Fix autothrottle not holding speed in a flight simulator by checking active modes, throttle bindings, lever detents, speed targets and aircraft limits.
In a flight simulator, autothrottle usually misses the selected speed because it is armed but not in an active speed mode, the flight-management system is commanding another target, a hardware throttle is overriding it, or the aircraft lacks enough thrust or drag for the chosen vertical path. Check the mode annunciator first.
Across MSFS, X-Plane, Prepar3D and FSX, the exact labels and logic depend on the aircraft. Autothrottle, autothrust and automatic throttle control are related terms, but they do not always operate in the same way.
What does the selected speed actually control?
The selected speed controls the aircraft only when the relevant automation mode is active and accepting that target. Turning the speed knob does not by itself command the engines.
Read the flight-mode annunciator rather than relying on an illuminated autothrottle switch. An armed indication means the system is ready but may not yet be controlling thrust. Fixed-thrust modes such as climb thrust or idle can leave pitch responsible for speed, while managed or VNAV modes may follow a flight-management target instead of the number selected on the panel.
The target is normally indicated airspeed or Mach, not groundspeed. Wind can therefore make the groundspeed differ substantially while the autothrottle is working correctly. Protection, go-around, take-off, flare, stall-recovery and engine-out modes can also override ordinary speed control.
How do I fix autothrottle not holding speed?
Work through the automation, target source, throttle hardware and aircraft energy state in that order.
- Confirm that the aircraft has automatic thrust. Many light-aircraft autopilots provide IAS or FLC modes that control speed with pitch, leaving power entirely manual. Military aircraft may use specialised logic; our explanation of F/A-18 speed-capture and controller-binding behaviour shows why a generic airliner procedure does not always apply.
- Read the active mode annunciation. Look for an active speed or Mach mode, not merely an armed A/T or A/THR indication. A thrust, hold or idle mode may deliberately use a fixed thrust command while another system manages speed.
- Verify the target source and units. Check whether the aircraft is using selected speed, an FMS-managed target, IAS or Mach. Some aircraft require a push, pull or separate mode selection before the panel value becomes active.
- Place the thrust levers correctly. Airbus-style autothrust normally expects the levers in an appropriate operating detent, while Boeing-style systems often move the simulated levers with servos. For that aircraft family, use our guide to A320 thrust-lever detents, calibration and mode logic.
- Eliminate controller interference. Remove duplicate throttle assignments, check separate engine and reverse axes, add a small dead zone to a noisy control and make sure no latching switch is repeatedly sending an autothrottle-disconnect command. If the throttle itself behaves incorrectly even with automation off, follow the full throttle-axis and binding checks.
- Make the requested speed achievable. Full thrust with falling speed indicates excessive climb demand, drag or insufficient performance. Idle thrust with rising speed means the descent is too steep or the aircraft needs more drag; autothrottle cannot command less than idle.
- Reset and isolate the fault. Disconnect autothrottle, stabilise manually in level flight at a sensible speed, then arm and engage the correct mode again. If the fault remains, test a default aircraft with secondary controllers disconnected and normal simulation rate. That separates an aircraft-specific configuration problem from a global control conflict.
| What you see | Likely cause | What to change |
|---|---|---|
| A/T is armed but thrust does not respond | No active speed mode, or engagement conditions are unmet | Select the proper mode and verify the annunciator |
| The panel speed differs from the commanded marker | Managed FMS speed or Mach mode is active | Switch to selected speed using the aircraft's procedure |
| Thrust jumps or autothrottle disconnects | Noisy axis, duplicate binding or disconnect switch | Clear conflicts, recalibrate and add a small dead zone |
| Maximum thrust with an increasing underspeed | The climb or configuration exceeds available performance | Reduce climb demand and verify configuration |
| Idle thrust with an increasing overspeed | The descent path is too steep for idle thrust alone | Shallow the descent or add approved drag |
| Speed, pitch and altitude repeatedly seesaw | Unsuitable vertical mode, poor trim or conflicting input | Stabilise the aircraft and correct the vertical-mode setup |
Why does autothrottle lose speed during a climb?
Autothrottle cannot hold speed when the selected climb rate demands more energy than the engines can supply. In a fixed climb-thrust mode, the autopilot may need to lower the nose to recover speed; a rigid vertical-speed selection can instead drive the aircraft towards an underspeed.
Where supported, choose FLC, FLCH or an appropriate VNAV mode when speed protection should take priority over a precise climb rate. Choose vertical-speed mode only when the requested rate is comfortably attainable. If altitude and speed oscillate together, check the related pitch, trim and vertical-mode causes of altitude oscillation.
Why does autothrottle allow an overspeed in descent?
An aircraft can accelerate with the engines already at idle because gravity is supplying the energy. Autothrottle cannot produce negative thrust, so it cannot correct an overly steep descent by itself.
Reduce the descent rate, level temporarily or use speedbrakes and configuration changes in accordance with the aircraft's operating limits. A tailwind, late descent or restrictive vertical path can leave even correctly functioning VNAV and autothrottle unable to recover the selected speed without pilot intervention.
Throttle-lever behaviour differs by aircraft
A moving virtual throttle is not required for every automatic-thrust system. Boeing-style autothrottles commonly move the cockpit levers, whereas Airbus autothrust varies commanded thrust while the levers remain in their detents. Most desktop throttle quadrants are not motorised, so their physical levers will not follow the simulated ones.
Small, temporary speed changes are normal during turns, turbulence, configuration changes and engine spool-up. Persistent divergence, repeated disconnects, continuous thrust hunting or operation stuck at maximum or idle indicates a mode, binding or energy-management problem rather than ordinary variation.