A/THR Architecture and Engagement

The guidance group (articles 05–15) managed attitude and trajectory; the autothrust manages energy. It is the FG's thrust half inside the FMGEC, and it takes two articles: this one — who it is and how it comes on duty; article 17 — how it works and how it stands down.

1. One master, one standby — and the lever as a letter of authority

Per FCOM DSC-22_30-50:

The FMGECs independently compute the A/THR commands and only one FMGEC controls the active A/THR, referred to as the master FMGEC. If one AP is engaged, the A/THR uses the system data associated to the engaged AP. If no AP/FD is engaged or both APs are engaged, FMGEC1 has the priority.

Both computers always compute; one commands — the master FMGEC, which follows the engaged AP, and defaults to FMGEC 1 with none or both engaged (article 05's "AP1 is always active", thrust edition). Who talks to the FADECs depends on the state:

When the A/THR is active, the A/THR thrust target is sent to the FADECs. The FADECs computes thrust commands, in accordance with the aircraft environment and status. The thrust commands are then sent to the engines. When the A/THR is armed, the FADECs compute the thrust commands in accordance with the thrust lever positions, and the selection of PERF TAKEOFF page. When the A/THR is disconnected, the FADECs compute the thrust commands in accordance with the thrust lever positions.

Read the armed and disconnected lines twice: in both, thrust comes from the levers — the difference is only the standby badge. Active is the one state where the FMGEC's target drives the engines. And why those levers never move by themselves — Airbus's three official reasons. Per FCTM AOP-10-30-10:

Airbus has selected the non-back-driven thrust lever concept: ‐ The flight crew can easily and intuitively monitor the energy of the aircraft via current energy cues (speed, speed trend, HUD chevrons , engine parameters), and not via ambiguous thrust levers movement ‐ When the autothrust is engaged, the Thrust Lever Position determines the maximum authorized thrust that may be commanded by the autothrust ‐ When the flight crew uses manual thrust, the Thrust Lever Position determines the current thrust (as on any aircraft not equipped with autothrust).

The system-level idea to internalise: the CL detent is not a "climb thrust switch" — it is a letter of authority. It grants the A/THR the right to command anything from idle up to CLB thrust, inside which it roams freely while the lever sits still. Monitor thrust on the E/WD's N1/EPR needles and target circles (article 17) — never on the lever.

2. Three states, two mode families

Per FCOM DSC-22_30-50:

The A/THR is either armed, active or disconnected. When active, two different types of A/THR modes are available: ‐ The SPEED/MACH mode: The A/THR continuously adjusts the thrust in order to maintain a speed/Mach target (e.g. during cruise, and approach). ‐ The THRUST modes: The A/THR controls a fixed thrust, in accordance with the engaged THRUST mode.

And you never choose between the families — the vertical mode does:

The A/THR modes are automatically linked to the AP/FD vertical modes: ‐ When an AP/FD vertical mode controls the trajectory (e.g. altitude acquire modes, altitude hold modes, V/S / FPA, G/S, F-G/S ), the A/THR is in SPEED/MACH mode ‐ When an AP/FD vertical mode adjusts the aircraft pitch in order to maintain a speed/Mach target (e.g. climb, descent), the A/THR is in THRUST mode.

Vertical mode owns the trajectory → A/THR owns the speed. Vertical mode owns the speed (pitch trades) → A/THR holds a fixed thrust. At any instant, the speed has exactly one owner — article 06 said the thrust levers are a mode selector; here is the other half: the vertical mode is the A/THR's mode selector.

3. The thrust levers — four detents, two buttons

Per FCOM DSC-22_30-50:

The thrust levers have: ‐ Four detents: IDLE, CL, FLX-MCT, TOGA ‐ Two instinctive disconnect pushbuttons. The flight crew uses the thrust levers to: ‐ Arm, activate, or disconnect the A/THR ‐ Engage the takeoff and go-around modes ‐ Manually control the thrust of each engine, when the A/THR is disconnected ‐ Engage reverse thrust.

Four jobs in one handle: A/THR state switch, takeoff/go-around igniter (article 07, article 14), manual thrust control, reverser handle. Engine-out, the geometry stretches. Per FCOM DSC-22_20-10-50:

The system extends the active range of the active engine from idle to maximum continuous thrust (MCT instead of CL thrust). The Flight Mode Annunciator requests maximum continuous thrust on the live engine, at a time that depends on when the engine-out occurs.

