Airbus a320 Flight Control Unit (FCU) Functions Explained

Airbus a320 Flight Control Unit (FCU) image download


The FCU (Flight Control Unit) which is installed in the center of glare-shield is used for the automatic flight controls. The FCU has two channels (for safety reason) each of which can independently command the central panel. If one channel fails, the other channel can control all the required functions.

FCU Philosophy in Auto flight

When aircraft fly in auto flight mode, there are two types of guidance systems that control the aircraft. The Flight Management Guidance System (FMGS) control the auto flight in one System. The other system relies & follows the target quantity manually entered by Pilots. The FCU windows display dashes and the white dots next to those windows light up when the aircraft uses target quantities from the FMGS. The windows display the selected numbers and the white dots do not light up when the aircraft employs target quantities entered by the pilot.

Selector Knobs

The Flight Control Unit (FCU) has four selector knobs for the Input/Change the target quantities.

  • HDG - TRK
  • ALT
  • V / S - FPA

FCU's selector knobs can be turned, pushed in, and pulled out. To arm or engage managed guidance for a specific mode, users simply turn the selector knob associated with that mode.

Turn the selection knob to set the desired value, then pull the knob out to activate the selected guidance mode with a target value equal to the intended value.

The value displayed when a selector knob is spun without being pulled is the sum of the current target and the turn action value.


Selector knob for SPD/MACH: The speed ranges from 100 to 399 knots, while the Mach number ranges from 0.10 to 0.99. A single spin of the knob equals approximately 32 knots or 0.32 Mach.

SPD/MACH Pushbutton

By pressing the SPD/MACH button, the SPD target is switched to the equivalent MACH target, and vice versa.


Selector knob for HDG/TRK The display range is 0° to 359°.

One turn of the knob equals 32 degrees (1 degree per click).

LOC pushbutton

The LOC (Localizer) mode is armed, engaged, or disengaged by pressing the LOC pushbutton.

HDG V/S- TRK FPA pushbutton

This pushbutton is used by the pilot to choose between HDG and TRK (associated with FPA).

Pushing it will do the following functions:

  • Displays the FPV (Flight Path Vector) on the PFD (Primary Flight Display) or deletes it.
  • This button changes the FD crossbar display to the aircraft Flight Path Director and vice versa on the PFD.
  • Also Changes heading reference into track reference in the HDG/TRK window and vice versa.
  • Also Changes vertical speed reference target into flight path angle reference target in the V/S-FPA window and vice versa. 

ALT Knob

The pilot's target value is always displayed in the FCU Altitude window. The ALT window never displayed dashes.

Inner and outer altitude selector knobs have the display range from 100 to 49000 feet.

There are two positions on the outer ALT knob of are 100 and 1000.

Depending on the position of the outer knob, the inner ALT knob adjusts the altitude in the FCU windows in increments of 100 or 1000 feet.

Metric ALT Pushbutton

Metric ALT Pushbutton converts the altitude in meter from feet or vice versa. Pressing the pushbutton will display the current altitude FCU/FM altitude targets in meters on the (PFD)Primary Flight Display. or FCU altitude in meters on ECAM page.

AP1 AP2 pushbuttons

AP1 AP2 pushbuttons are used to engage or disengage the autopilots.

When the autopilot is turned on, the buttons turn green.

A/THR Pushbutton

This pushbutton is used to arm, activate, and disconnect the auto thrust (A/THR).

If the A/THR is armed or operating, this button glows green.

EXPED pushbutton:

The expedite mode is activated by pressing EXPED Pushbutton.

LOC Pushbutton

The LOC (Localizer) mode can be armed, engaged, or disengaged by pressing the LOC pushbutton

APPR Pushbutton

If an ILS approach is selected in the active F-PLN, APPR pushbutton arms, disarms, engages, or disengages the approach modes: LOC and G/S modes. If a non-precision approach is selected in the active F-PLN, APP NAV-FINAL modes are used.

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