A320 Autothrust and Thrust Levers

The A320's thrust levers do two jobs. Set by hand, they select a thrust rating limit that the FADEC applies, from idle to take-off/go-around thrust; within the autothrust range, their position becomes the ceiling of what the autothrust (A/THR) may command. Unlike a Boeing autothrottle, the A/THR never moves the levers. The autothrust commands come from the flight guidance part of the flight management and guidance computers (FMGCs), and the FADECs turn them into fuel flow.

Most of the logic below is common to the A320 family. Thrust figures and EGT limits are those of the CFM56-5B. The engines themselves are described in A320 engines and FADEC and the general principles in autothrottle.

Non-back-driven thrust levers

There is no mechanical linkage between the thrust levers and the engines. Each lever's position is sensed electrically and sent to its FADEC as a thrust lever angle, which the FADEC treats as the crew's intent. Airbus chose non-back-driven thrust levers so that the crew judge the aircraft's energy from current cues, speed, speed trend and engine parameters, rather than from levers that move by themselves.

The levers belong to the pilot flying. If an ECAM, QRH or OEB procedure calls for a lever action, the pilot flying points to the lever, the pilot monitoring confirms, and the pilot flying moves it. During the take-off the captain keeps a hand on the levers until V1. Reverse, behind the idle stop, is reached by lifting the reverser latching levers on the front of the thrust levers.

Detents and thrust rating limits

Each thrust lever has four forward detents, TOGA, FLX/MCT, CL and IDLE, and carries an instinctive disconnect pushbutton. For each position the FADEC computes an N1 rating limit (an EPR limit on the V2500), shown in green on the engine/warning display, with the thrust limit mode and any FLEX temperature in blue. A lever set between two detents gets the limit of the higher detent. The limits available depend on the situation:

MCT is therefore not available on the ground with the engines running, and there the displayed limit is TOGA whatever the lever position, unless a FLEX temperature is entered, when it is the FLEX N1 up to the FLX/MCT detent. The CL detent gives climb thrust (CLB), the maximum for the climb. TOGA may be used for 5 minutes with all engines operating and 10 minutes with one inoperative, with an EGT limit of 950 °C; MCT is not time-limited, at 915 °C. MREV is maximum reverse.

Only FLX/MCT and TOGA are approved for take-off. The pilot flying sets about 50 % N1 (1.05 EPR) and then the chosen detent, and the pilot monitoring checks that each engine has reached its rating limit before 80 kt and calls "thrust set".

Some engines have a thrust bump function. On the A320-214, for example, modification 38946 introduces it and the CFM56-5B4 becomes the CFM56-5B4/P1, and the type certificate data sheet does not allow bump and non-bump engines to be mixed on one aircraft. The A320's published VMCA of 110 kt and VMCG of 111 kt in CONF 1+F at sea level are the values without thrust bump.

Flexible take-off thrust

Turbofans are flat rated: they give their full rated thrust up to a flat-rating temperature, TREF, and less above it. A flexible take-off exploits this. The crew enter an assumed temperature, the flexible temperature (TFLEX), on the MCDU PERF TAKE OFF page, and the FADEC gives the thrust the engine would produce at that temperature. TFLEX may not be:

• higher than the maximum flexible temperature (TMAXFLEX), ISA + 53 °C;
• lower than TREF;
• lower than the actual outside air temperature.

FLEX is not permitted on a contaminated runway, and it may not be combined with a derated take-off. A derate is a lower certified rating; during a derated take-off TOGA may not be selected unless an abnormal or emergency procedure calls for it, and a derate may be used on dry, wet or contaminated runways (see reduced and derated take-off thrust).

FLEX performance assumes an engine failure at V1 with FLX thrust on the remaining engine, so no lever action is needed; TOGA remains available. The rating changes from FLX to MCT only when the lever is moved to TOGA or CL and back to FLX/MCT, normally at green dot speed when LVR MCT flashes. Moving the lever out of the detent without reaching TOGA or CL changes nothing, and once MCT has been set, FLX is no longer available for the rest of the flight.

Autothrust arming and activation

The A/THR is always in one of three states: armed, ready to take over; active, controlling thrust; or disconnected. Arming needs at least one FMGC, one flight augmentation computer (FAC), two ADIRUs, two FADECs, one flight control unit channel and one landing gear control interface unit, and the A/THR must not have been disabled by the crew. With no autopilot or flight director engaged, FMGC 1 controls the A/THR.

