Autothrottle

An autothrottle (A/T) controls engine thrust so that the pilots do not have to move the thrust levers by hand. Depending on the mode engaged, it either varies thrust to hold a target airspeed or Mach number, or sets a fixed thrust, such as the climb limit, idle, or take-off and go-around thrust, while the autopilot or flight director holds the speed with pitch. On the Boeing 737 the automatic flight system consists of the autopilot flight director system (AFDS) and the autothrottle.

Airbus calls the function autothrust (A/THR) and builds it differently: the thrust levers stay where the pilot put them, and their position sets the most the system may command. The vocabulary differs, but the logic of speed modes and thrust modes is the same, and so is the lesson of several accidents: an autothrottle does exactly what its mode tells it to, which is not always what the crew believes.

How the system works

The autothrottle computer turns a target into engine commands. EASA training texts describe the target as fan speed (N1), engine pressure ratio (EPR) or an airspeed set on the mode control panel. The limits come from the flight management computer (FMC). On the 737 the FMC normally calculates the active N1 limit shown on the thrust mode display, and in N1 mode the autothrottle holds thrust at that limit. With the N1 SET knob in AUTO it will not set thrust above the displayed limit; if the FMC stops supplying limits, the display shows A/T LIM.

On Boeing aircraft servo motors move the thrust levers, so their position always shows the thrust commanded. With the A/T ARM switch at ARM, the 737 autothrottle engages by itself when LVL CHG, ALT ACQ, V/S, VNAV, ALT HOLD, glideslope capture or TO/GA engages. The ARM switch is held magnetically and drops to OFF when the autothrottle disengages.

On the A320 family the levers never move under autothrust. They have detents, from front to back TOGA, FLX/MCT, CL and IDLE, with reverse behind idle. With both engines running the A/THR is active from just above idle up to the CL detent; forward of CL the FADEC gives the fixed thrust of the detent and the autothrust commands nothing. Within its range the lever position is the ceiling of what it may command. The A/THR is always in one of three states: armed, active or disconnected.

Speed and thrust modes

The central idea is a division of labour. When the pitch channel holds the speed, the autothrottle sets a fixed thrust; when the pitch channel holds a path, the autothrottle holds the speed.

On the 737 the engaged autothrottle modes, shown in green, are N1, GA, RETARD, FMC SPD, MCP SPD and THR HLD; ARM is shown in white. Speed mode holds the speed in the IAS/MACH window, or a performance or limit speed, and engages automatically with ALT ACQ, ALT HOLD, V/S or glideslope capture. With VNAV engaged the autothrottle shows FMC SPD, N1, RETARD or ARM; in a VNAV path descent it stays at idle but can call up FMC SPD if the groundspeed is too low to hold the path. Airbus uses SPEED or MACH when the vertical mode controls the trajectory, and THRUST modes (CLB, MCT, IDLE or LVR) when the pitch mode holds the speed. THR IDLE goes with DES, OP DES, EXP DES and RETARD.

The pairing explains practical rules. On the Embraer E190-E2 the speed brakes are used in the FLCH mode rather than in V/S, because in V/S the autothrottle is holding the speed and would simply add thrust. A crew who selects a mode without reading the flight mode annunciator (FMA) can end up with neither axis holding the speed, which is why crews are trained to check the FMA after every selection and to keep airspeed in the scan.

TOGA thrust and the TO/GA switches

Take-off/go-around thrust (TOGA) is the maximum rated thrust for take-off and go-around. On the A320 it is set by pushing the levers fully forward into the TOGA detent, and the FADEC's engine limit protection prevents an overboost. Reduced take-off thrust, by assumed temperature (FLEX) or derate, is a separate choice (see A320 FLEX take-off).

Boeing uses TO/GA switches on the thrust levers. For take-off the 737 crew advances the levers to about 40% N1, lets the engines stabilise, then pushes a TO/GA switch. The autothrottle changes from ARM to N1 and drives the levers to take-off thrust, while the flight directors give take-off pitch commands (see flight director and autoflight modes). At 84 KIAS the mode becomes THR HLD: the autothrottle can no longer move the levers, though the pilots can. On the 737 NG it stays there until 800 ft above field elevation, then shows ARM, and climb thrust is selected by pushing N1. The 737-400 description in EASA training texts gives a different rule: THR HLD until both 400 ft radio altitude and about 18 seconds after lift-off. The E190-E2 has a similar HOLD mode, which disengages the lever servos from 60 kt to 400 ft, so take-off thrust must be set before 60 kt.

The A320 has no TO/GA switch. The pilot sets FLX/MCT or TOGA with the levers, the FMA shows a manual thrust mode (MAN TOGA with the levers in the TOGA detent), and the FADEC delivers the rating of the detent while the autothrust waits, armed but not active. At the thrust reduction altitude LVR CLB flashes until the levers are brought back to the CL detent, where the autothrust becomes active.

