What Is the Purpose of the Flight Director System?

Aircraft are equipped with many controls, such as switches, levers, systems, and instruments, allowing them to traverse the sky with ease. For instance, the autopilot feature in aircraft is required to perform numerous functions like finding a specific course, changing altitudes, and following navigation sources with crosswinds. Most importantly, a flight director system is built with the computing ability to carry out all of these tasks, providing signals to assist pilots or autopilot control inputs to stay along a predetermined and computed flight path.

To understand how the flight director functions, this blog will go over the systems that allow it work optimally. For example, an ADC and flight data computer transmit data to the flight director. The ADC accounts for altitude, airspeed, and temperature data, as well as heading data from magnetic flux valves, Horizontal Situation Indicator readings, and more. The flight data computer changes this information, alongside other metrics, into a command signal.

The command bars on the attitude indicator displays a command signal that presents the required pitch and roll inputs. This may look like two v-shaped symbols or cross bars that guide aircraft to their desired location. Furthermore, an aircraft symbol on the attitude indicator is aligned with the command bars as well. It is important to note that autopilot must be enabled in order to control and maneuver the aircraft along a chosen track and altitude. To better understand the flight director system, the next section will outline its major components.

Flight Director Indicator (FDI)

This is the first major component of the flight director system that aids the pilot in maintaining pre-decided pitch values. The pitch command control adjusts the intended pitch angle that is needed for ascent or descent. The calculated altitude is then shown on the command bars in order to maintain the pre-selected pitch angle through a variety of modes, including the HDG, VOR/LOC, and various others.

The ILS glide slope enables the auto mode to activate an entirely automated pitch selection algorithm relative to aircraft performance and wind conditions. While climb is initiated by turning the control clockwise, a fall is achieved by turning the control in a counter-clockwise direction. The glide slope (GS) enables pilots to control an aircraft’s glide slope using the pitch command signals. The go around (GA) mode presents climb command data combined with automated throttle/speed control.

To enable the HDG and VOR/LOC modes, the ALT HOLD (altitude hold) option can be utilized. Before the aircraft enters the glide path, the AUTO APP mode switch can also be used. When activated, pitch commands are dependent on the altimeter’s current barometric altitude.

Horizontal Situation Indicator (HIS)

The HSI is responsible for providing pilots with aircraft navigational aids. It combines data from the heading indicator, radio magnetic indicator (RMI), track indicator, and range indicator. Additionally, it may show VOR, DME, ILS, and ADF data. Just below the heading lubber line, a spinning compass card presents the heading value. The heading pointer transmits magnetic bearing data from the aircraft to a ground station. Meanwhile, a fixed aircraft icon and floating track bar displays the aircraft’s position along a predetermined track.

Flight Director Computer

The flight director computer receives data from the attitude gyroscope, radar altimeter, compass system, barometer sensors, and VOR/localizer glideslope receiver. It uses these controls to allow the pilot to fly in a specified direction, keep the appropriate pitch attitude during flight, maintain altitude, stay along a predetermined VOR track, and use an ILS glide slope to fly.


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