How autopilot airspeed control, IAS and vertical-speed modes divide pitch and thrust, with autothrottle, autothrust and throttle-axis fixes.
In a general flight simulator, autopilot airspeed control works through either pitch or thrust. IAS/FLC modes usually pitch the aircraft to hold speed while thrust is set separately; altitude, vertical-speed and path modes use pitch for flight path, so autothrottle, autothrust or the pilot must control speed with engine power.
This principle applies across Microsoft Flight Simulator, X-Plane, Prepar3D, FSX and other general simulators. The exact mode names, protections and depth of simulation depend on the aircraft. The Flight Mode Annunciator, or equivalent autopilot status display, tells you which system is actually controlling pitch and thrust.
How does an autopilot work?
An autopilot is a closed-loop control system that compares the selected target with sensor data, commands a correction and continually measures the result.
If altitude is below the selected value, for example, the pitch channel commands the elevator or simulated flight-control system to raise the flight path. Roll modes use the same principle for heading, track or bank. Our explanation of autopilot feedback loops and vertical modes covers this wider control process.
Three related autopilot systems are often confused:
- Flight director: calculates and displays pitch and roll guidance for the pilot to follow.
- Autopilot: follows that guidance by controlling the aircraft’s flight controls.
- Autothrottle or autothrust: controls engine power. It is normally a separate system and may remain available when the autopilot is disconnected.
A speed selected on the control panel is only a target. It does not prove that either the autopilot or autothrottle has captured and is holding it.
Which system controls airspeed in each autopilot mode?
The active vertical mode decides whether pitch controls airspeed, altitude, vertical speed or a defined flight path.
| Active mode or phase | What pitch controls | How airspeed is controlled |
|---|---|---|
| Altitude hold | Selected altitude | Autothrottle adjusts thrust, or the pilot moves the throttle |
| Vertical speed | Selected climb or descent rate | Autothrottle or pilot-controlled thrust must manage speed |
| IAS, FLC or FLCH | Selected IAS or Mach | Pitch holds speed; thrust is set for climb, descent or level flight |
| VNAV speed mode | Managed or programmed speed | Pitch commonly controls speed while thrust follows a climb or idle command |
| VNAV path, glideslope or glidepath | Programmed or radio-defined path | Autothrottle normally controls speed, subject to thrust limits |
| Take-off or go-around | Attitude, speed guidance or flight-director command | The thrust system usually commands a rating rather than holding an exact speed |
An active autothrottle does not always mean speed hold. Modes such as climb thrust, idle, thrust reference and take-off/go-around command an engine setting; pitch may be responsible for airspeed at that moment.
What does IAS mode mean on an autopilot?
IAS mode uses pitch to maintain a selected indicated airspeed, so the aircraft’s climb or descent rate is allowed to change.
If the aircraft becomes too slow, the autopilot lowers the nose; if it becomes too fast, it raises the nose. The pilot or thrust system must provide a suitable power setting. IAS mode does not normally hold altitude at the same time.
Flight level change, labelled FLC, FLCH or a similar abbreviation, uses the same broad speed-on-pitch principle. Airliner systems may automatically select climb or idle thrust, while many general-aviation autopilots leave power entirely to the pilot. At higher altitude, the aircraft may change from IAS to Mach control.
How does vertical-speed autopilot mode affect airspeed?
Vertical-speed mode holds the selected climb or descent rate with pitch, leaving airspeed dependent on thrust and drag.
A common mistake we see is selecting an aggressive climb rate and expecting the autopilot to protect the selected speed. If maximum thrust cannot support that rate, speed decays. During descent, idle thrust may still be insufficient to prevent acceleration if the selected descent rate is too low or the aircraft has excess energy.
- Choose IAS/FLC when speed has priority. Accept that the climb or descent rate will vary.
- Choose vertical speed when rate has priority. Monitor airspeed and adjust power or reduce the demanded rate.
- Choose altitude or path modes when position has priority. Autothrottle or the pilot must then manage speed.
Does selected or managed speed change who controls it?
Selected and managed speed identify where the target came from, not whether pitch or thrust will achieve it.
Selected speed is entered directly on the mode-control panel. Managed speed is calculated by the flight-management system from the route, flight phase, constraints and performance data. In either case, the active vertical and thrust modes determine how the aircraft follows that target.
Can autopilot hold speed without autothrottle?
Autopilot can hold speed without autothrottle only by using pitch and allowing the flight path or vertical rate to change.
In IAS or FLC mode, the autopilot pitches for speed while the pilot sets engine power. This is normal in many general-aviation aircraft equipped with an autopilot but no automatic thrust system.
Once altitude hold is active, pitch is committed to maintaining altitude. The aircraft cannot independently hold both altitude and airspeed with elevator alone, so the pilot must move the throttle to correct speed. The same limitation applies in vertical-speed and path modes when autothrottle is unavailable.
Do autothrottle and autothrust mean the same thing?
Autothrottle and autothrust perform the same broad task—automatic engine-power control—but their lever logic can differ.
Auto throttle is normally written as one word: autothrottle. Traditional autothrottle systems often use servos to move the cockpit thrust levers. Airbus-style autothrust usually leaves the levers in fixed detents while the engine-control computers vary thrust within the permitted range.
