IAS hold vs Mach hold explained: learn what each aircraft autopilot mode controls, when to switch at crossover, and why pitch or thrust changes.
IAS hold commands the autopilot to maintain a selected indicated airspeed; Mach hold commands it to maintain a selected Mach number. IAS is normally used lower in the climb and during descent, while Mach is used at high altitude. Depending on the aircraft, pitch, thrust, or a coordinated combination controls speed.
IAS hold vs Mach hold at a glance
In real-world aviation, the practical difference is the speed reference the system maintains as altitude changes.
| Characteristic | IAS hold | Mach hold |
|---|---|---|
| Reference | Indicated airspeed, usually displayed in knots | Aircraft speed divided by the local speed of sound |
| Typical use | Lower and medium-altitude climb, and lower descent | Upper climb, high-altitude cruise and initial descent |
| Effect during a climb | At constant IAS, Mach generally increases | At constant Mach, IAS generally decreases |
| Main operational reason | Useful for low-speed margins, handling and limits expressed as IAS | Useful for compressibility and high-speed limits expressed as Mach |
IAS and Mach are not interchangeable readings of the same scale. Our explanation of how IAS, Mach and crossover altitude relate covers the underlying airspeed measurements.
How does the autopilot hold IAS or Mach?
The selected reference does not by itself reveal whether pitch or throttle controls speed; that depends on the active vertical mode and the aircraft's automation design.
- Speed controlled by pitch: In flight-level-change or open-climb-style modes, thrust is normally set for the climb or descent while the autopilot adjusts pitch to maintain IAS or Mach. If the aircraft becomes slow, it lowers the nose; if it becomes fast, it raises the nose.
- Speed controlled by thrust: In altitude hold or some path modes, pitch is already maintaining altitude or trajectory. An autothrottle or autothrust system therefore changes engine power to hold the selected IAS or Mach.
- Coordinated control: More sophisticated systems may use both pitch and thrust during captures, mode transitions and envelope protection.
An aircraft without automatic thrust cannot normally hold altitude and airspeed independently using pitch alone; the pilot must manage power. Our overview of how autopilot sensors, modes and servos divide the work explains this control architecture in more detail.
With the autopilot disconnected, an IAS or Mach mode may still drive the flight-director command bars, but it will not move the controls. This is part of the distinction between flight-director guidance and autopilot control.
When should IAS hold change to Mach hold?
Aircraft normally climb at a scheduled IAS until reaching crossover altitude, then continue at a scheduled Mach number; the sequence is reversed during descent.
If IAS were held throughout a high-altitude climb, Mach would continue rising and could approach the aircraft's maximum operating Mach, MMO. Mach hold prevents that trend by allowing IAS to decrease as the aircraft climbs. During descent, changing back to IAS helps manage the indicated-speed limit, VMO. These limits and their notation are covered in our guide to V-speeds and aircraft operating-speed limits.
What is crossover altitude?
Crossover altitude is the pressure altitude at which the scheduled IAS and Mach values represent the same flight condition. It is not one fixed altitude for every flight: each IAS/Mach pair has its own crossover point.
Many airliners change the displayed speed reference automatically. Where a manual change is required, the basic check is:
- Use the published schedule. Select the IAS and Mach values specified for that aircraft, weight and operating procedure.
- Match the references. Change modes near the point where the present IAS corresponds to the intended Mach number.
- Check the flight-mode annunciator. Confirm that IAS or Mach mode is active rather than merely selected or armed.
- Monitor the response. Watch the speed trend, pitch and thrust for an unexpected mode reversion or target mismatch.
When the values are matched correctly, changing from IAS to Mach should not cause an acceleration or deceleration; only the reference being held changes.
Why does the aircraft pitch when IAS or Mach hold is selected?
A large pitch change usually indicates a mismatched target, insufficient energy or a different active mode from the one expected.
- Mismatched IAS and Mach targets: Switching to a preselected Mach value that does not correspond to the present IAS commands an immediate correction. Match the target before changing modes.
- Insufficient thrust: Speed-on-pitch logic sacrifices climb rate to preserve speed. Near the aircraft's performance ceiling, the autopilot may lower the nose substantially or stop climbing because it cannot create energy.
- Incorrect mode assumption: Pressing a selector requests a mode, but the flight-mode annunciator shows what is actually active. Mode names and arming logic differ between aircraft.
- Autothrottle not controlling speed: In altitude hold, selecting a speed does nothing by itself if automatic thrust is unavailable or disengaged. Power must then be adjusted manually.
- Operating limit reached: IAS or Mach hold is not automatically an overspeed or stall-protection system. Protection capability varies, so VMO, MMO and low-speed indications must still be monitored.
A mistake we see regularly is expecting the autopilot to hold speed, altitude and climb rate simultaneously without enough thrust or a separate automatic-thrust channel. The exact behaviour, permitted modes and changeover procedure are aircraft-specific; the approved aircraft documentation and operating procedures take precedence.