How do autopilot airspeed control and autothrottle work?

In flight simulators, autopilot airspeed control uses either pitch or engine thrust. The autopilot can raise or lower the nose to hold a target speed, while autothrottle or autothrust adjusts engine power. Which system controls speed depends on the aircraft, vertical mode, flight phase and active automation mode.

This principle applies across general flight simulators, although individual aircraft model the systems with different levels of fidelity. Our explanation of the feedback loops behind aircraft autopilot modes covers the wider relationship between sensors, guidance commands and flight controls.

Which system controls airspeed in each autopilot mode?

The active vertical mode determines whether pitch is controlling altitude, vertical speed, flight path or airspeed. An aircraft cannot independently hold both altitude and speed with elevator alone; maintaining both requires thrust control as the second control channel.

An illuminated autothrottle system is therefore not necessarily holding airspeed. It may be maintaining a fixed thrust rating such as climb thrust, idle or go-around thrust while the autopilot uses pitch to manage speed.

Can autopilot hold speed without autothrottle?

Yes, but only by allowing the aircraft's flight path to change. In flight level change, IAS hold or a similar mode, the autopilot pitches up when the aircraft is too fast and pitches down when it is too slow; the pilot must set suitable engine power.

In altitude hold, pitch is already committed to maintaining altitude. An aircraft without autothrottle cannot also correct speed independently, so the pilot must move the throttles. This is the normal arrangement in many general-aviation aircraft fitted with an autopilot but no automatic thrust system.

What do selected and managed speed mean?

Selected speed comes directly from the value chosen on the mode-control panel. Managed speed comes from the flight-management system, using the route, performance data, altitude constraints and flight phase.

These terms identify the source of the target, not necessarily how the aircraft will achieve it. On an Airbus, for example, managed or selected speed can be paired with autothrust or with a pitch-controlled climb or descent mode. The simulator's push-and-pull behaviour is explained in our guide to selected and managed A320 autopilot modes in MSFS.

How should I engage automatic speed control?

Use the mode annunciations, not the number displayed in the speed window, to confirm what the aircraft is actually doing.

1. Set a sensible target. Select IAS or Mach as appropriate, or confirm that the flight-management system contains the managed speed you expect.
2. Make thrust available. Arm or engage autothrottle where required. On Airbus-style systems, place the thrust levers in the correct detent so autothrust has authority; our A320 thrust-lever detent explanation covers that arrangement.
3. Select a compatible vertical mode. Use altitude hold or a path mode when autothrottle should regulate speed. Use flight level change or IAS mode when pitch should regulate speed.
4. Read the Flight Mode Annunciator. Check the active pitch, thrust and speed modes. An armed mode is waiting for its capture condition and is not yet controlling the aircraft.
5. Monitor the speed trend and thrust limits. Automation cannot overcome insufficient thrust, excessive drag or an excessively steep descent path.

Why is autothrottle not holding the selected speed?

The most common mistake we see is treating a speed shown on the control panel as proof that speed mode is active. The Flight Mode Annunciator is the authoritative indication.

• The mode is only armed. The aircraft is waiting to capture an altitude, path or other condition before speed control changes hands.
• Autothrottle is controlling thrust rather than speed. Climb thrust, idle, retard and take-off/go-around modes command power settings, not an exact airspeed.
• The thrust levers are in the wrong position. Airbus autothrust may require a particular detent, while other aircraft use moving servo-driven levers.
• A hardware throttle axis is overriding the system. Axis noise, duplicated bindings or moving the physical throttle can cause thrust to jump or disconnect automation.
• The selected vertical rate is unrealistic. A steep climb can demand maximum thrust and still lose speed; a steep descent can reach idle thrust and continue accelerating.
• The aircraft is following managed constraints. VNAV or managed speed may command a different target from the one expected, subject to the aircraft's mode logic.
• IAS and Mach modes have changed. At higher altitude, many systems transition between indicated airspeed and Mach. Verify both the unit and target.
• The simulated aircraft has limited system depth. Basic aircraft may approximate autothrottle behaviour, while high-fidelity add-ons model thrust limits, mode reversion and protections more closely.

If speed diverges, first read the annunciations, then reduce any excessive climb or descent demand and correct the thrust-lever position. Disconnect the automation and fly a stable pitch-and-power setting if the mode remains unclear. For the Airbus-specific causes, use our A320 autopilot and autothrust troubleshooting checks.

Do autothrottle and autothrust mean the same thing?

They perform the same broad job but can use different cockpit logic. Traditional autothrottle systems often drive the physical thrust levers through servos. Airbus autothrust normally leaves the levers in fixed detents while the engine-control computers vary thrust within the permitted range.

That distinction matters when using a simulator throttle quadrant. A lever that does not move on screen may be normal for one aircraft but evidence of an unengaged system in another. In every case, the active thrust mode and engine indications tell you more than lever movement alone.