TOGA is also the thrust of the emergency manoeuvres. In a windshear escape the 737 pilot pushes TO/GA and checks that thrust advances to go-around power. On the A320, alpha floor commands TOGA whatever the lever position when the angle of attack passes a threshold between alpha prot and alpha max; it is available from lift-off down to 100 ft radio altitude on approach (see fly-by-wire). The 737 has no alpha floor.

View full schematic

Go-around mode

On the 737, with the A/T ARM switch at ARM, the autothrottle go-around mode is armed when the aircraft descends below 2,000 ft radio altitude, or below 15,500 ft pressure altitude if both radio altimeters have failed, whether or not the AFDS is engaged. A push on either TO/GA switch below 2,000 ft engages GA; at or above 2,000 ft the autothrottle engages in N1 and goes straight to the full go-around limit. The flight director commands 15° nose-up until a programmed rate of climb is reached. EASA texts note that the mode ends when another pitch mode is selected or altitude acquire engages. An automatic go-around by the autopilot itself needs both autopilots engaged and is armed once FLARE is armed (see autoland).

On the A320 the go-around starts with the levers: moved to the TOGA detent, they give TOGA thrust and, with the flap lever out of 0, engage SRS and GA TRK or NAV. The FMA reads MAN TOGA, SRS, GA TRK or NAV, and A/THR. At the go-around thrust reduction altitude the levers come back to CL and the autothrust becomes active again (see go-around and missed approach).

Reduced go-around thrust

Full go-around thrust must give an acceptable climb even with an engine failed. With all engines running at a light landing weight it gives far more climb than a missed approach needs, and the level-off altitude arrives very quickly. Reduced go-around thrust addresses this and also reduces engine wear.

On the 737 the first TO/GA push below 2,000 ft radio altitude advances the thrust towards a reduced go-around N1 that gives 1,000 to 2,000 ft/min rate of climb. A second push, once reduced thrust is reached, advances it to the full go-around N1 limit. With an engine failed, EASA texts note, the autothrottle goes to full thrust immediately.

Airbus has added a Soft Go Around thrust setting to some A320-family aircraft, the A321XLR among them. It is available only after the go-around has been started and needs a second, deliberate action by the crew. Because a soft all-engine go-around may climb more shallowly than a full-thrust go-around with one engine out, EASA's special condition B-12 requires the published approach climb performance to be the lower of the two, or a clear and unmistakable alert when soft thrust gives the lower gradient.

Protections, alerts and disconnection

The 737 autothrottle guards the low-speed end on approach. In a non-VNAV mode, with the gear down, flaps 15 or more and below 800 ft, it advances the levers to keep the speed 5 kt above the minimum speed computed by the stall management computer. This is a speed floor held with thrust, not an angle-of-attack protection. When a gear, flap or VMO/MMO placard is reached while the AFDS is not in a speed mode and the autothrottle is armed but not controlling speed, the autothrottle reverts to SPEED and holds just below the limit.

Disengagement on the 737 comes from the A/T ARM switch, a disengage switch on either lever, a detected fault, or automatically two seconds after touchdown. A disengagement flashes the red A/T disengage lights, except the automatic one at touchdown. A flashing amber light is an airspeed warning; in the 737-400 description of EASA training texts it comes on in flight, with the flaps not up and the autothrottle in a speed mode, when the speed is not within +10 or -5 kt of the target.

The A320 is disconnected with the instinctive disconnect pushbutton on the levers. Held for more than 15 seconds, it removes the A/THR, including alpha floor, for the rest of the flight. After a non-standard disconnection, thrust lock freezes the thrust at its last value: THR LK flashes on the FMA and ENG THRUST LOCKED repeats every 5 seconds until the levers are moved. After alpha floor, TOGA LK keeps TOGA thrust until the crew disconnects the A/THR.

Automation can also be the problem. The 737 recall items for engine fire, engine overheat and engine limit or surge begin by disengaging the autothrottle, so that the thrust lever stays where the pilot puts it. Unreliable airspeed procedures on both types begin by disconnecting autopilot and autothrottle, which would otherwise chase a false speed (see unreliable airspeed). EASA training texts say the autothrottle is not used in severe turbulence, and the FAA adds that some types recommend disconnecting it; the flight manual governs.

Mode awareness: two accidents

On 25 February 2009 Turkish Airlines Flight 1951, a Boeing 737-800, was approaching Amsterdam when the left radio altimeter began to read -8 ft. The autothrottle took its height from that altimeter, entered its RETARD flare mode and closed the thrust levers while the autopilot held the glideslope. The speed decayed unnoticed until the approach to the stall, and the aircraft crashed short of the runway (see radio altimeter).

At San Francisco in 2013, Asiana Airlines Flight 214, a Boeing 777-200ER, flew a visual approach. When the pilot flying moved the thrust levers to idle during the descent, the autothrottle entered HOLD mode and stopped controlling speed, which the crew did not notice. The NTSB cited the unintended deactivation of automatic speed control among its findings (see stabilised approach).

Both crews had an autothrottle doing what its logic dictated. The defences are the same on every type: call and check every FMA change, keep speed and thrust in the scan on a coupled approach, and when the automation surprises the crew, reduce the level of automation and fly the aircraft.

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.