See how autothrottle works in an airliner, how it controls speed and thrust, why Airbus and Boeing differ, and why selected speed may drift.
An airliner’s autothrottle automatically commands engine thrust to maintain a selected speed, Mach number or thrust setting. Using air-data, engine and flight-control inputs, its computers compare the target with actual performance and adjust thrust. It controls thrust only; the autopilot controls flight path, and either system can operate without the other.
In Aviation & Real-World Flying, autothrottle is a generic term covering several designs. Airbus usually calls its system autothrust, while Boeing commonly uses autothrottle. The exact modes and protections vary, so the flight-mode annunciator, or FMA, is the authoritative indication of what the system is actually doing.
What does an autothrottle actually control?
An autothrottle controls the engines’ thrust command, either by moving the thrust levers or by sending an electronic demand to the engine-control computers.
- A target is established. This may be a selected airspeed, a Mach number, an FMS-managed speed or a calculated thrust limit for take-off or climb.
- The aircraft measures its condition. Inputs can include indicated airspeed, Mach, altitude, acceleration, engine parameters, configuration and flight-guidance mode.
- The control law calculates a correction. If the aircraft is below a speed target, it normally requests more thrust; if it is fast, it reduces thrust.
- The engines receive the command. A lever servo or electronic engine control changes fuel flow while respecting engine limits.
- The system monitors the result. It continually updates the command as speed, drag, wind and flight path change.
Large turbofans take time to spool up or down, so the response is not instantaneous. Autothrottle logic accounts for trends and avoids chasing every tiny fluctuation, but turbulence and abrupt configuration changes can still produce temporary speed movement.
How does autothrottle work with the autopilot?
Autothrottle and autopilot are separate systems whose jobs change according to the active vertical mode. Our explanation of how autopilot channels and automated thrust divide the work covers that distinction in simulator terms.
| Flight condition | Autothrottle’s job | What mainly controls speed |
|---|---|---|
| Level flight with speed mode active | Varies thrust | Autothrottle |
| Climb at a fixed climb-thrust setting | Maintains the permitted climb thrust | Aircraft pitch |
| Idle descent | Commands idle or near-idle thrust | Pitch, flight path and drag |
| Approach with speed mode active | Varies thrust as configuration and wind change | Autothrottle, within available thrust limits |
This division causes frequent confusion. During a climb, the autothrottle may be correctly holding climb thrust while the autopilot changes pitch to maintain speed. In a steep descent, the system may already be at idle; it cannot reduce thrust any further, so pitch, path changes or speedbrakes must control acceleration.
How do Airbus autothrust and Boeing autothrottle differ?
Using the A320 and 737 as familiar examples, Airbus autothrust normally leaves the thrust levers in a detent, whereas Boeing autothrottle normally moves the cockpit thrust levers with a servo.
| Feature | Airbus A320-style autothrust | Boeing 737-style autothrottle |
|---|---|---|
| Lever behaviour | Levers normally remain in a selected detent | Virtual or cockpit levers normally move as thrust changes |
| Engine command | Electronic system varies thrust within the authority allowed by the lever position | Servo moves the levers to command the required thrust |
| After take-off | Pilots normally move the levers to the climb detent at the appropriate point | The system normally reduces from take-off to climb thrust through its active mode |
| Flare | A callout prompts the pilots to move the levers to idle | The system commonly drives the levers towards idle, depending on type and mode |
The Airbus lever position establishes a thrust range or limit rather than showing the precise thrust being produced at every moment. Our guide to A320 thrust-lever detents and FADEC commands explains that operating logic, while the comparison of A320 and 737 automation behaviour gives the wider cockpit context.
In a home simulator, a non-motorised physical throttle will not move by itself even when the simulated Boeing levers do. Conversely, stationary virtual levers in an A320’s climb detent are normal; the engine indications should change as autothrust adjusts power.
What does autothrottle do during each phase of flight?
The autothrottle’s purpose changes with the phase of flight and the selected guidance mode.
- Take-off: it commands or maintains a calculated take-off thrust setting. It is not trying to hold rotation speed while the aircraft runs along the runway.
- Climb: it often holds a climb-thrust limit while pitch controls airspeed or Mach.
- Cruise: it normally varies thrust to maintain selected or FMS-managed speed or Mach.
- Descent: it may hold speed with variable thrust or command idle while the vertical guidance controls the descent.
- Approach: it adjusts thrust as flaps, landing gear, wind and glide path alter the required power. The crew must still verify that the target speed is appropriate.
- Landing: retard logic is aircraft-specific. Some systems close the thrust levers automatically; others require the pilots to select idle when prompted.
Why is the autothrottle not holding the selected speed?
An autothrottle that misses the selected speed is often obeying a different mode, sitting at a thrust limit or receiving an unintended control input rather than suffering a system failure.
- Read the FMA. Check whether the system is active, merely armed or disconnected, and identify whether it is in speed, thrust or idle mode.
- Identify which system controls speed. In climb-thrust or idle-descent modes, pitch rather than autothrottle may be responsible for maintaining airspeed.
- Check the target source. Confirm selected versus managed speed and IAS versus Mach. The displayed target may have changed during a mode transition.
- Check lever and arming states. An A320 normally needs the levers in the correct detent for normal autothrust authority. Boeing logic depends on the autothrottle arm state and active mode.
- Look for an authority limit. At maximum permitted thrust the aircraft may still lose speed in a steep climb; at idle it may accelerate in an overly steep descent.
- Inspect simulator controls. Duplicate throttle bindings, axis noise, an incorrectly calibrated Airbus detent or assistance features can repeatedly override the automation. These practical A320 autothrust engagement checks for MSFS cover the usual simulator-specific causes.
In an actual aircraft, crews follow the type-specific checklist or quick-reference procedure rather than attempting an improvised reset. Mode names, re-engagement restrictions and failure indications are not universal.
Can pilots disconnect or override autothrottle?
Pilots can disconnect autothrottle and control thrust manually while leaving the autopilot engaged. The transfer must be deliberate: they confirm the mode change, match or position the levers as required by the aircraft type, and monitor speed and engine indications.
Autothrottle is also not universal stall protection. Some aircraft provide separate low-speed, overspeed or angle-of-attack protections, but their availability depends on aircraft design, active mode and system health. Pilots remain responsible for recognising an inappropriate thrust command and intervening when necessary.