot based on their functioning.

The Autopilot accounts for the required attitude shown on the attitude indicator, which is evaluated by the Flight Director. In contrast, the autopilot operates the aircraft’s control surfaces to alter the aircraft’s altitude, direction, and altitude.

So, the autopilot actually controls the aircraft, while the FD calculates and displays the planned fly route to the pilot like the brain of the operation. In another reference, the pilot can activate the Flight Director and adhere to its suggested attitude to fly the aircraft manually, all without ever engaging the autopilot.

Single Cue vs. Dual Cue Flight Director

Single Cue Flight Director

There are two major types of the Flight Director system based on the method of data visualization. The first one is referred to as the Single Cue Flight Director.

The Single Cue, often known as the V-bar, gives the pilot an immediate indication of where to position the aircraft in roll and pitch.

Image Source: U.S. Dept. of Transportation, FAA, ADI, marked as public domain, more details on Wikimedia Commons

In this display, the “V bars” show which way to go: up, down, bank left, or bank right. The pilot only needs to match the airplane icon to the “V bars.”

Dual Cue Flight Director

The second major type is the Dual Cue, which is also the widely used one globally.

The split cue or cross-bars, also known as the dual cue, has the benefit of permitting separate Flight Director command following, such as during a circling approach. The vertical bar in this display indicates a command to the left or right.

Image Source: Photographer: Reddy & Garcia, VMS Artificial Horizon, marked as public domain, more details on Wikimedia Commons

Left orientation of the vertical bar necessitates the aircraft to bank in the left direction. If the pilot complies with the order, the vertical line will move to the center. The horizontal bar is the same; as it falls, place the airplane symbol on it. If the command is executed, the flat bar will move to the center.

Esha Chakraborty

I have a background in Aerospace Engineering, currently working towards the application of Robotics in the Defense and the Space Science Industry. I am a continuous learner and my passion for creative arts keeps me inclined towards designing novel engineering concepts. With robots substituting almost all human actions in the future, I like to bring to my readers the foundational aspects of the subject in an easy yet informative manner. I also like to keep updated with the advancements in the aerospace industry simultaneously. Connect with me with LinkedIn - http://linkedin.com/in/eshachakraborty93

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