Flight Director Systems on Aircraft

Aircraft · 4 min read · Dec 15, 2022
flight director

Every commercial airliner is equipped with a flight director system. The flight director’s primary purpose is to assist the pilot in flying the airplane, and it works whether the pilot is handling the flight manually or has set the auto pilot mode.

If you want to learn more about the flight director systems, we encourage you to keep reading as we share all the details.

What does a flight director system do?

A flight director system shares crucial information in its purpose to assist the pilot during the flight. However, we could summarize it by saying that a flight director computes the aircraft’s attitude required to attain and maintain a preselected flight condition. Then, it displays it to the pilot on the attitude indicator.

In other words, a flight director system constantly monitors the speed, altitude, and heading of the aircraft. It then compares these parameters to the desired values set by the pilot. If the aircraft deviates from the desired values, the flight director system will compute and display the necessary corrections. The pilot can then follow these corrections to get back on course.

Aircraft cockpit with illuminated instruments and flight director display, flight director's display shows information.

How Flight Director Computes Attitude

The flight director system computes the aircraft’s attitude by referencing data from the following sources:

  • Air Data Computer – This instrument type constantly measures static pressure, total pressure, and temperature. It uses this data to determine airspeed, altitude, and vertical speed.
  • Inertial Reference Unit – This instrument type measures acceleration and gyroscopic precession. It uses this data to determine the proper pitch, roll, and yaw rate.
  • Navigation System – This device compares the aircraft’s current position with navigational reference points. It uses this data to compute the heading, track, and ground speed.

Flight director’s display

Flight director’s display varies from aircraft to aircraft. Also, the exact form of the display varies depending on the type of instrument. However, a flight director’s display will always have the following components:

  • An attitude indicator with a horizontal situation indicator.
  • A fixed aircraft symbol.
  • Pitch and bank command bars where the pilot can see the proper pitch and bank angles required.
  • A glide slope indicator.
  • A localizer deviation indicator.
  • A slip indicator.
  • A warning flag for gyro, computer and glide slope.

Let’s see some of them in more detail.

Flight director helping the pilot navigate in the air.

Fixed aircraft symbol

The aircraft’s attitude is symbolized by its position in relation to the natural horizon, which is indicated by command bars. There are three flight modes that the pilot can choose from. To fly using the armed command bars, the pilot must place the aircraft symbol between both bars.

COMMAND BARS

The command bars give the pilot directional guidance by moving up for a climb, down for descent, and left or right for lateral movement. They also compute the angle of bank for standard-rate turns so that the pilot can fly to a selected beading or track.

In addition, the bars display pitch commands that enable the pilot to capture and follow an ILS glide slope, maintain a preselected pitch attitude, or stay at a selected barometric altitude.

The pilot follows the directions given by the command bars, lining up the fixed symbol with them. They can be moved out of view when not needed.

GLIDE SLOPE INDICATOR

The glide slope deviation pointer tells you how far off the center of the instrument landing system (ILS) glide slope your aircraft is, and the glide scale shows where your aircraft is in relation to that centerline.

LOCALIZER DEVIATION POINTER

The deviation pointer, which symbolizes the center of the ILS localizer, becomes visible to the pilot when he/she has acquired the glide slope. The expanded scale movement illustrates how far lateral deviation from the localizer is, and it’s about twice as touchy as bar in horizontal situation indicator.

SLIP INDICATOR

This provides indications if you are slipping or skidding.

How Flight Director Systems Display Attitude Information

Although the flight director’s display varies depending on the aircraft, the flight director system always displays the aircraft’s attitude on the attitude indicator. The attitude indicator shows both the pitch and roll of the aircraft. The pitch is represented by a horizontally moving needle and the roll is represented by a vertically moving bar which is one of the so, called command bars.

Pilots use this information to keep the aircraft flying straight and level. They can also use it to make turns. When making a turn, pilots will see both the turn coordinator (a ball that moves left or right) and the heading indicator (a compass that shows which way the plane is pointing) move in unison.

The further away from level flight an aircraft gets, the more those two instruments will disagree with each other. That disagreement is called slip or skid, and it’s an important indication of how much cross-control a pilot needs to apply in order to maintain level flight.

Advanced flight director screen.

Final remarks

Flight directors are an essential part of every commercial airliner or any other aircraft for that matter. They are responsible for computing the aircraft’s attitude required for their flight and displaying it to the pilot on the attitude indicator. Also, they manage other information that is critical for flying, such as bank angles required for the flight path and conditions, wind drift correction, and other additional information.

In short, the flight director’s primary purpose is to assist the pilot in flying the aircraft. So, by understanding how flight director systems work, pilots can better use this critical tool to fly safely.

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Carlos Collantes
Carlos Collantes
A mechanical engineer and aviation enthusiast dedicated to share some knowledge by creating top-notch content, especially in engineering and aviation topics.

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