4. Arming — four prerequisites, several doors

The prerequisites (all required). Per FCOM DSC-22_30-50:

A/THR can only arm when all of the following are applicable: ‐ At least one FMGEC is available, and ‐ At least one FCU channel is available, and ‐ Two ADIRUs are available, and ‐ The A/THR is not manually disabled (Instinctive Disconnect pb has not been pressed for more than 15 s), and

— plus, from the same list: all the engines are operative in the same rating mode: EPR or N1, and a maximum of one engine is operative in N1 degraded mode. Note the second prerequisite is why total FCU failure costs the A/THR (article 02), and the fourth is the 15-second manual disablement — the "red line" developed in article 17.

The doors in. Per FCOM DSC-22_30-50:

The A/THR arms, if one of the following occurs: ‐ At takeoff, the flight crew sets at least one thrust lever: • To the TOGA detent, or • To the FLX-MCT detent, when a FLX TO temperature or a DRT TO level (Derated Takeoff ) is entered in the PERF TAKEOFF page. SRS TO engages

Takeoff thrust is the arming action (the same push that lights SRS TO). The other doors: TOGA at go-around (article 14); the FCU A/THR pushbutton with the levers outside the active range (above 100 ft RA or with all RAs failed, and not in LAND/FLARE); and a TCAS RA with the levers outside the range (article 15's disconnected→armed branch). (A Soft GA arming clause exists in print; the function is not fitted on the baseline airframe.)

While armed, the FMA speaks the MAN family in the boxed first lines — MAN TOGA, MAN FLX +XX, MAN DTO, MAN MCT, MAN THR — with three footnotes worth exam money:

(2) If the flight crew sets the thrust levers to FLX-MCT, MAN MCT will appear on FMA. (3) If the flight crew sets the thrust levers between CL and TOGA, but not at the CL, FLX-MCT, or TOGA detent, MAN THR will appear on FMA. (4) If the flight crew sets the thrust levers directly to FLX-MCT and not TOGA then FLX-MCT, the MAN THR will appear on the FMA.

No FLX temperature entered but the lever at FLX-MCT: MAN MCT. Lever parked between detents: MAN THR — the system speaks detents; between them is just manual thrust. And footnote 4's sequence-sensitivity (TOGA first, then back) belongs to the un-fitted Soft GA path.

5. Activation — entering the active range

Per FCOM DSC-22_30-50:

The thrust lever positions determine the maximum thrust that the A/THR can command (except in A.FLOOR). The normal position of the thrust levers is with A/THR active is: ‐ The CL detent (AEO) ‐ The FLX-MCT detent for the operative engine (OEI).

The letter of authority again — with its single exception named (A.FLOOR ignores authorisations when it is saving you; article 17). The active range, precisely:

AEO Both thrust levers are above idle, and at or below the CL detent. The A/THR also activates when: ‐ One thrust lever is above the idle detent, and at or below the CL detent, and ‐ The other thrust lever is • At the idle detent, or • Above the CL detent, and at or below the FLX-MCT detent.

Both levers above idle and at/below CL — with two tolerance branches: one lever normal and the other at IDLE, or the other between CL and FLX-MCT (asymmetry tolerated). Engine-out, the range for the live engine runs idle to FLX-MCT (section 3's extension made concrete). And the two-step that puzzles every FLX-takeoff student:

If the engine-out occurs during a FLX TO or a DRT TO (Derated Takeoff ), the flight crew activates the A/THR by: 1. Setting the thrust lever of the operative engine below FLX-MCT, 2. Then, settings the thrust lever of the operative engines back to FLX-MCT.

Why "down, then back up"? Because activation is a boundary crossing: armed means levers outside the range, and bringing them into the range activates. At a FLX takeoff the lever is already sitting on the boundary (FLX-MCT) — so it must leave and re-enter. Understand the crossing rule and the odd manoeuvre becomes a corollary, not a memory item.

Other activation doors: the FCU A/THR pushbutton with the levers already in the range (above 100 ft RA or all RAs failed, not in LAND/FLARE); a TCAS RA with levers in range → straight to active (article 15); and the unconditional one — A.FLOOR engages regardless of the initial A/THR status, and the position of the thrust levers. Activation announces itself: FCU button lit, thrust targets on the E/WD, white A/THR on FMA line 3 of column 5, and the active mode in column 1 (THR CLB, SPEED… — article 17's table).

6. The revolving door — active back to armed

Per FCOM DSC-22_30-50:

AEO All the thrust levers are above the CL detent. Or at least one thrust lever is above the FLX-MCT detent. OEI The thrust lever of the operative engine is above the FLX-MCT detent. When the A/THR becomes armed, the thrust immediately increases in accordance with the thrust lever position (above CL detent (AEO) or above FLX-MCT detent (OEI)). The FMA displays MAN THR on Line 1 of the 'A/THR Modes and Messages' column.

