Why does autothrottle overshoot or miss selected speed?

Autothrottle overshoots or misses selected speed because it commands thrust, not airspeed directly. Pitch changes, aircraft inertia, wind, drag configuration, thrust limits and the active flight-guidance mode can all outrun its response. In simulators, incorrect throttle detents, noisy axes or duplicate control bindings add another common cause.

In Aviation & Real-World Flying, the selected speed is a control target rather than a guarantee. The system must wait for an engine response and then for that thrust change to accelerate or decelerate the aircraft. Our explanation of how airliner autothrottle converts speed error into thrust covers that basic control loop.

Why does autothrottle not hold speed exactly?

Autothrottle deliberately responds with some damping and delay so that it does not chase every one-knot fluctuation. Jet-engine spool time, aircraft momentum and filtered air-data signals mean thrust can begin changing before the speed reverses, then remain effective after the target is crossed.

It also has limited authority. At idle it cannot produce negative thrust, while at maximum or climb-limited thrust it cannot command more power. A steep descent, strong tailwind change, excessive climb demand, configuration change or poor energy state can therefore move the speed away from the target despite correct system operation.

Which flight mode is actually controlling the speed?

The flight mode annunciator, or FMA, is the decisive indication; an illuminated autothrottle switch may mean only that the system is armed. A common mistake we see is assuming that autothrottle must be controlling speed whenever it is engaged.

On Boeing types, a mode such as MCP SPD generally means the autothrottle is varying thrust for speed, while modes such as N1, THR REF, THR HLD or IDLE represent different thrust logic. Exact labels vary by aircraft, so use the type-specific documentation and our summary of 737 thrust modes and FMA indications.

Airbus calls the system autothrust. SPEED or MACH indicates active speed control, whereas modes such as THR CLB or THR IDLE command a thrust condition while pitch or the managed flight path handles the energy state. The thrust levers normally remain in a detent, making correct A320 lever calibration and detent use essential in a simulator.

What causes autothrottle speed overshoot in a simulator?

Simulator overshoot is often caused by an input or mode conflict rather than faulty autothrottle logic. Check these common failures:

• Duplicate throttle assignments: two controllers, or both a combined axis and individual engine axes, send competing inputs.
• Axis jitter: a worn potentiometer continually reports small movements, which some aircraft interpret as manual thrust intervention.
• Incorrect detents: an Airbus-style model may not recognise CL, FLX/MCT or idle correctly until its throttle calibration is completed.
• Selected versus managed speed: the displayed target may come from the flight-management system rather than the value the pilot expected to select.
• Wrong vertical mode: vertical speed, flight-level change, VNAV or managed descent can divide control of pitch, path, thrust and speed differently.
• Assistance conflicts: automatic piloting or assisted throttle features can compete with the aircraft's own systems.
• Accelerated simulation: some complex aircraft become less stable when time acceleration gives their control loops too few useful update cycles.

How do I troubleshoot an autothrottle that misses speed?

1. Control the aircraft first. If speed is becoming unsafe, use the approved pitch, thrust and automation-disconnection procedure for that aircraft. In real-world flying, the aircraft manual and operator SOP take precedence.
2. Confirm the actual target. Check the PFD speed marker, IAS or Mach selection, managed versus selected state and any active speed constraint.
3. Read every FMA change. Establish whether thrust or pitch is controlling speed and whether autothrottle is active, armed, limited or disconnected.
4. Check available authority. If thrust is already at idle or its upper limit, change the path, pitch demand or drag configuration rather than waiting for more thrust response.
5. Stabilise the test. Return to normal simulation rate and use steady weather while diagnosing the system. Abrupt gusts can create legitimate indicated-airspeed changes.
6. Isolate the hardware. Remove duplicate bindings, add only enough dead zone to stop jitter and temporarily disconnect secondary controllers.
7. Calibrate aircraft-specific detents. Do this inside the aircraft's own configuration system when provided; a simulator-wide calibration alone may not define the required detent positions.

How much speed variation is normal?

A brief deviation of a few knots can be normal during altitude capture, configuration changes, turbulence or a large thrust transition. The expected tolerance depends on the aircraft, phase of flight and approved operating procedure; there is no universal number that makes every approach acceptable.

Persistent divergence, repeated hunting, an engine remaining at an inappropriate limit or movement towards stall or overspeed is not normal. On approach, use the aircraft's stabilised-approach criteria rather than assuming autothrottle will recover before touchdown.

What if the aircraft is fast with autothrottle at idle?

Autothrottle cannot remove more energy once it has commanded idle thrust. The solution is usually an earlier deceleration, a shallower descent, a revised vertical mode or approved additional drag. If speed brakes are appropriate for the aircraft and phase of flight, follow the guidance on using speed brakes without destabilising the flight path.

If the aircraft is slow, remember that large jet engines need time to spool up. Reduce an excessive pitch demand and follow the type-specific recovery or go-around procedure instead of waiting for autothrottle alone. In a simulator, also anticipate a possible thrust jump when disconnecting automation if the physical throttle is far from the commanded position.