The terminology is not an absolute guarantee of how every aircraft is built or simulated. Check the thrust-mode annunciation and engine indications rather than assuming that the virtual levers must move. Our phase-by-phase explanation of airliner autothrottle behaviour explains what to expect during climb, cruise, descent and approach.
What does THR HOLD mean?
THR HOLD or HOLD generally means the autothrottle servo has stopped positioning the thrust levers, allowing the pilot to move them manually without an immediate servo response.
This annunciation is commonly associated with a particular part of the take-off sequence on some aircraft. It does not mean airspeed hold, and it does not guarantee that an exact thrust value is being maintained. The detailed trigger and re-engagement logic are aircraft-specific, so the displayed thrust mode takes precedence over assumptions based on the switch position.
Thrust vs throttle: what does a throttle axis control?
Throttle is the pilot’s input or lever position; thrust is the force produced by the engine.
On turbine aircraft the cockpit control is usually called a thrust lever, while piston aircraft commonly use a throttle and some turboprops use power levers. A simulator throttle axis maps an analogue controller position to that cockpit input. It does not command a fixed amount of thrust because altitude, airspeed, engine state and control-system logic all affect the resulting output.
A non-motorised hardware quadrant cannot follow servo-driven virtual levers. When autothrottle disconnects, a mismatch between the physical and virtual positions can therefore cause a sudden thrust jump. Airbus-style detent systems have a different problem: the hardware axis must be calibrated so each physical detent corresponds to the simulated one.
What does LVL mode mean on an autopilot?
LVL mode normally commands wings level and a pitch mode intended to arrest the climb or descent; it is not usually an automatic speed or throttle mode.
Some systems target approximately zero vertical speed, while others use aircraft-specific level-flight logic. LVL does not necessarily return the aircraft to the altitude at which the button was pressed, and it cannot correct badly mismatched power by itself. Unless the aircraft also has active autothrottle, the pilot remains responsible for thrust and airspeed.
Because LVL behaviour is not standardised across every real or simulated autopilot, confirm the active roll, pitch and thrust modes after pressing it.
How should automatic speed hold be engaged?
Automatic speed control should be engaged by matching the speed target, vertical mode and thrust system, then confirming all three on the annunciator.
- Stabilise the aircraft. Set a sensible pitch-and-power condition before asking the automation to capture a target.
- Set the target and unit. Select the required IAS or Mach, or verify the managed target supplied by the flight-management system.
- Prepare the thrust system. Arm or engage autothrottle where required. Place fixed-detent thrust levers in the correct operating detent; on other aircraft, confirm the autothrottle has servo authority.
- Select the mode by priority. Use IAS/FLC when pitch should hold speed. Use altitude, vertical-speed or path modes when thrust should hold speed.
- Read the Flight Mode Annunciator. Check the active pitch and thrust modes. An armed mode is waiting for a capture condition and is not yet controlling the aircraft.
- Monitor the first response. Verify that pitch or thrust moves in the expected direction and that no thrust, drag or speed limit has been reached.
Why is autothrottle not holding the selected speed?
The Flight Mode Annunciator usually reveals why autothrottle is not holding speed; the selected number and illuminated switch alone are not proof of an active speed mode.
- The system is armed, not active. Wait for the required capture condition or select a compatible mode, then confirm the annunciation changes.
- Thrust rather than speed is being held. Annunciations such as climb thrust, idle, thrust reference, hold or take-off/go-around do not promise exact airspeed control.
- The wrong vertical mode is active. In IAS/FLC, pitch controls speed. In vertical-speed or path mode, thrust must do so.
- The selected rate is beyond the aircraft’s performance. Reduce an excessive climb rate, steepen or shallow the descent as appropriate, and use drag devices when the procedure permits.
- A thrust limit has been reached. Maximum thrust cannot prevent every low-speed condition, and idle thrust cannot prevent every descent overspeed.
- The levers are in the wrong detent. Fixed-detent autothrust may have no authority, reduced authority or a different commanded range until the levers are positioned correctly.
- A throttle axis is overriding the automation. Clear duplicated assignments, check separate engine axes, correct reversed axes and add a small dead zone if controller noise repeatedly changes the command.
- The physical and virtual throttles disagree. Avoid moving a non-motorised quadrant while servo-driven autothrottle is active unless that aircraft supports manual override.
- Managed speed or an IAS/Mach transition changed the target. Check the commanded unit, flight phase and any active speed constraint.
- The aircraft has simplified or incomplete system modelling. Basic default aircraft and add-ons may approximate mode transitions, thrust limits or protections differently from higher-fidelity models.
If the behaviour remains unclear, disconnect the automatic thrust system, establish a stable manual pitch-and-power setting and diagnose it away from a critical phase of flight. The simulator autothrottle engagement checklist covers controller conflicts, detents, armed modes and aircraft limitations in more detail.
Can autothrottle work without the autopilot?
On many aircraft, autothrottle can control speed while the pilot hand-flies or uses only the flight director.
The pilot remains responsible for pitch, bank and flight path, while the thrust system responds to its active mode. This can reduce speed workload, but large manual pitch changes may drive the engines to idle or maximum thrust. Availability and disconnect logic vary by aircraft; our guide to using autothrottle while manually controlling the aircraft explains the practical separation between the systems.