Push the levers out of the range and the A/THR steps back to armed — and the thrust immediately jumps to the lever position. Pushing past CL is not "granting more authority"; it is taking over: past the boundary the lever stops being a ceiling and becomes the command. (This is the mechanism behind article 17's disconnect technique — match the levers to the current thrust before pressing the instinctive disconnect.) Bring the levers back into the range and it re-activates — a true revolving door.

7. Operating the lifeline

The standard takeoff, as an A/THR biography (FCOM's own scenario): levers IDLE → push to TOGA: A/THR arms, thrust rises to TOGA, FMA reads MAN TOGA, SRS TO engages → at thrust-reduction altitude LVR CLB flashes (article 17's flashing family) → levers to CL: A/THR activates, THR CLB. The point of armed: through the takeoff the thrust is yours (lever position, TOGA/FLX undiluted by automation); the handover happens at CL.

Engine-out pocket card: live engine to MCT (the FMA prompts, timing situation-dependent), failed engine to IDLE; engine failure during a FLX/DRT takeoff → down-and-back-up on the live lever. Interfaces: A/THR lost entirely still leaves CAT2 (its equipment-table column reads 0 — article 12) but bars CAT III; dual-FCU-channel loss takes the A/THR with it (prerequisite two).

[!warning]- Three misconceptions this article corrects (1) The CL detent is not a climb-thrust switch — it is a letter of authority: the A/THR commands anything from idle to CLB beneath a motionless lever; monitor thrust on the E/WD, never the levers. (2) Armed and disconnected produce the same thrust (lever position) — armed is just the standby badge; the FMGEC's target reaches the engines only when active. (3) Pushing the levers past CL while active does not give the A/THR more headroom — it hands control back to you, and thrust jumps to the lever position at once (MAN THR).

Self-test

[!note]- Q1. The master-FMGEC rule in three sentences?

Both FMGECs compute A/THR commands independently; only the master controls the active A/THR. The master follows the engaged AP. With no AP/FD engaged, or both APs engaged, FMGEC 1 has priority.

[!note]- Q2. Whom do the FADECs obey when the A/THR is armed? And when active?

Armed: the thrust lever positions (plus the PERF TAKEOFF page selection for FLX/DRT). Active: the A/THR thrust target from the master FMGEC. (Disconnected: levers again — armed and disconnected differ only in the standby status.)

[!note]- Q3. Airbus's three official reasons for non-back-driven levers — and where do you monitor thrust with A/THR active?

Energy is monitored via energy cues (speed, trend, engine parameters), not ambiguous lever motion; with autothrust engaged the lever position is the maximum authorised thrust; with manual thrust the lever position is the current thrust. Monitor on the E/WD needles and targets.

[!note]- Q4. The four arming prerequisites — and how does the 15-second rule appear among them?

At least one FMGEC; at least one FCU channel; two ADIRUs; and the A/THR not manually disabled — i.e. the instinctive disconnect button not held for more than 15 s (plus: engines in a common rating mode, at most one in degraded N1).

[!note]- Q5. The two-engine active range — and its two tolerance branches?

Both levers above idle and at or below CL. Tolerated: one lever in range while the other is at IDLE, or while the other is between CL and FLX-MCT.

[!note]- Q6. After an engine failure on a FLX takeoff, the two-step to activate the A/THR — and why?

Live-engine lever below FLX-MCT, then back to FLX-MCT. Activation is a boundary crossing into the active range; a lever already parked on the boundary must leave and re-enter.

[!note]- Q7. Lever parked between CL and TOGA, on no detent — what does the FMA show?

MAN THR — the system recognises detents; between them is plain manual thrust.

[!note]- Q8. Active, and you push both levers past CL — what does the thrust do, and what does the FMA show?

Thrust immediately increases to match the lever position; the A/THR drops to armed and the FMA shows MAN THR. Past the boundary, the lever is the command.

[!note]- Q9. Which two events activate the A/THR without the usual lever/button conditions?

ALPHA FLOOR — regardless of A/THR status and lever position; and a TCAS RA with the levers already in the active range (article 15).

Key takeaways

References

Master-FMGEC rule, three-state data flow, mode families and their linkage per FCOM DSC-22_30-50 (general); lever detents and functions, arming prerequisites and doors, MAN-family annunciations and footnotes, active-range definitions, the FLX/DRT engine-out two-step, non-lever activations and the active↔armed transitions per FCOM DSC-22_30-50 (arming/activation sections); the takeoff scenario per its operational-scenarios section. Non-back-driven lever rationale per FCTM AOP-10-30-10; engine-out range extension per FCOM DSC-22_20-10-50. Soft GA clauses are quoted as printed; the function is not installed on the baseline airframe of this series (article 14). The letter-of-authority framing is an integrative synthesis.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.