A320 Autoflight System — AP, FD, ATHR, FCU and FMA Logic

Autopilot (AP): The A320 has two independent autopilots — AP1 and AP2. In normal operations only one is engaged at a time; both engage simultaneously only for CAT II & III approaches where independent monitoring is required. The AP sends commands to the flight control computers (ELAC, SEC, FAC) which move the control surfaces. Engaging the AP does not change what the FD shows — the AP simply follows the same guidance the FD was already presenting.

Flight Director (FD): The FD displays guidance bars on the PFD — pitch bar and bank bar — showing the pilot where to point the aircraft to follow the computed flight path. The FD has no authority over control surfaces. It is a display, not a controller. When flying manually with FD on, the pilot follows the bars. When the AP is engaged, the AP follows the bars automatically. Switching the FD off does not disconnect the AP.

Autothrust (ATHR): The ATHR controls thrust by commanding the FADECs. Unlike a conventional autothrottle, the A320's thrust levers do not move when ATHR is active — lever position defines the thrust envelope, not the commanded thrust. This is the most common source of confusion for pilots transitioning from aircraft with moving autothrottle levers. The levers set limits; the ATHR operates within them.

Flight Control Unit (FCU): The FCU on the glareshield is the crew's primary interface with the autoflight system. It controls target values (speed, heading/track, altitude, vertical speed/flight path angle) and switches between managed and selected guidance. Every FCU input either targets a specific value (selected) or hands control to the Flight Management System (managed).

Every autoflight mode is either managed or selected. This is the distinction that ATPL orals probe most consistently, because it reveals whether a candidate understands the system or has merely memorised mode names.

Managed guidance means the FMS is in control of that parameter. The FCU window for that parameter shows dashes rather than a numerical value. The FMA annunciates the mode in magenta. The aircraft follows the FMS computed profile — the lateral route, the climb or descent profile, the speed schedule.

Selected guidance means the pilot has assigned a specific value via the FCU. The FCU window shows the numerical value. The FMA annunciates the mode in green (active) or blue (armed). The aircraft tracks that value.

The Flight Mode Annunciator occupies the top strip of the Primary Flight Display and is divided into five columns. Reading the FMA correctly, every time, is non-negotiable for safe autoflight operation — and it is the most examined topic in A320 type rating oral assessments.

The five FMA columns, left to right:

Column 1 — Autothrust mode: What the ATHR is doing. Common modes: MAN THR (manual thrust, ATHR not active), SPEED (ATHR is maintaining a speed target), or a fixed thrust rating being commanded. The thrust rating appears below the mode — THR CLB, THR MCT, THR IDLE, etc.

Column 2 — Vertical mode: How the aircraft is managing altitude and vertical flight path. Modes include: ALT (holding an altitude), ALT* (capturing an altitude — transitioning), CLB / DES (managed climb or descent following FMS profile), OP CLB / OP DES (open climb or descent to selected altitude), V/S (vertical speed mode), FPA (flight path angle mode), G/S* (glideslope capturing), G/S (glideslope captured on ILS approach).

Column 3 — Lateral mode: How the aircraft is tracking laterally. Modes include: NAV (following FMS lateral route), HDG (tracking a selected heading), TRACK (tracking a selected track), LOC (localiser captured or capturing), RWY (runway track on takeoff).

Column 4 — Approach capability: The approach capability (CAT 1, CAT 2, CAT 3 SINGLE, CAT 3 DUAL) displayed during approach phase. AP OFF in amber indicates the autopilot has disconnected.

Column 5 — AP, FD and ATHR status: AP1 or AP2 engagement status, which flight directors are active (1 FD 2 means both are on, left and right), and whether the Autothrust is armed or active (A/THR).

The push/pull logic — the most common oral examination question

• Pull the altitude knob → Open Climb (OP CLB) or Open Descent (OP DES). The aircraft climbs or descends to the FCU altitude at maximum performance. A selected value is shown in the FCU window. This is the selected guidance mode for the vertical axis.
• Push the altitude knob → Managed CLB or managed DES. The FMS takes control of the vertical profile. Dashes appear in the altitude window — the FMS is managing. The aircraft follows the FMS computed climb or descent path, which may not use maximum performance.

V/S and FPA modes — and their risk

The ATHR operates in one of two fundamental modes at any time:

Speed mode: The ATHR adjusts thrust to maintain the target speed. This is the normal mode during cruise and approach. The FMA annunciates SPEED in column 1. The aircraft holds the speed; altitude is managed by the pitch axis.

Thrust mode: The ATHR commands a fixed thrust rating — CLB, MCT, IDLE, or TOGA — and holds it regardless of speed variation. Speed is then controlled by the pitch axis (the FD and AP pitch commands). This is the normal mode during climb and descent in managed guidance. The FMA annunciates the thrust rating (CLB, MCT, etc.) in column 1.

Alpha Floor is the ATHR protection mode that automatically applies TOGA thrust when the aircraft reaches a high angle of attack threshold, regardless of thrust lever position. Once activated, it locks to TOGA LK on the FMA — the crew must manually move thrust levers to TOGA and then select a lower setting to reset. Alpha Floor activation in normal operations is an abnormal event requiring crew action and understanding of the reset procedure.

The A320 autoflight system rewards pilots who understand the logic rather than memorise the modes. The FCU is the interface — every knob push or pull either takes authority away from the FMS or gives it back. The FMA is the readback — it tells you what the system is actually doing, and it boxes every mode change for 10 seconds specifically to demand your attention.

The managed vs selected distinction runs through every aspect of the system: managed speed shows dashes and annunciates in magenta; selected speed shows a number and annunciates in green. The ATHR is either managing speed or holding a thrust rating — not both simultaneously. The vertical mode is either following the FMS profile or tracking a pilot-selected value.

Understand those three distinctions clearly and the system becomes predictable. A predictable autoflight system is a safe one.