With both engines running the A/THR is active with the levers between just above idle and the CL detent; with one engine, between idle and MCT. Within that range the lever position sets the highest thrust the A/THR may command. Beyond it, in FLX/MCT or TOGA, the A/THR is armed but not active, and thrust is the manual rating of the detent, shown on the FMA as MAN TOGA, MAN FLX or MAN MCT.

A take-off therefore starts with the A/THR armed. At the thrust reduction altitude, 1,500 ft by default unless the operator sets another value, LVR CLB flashes on the FMA, and when the pilot flying brings the levers back to CL the A/THR becomes active. A go-around repeats the pattern: TOGA, then CL at the go-around thrust reduction altitude.

View full schematic

SPEED/MACH and THRUST modes

The A/THR mode depends on what the autopilot or flight director does with the pitch:

In managed descent the A/THR may be in THR IDLE on the idle path or in SPEED on a geometric segment. RETARD is available only in an automatic landing; the RETARD callout comes at 10 ft radio altitude in autoland and at 20 ft in a manual landing, and the A/THR disconnects when both levers reach idle.

In SPEED or MACH the A/THR never targets more than VMAX in a high-lift configuration, or VMAX − 5 kt clean. Below 3,200 ft radio altitude in CONF 1 or more it is more responsive above 150 ft and less responsive below, to avoid a long flare. The A/THR also protects the speed when the flight director is used without the autopilot: in a THR IDLE descent the flight director bars disappear at VLS − 2 kt, and in a THR CLB climb at VMAX + 4 kt, and the A/THR reverts to SPEED. When an A/THR thrust change is in progress, the N1 command arc on the N1 gauge shows the target N1 the needle is moving towards.

Disconnection and thrust lock

The standard disconnection uses the A/THR instinctive disconnect pushbutton on either lever. Held for more than 15 seconds, it disables the A/THR for the rest of the flight, alpha floor included, until the FMGCs are next powered up on the ground. Setting both levers to IDLE also disconnects it, as in the flare of a manual landing; a pilot who lands with manual thrust disconnects the A/THR by 1,000 ft above aerodrome level at the latest. The unreliable speed memory items set the A/THR OFF, after the autopilot, and in an emergency descent without A/THR the levers go to IDLE.

After a non-standard or involuntary disconnection, the thrust lock function freezes the thrust at its value before the disconnection. THR LK flashes on the FMA, and if the levers are not moved within 5 seconds ENG THRUST LOCKED appears on the ECAM, with a single chime and MASTER CAUT repeated every 5 seconds. Moving the levers ends the lock and gives manual thrust from the lever position.

Alpha floor and TOGA LK

Alpha floor is an autothrust function of the FACs' flight envelope protection. When the angle of attack becomes very high, between alpha prot and alpha max, it commands TOGA thrust whatever the lever position and the A/THR state, from lift-off down to 100 ft radio altitude on approach. The FMA shows A.FLOOR, and speedbrake extension is inhibited. When the condition has cleared and the speed is above VLS, the FMA shows TOGA LK in a flashing amber box and thrust stays at TOGA. The crew cancel it with the instinctive disconnect pushbutton once a safe speed is regained, then set the levers for the thrust they want.

Alpha floor is lost when the A/THR has been disabled for the flight, and in alternate law (see A320 flight control laws and protections). Extending the speedbrakes raises VLS, so crews keep a speed margin before using them, which also avoids triggering alpha floor.

Low energy warning

The low energy warning is the synthetic voice "SPEED, SPEED, SPEED", computed by the FACs. It sounds when the thrust is no longer enough to recover a positive flight path through pitch control alone, and repeats every 5 seconds until the thrust levers are advanced. It works in CONF 2, 3 and FULL and is inhibited above 2,000 ft and below 100 ft radio altitude, with TOGA selected, during alpha floor or a GPWS alert, in alternate or direct law, and with both radio altimeters failed.

Thrust lever messages

The levers matter elsewhere too. The ground spoilers extend only with both levers at or below idle, or reverse selected; speedbrake extension is inhibited with a lever above MCT; and on the ground the autopilot disengages if all levers are set above the MCT detent.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.