Aircraft autopilot altitude hold explained: barometric feedback, pitch and trim, altitude capture, power limits, drift and oscillation fixes.
Altitude hold on an aircraft autopilot maintains a barometric altitude by comparing the aircraft’s sensed altitude with a stored reference. If the aircraft rises or sinks, the pitch channel commands elevator and usually pitch-trim corrections to cancel the error. It normally controls pitch rather than thrust, so adequate power and airspeed remain essential.
In real-world aviation, altitude hold is one part of the automatic flight-control system. Sensors, flight-control computers and servos form a closed feedback loop; our guide to how autopilot feedback and control work explains that wider system.
What altitude does ALT HOLD use?
Most altitude-hold systems use a barometric altitude signal derived from static pressure, supplied by an air-data computer or a dedicated pressure sensor. They do not normally hold GPS altitude or height from the radio altimeter.
The reference may be the aircraft’s present altitude when ALT is engaged, or a preselected altitude acquired through an automatic capture sequence. Exact behaviour depends on the installation: a basic autopilot may only capture the present altitude, while an integrated flight-control system can level at a target entered on the mode-control panel.
Holding barometric altitude does not guarantee constant geometric height above terrain. Atmospheric pressure and temperature affect the relationship between indicated altitude and true height, so pilots use the required local or standard pressure setting and continue monitoring terrain clearance. Changing the barometric setting may also affect the held or displayed altitude on an integrated system.
How does selected-altitude capture work?
Selected-altitude capture changes the aircraft from a climb or descent mode into altitude hold while rounding out before the target.
- Select the target altitude. Entering a value does not necessarily make it active; it may only set the altitude selector or reminder.
- Use a climb or descent mode. The aircraft approaches the target in vertical-speed, airspeed, flight-level-change or VNAV mode.
- Arm altitude capture. Depending on the system, this happens automatically or requires a separate action. The flight-mode annunciator should show that capture is armed.
- Allow the autopilot to round out. The computer considers altitude error and vertical closure rate, then adjusts pitch before reaching the target. Annunciations such as ALT CAP, ALTS or ALT* may appear, although labels vary.
- Confirm ALT is active. After levelling, the active mode should change to ALT or ALT HOLD. Where fitted, the flight-mode annunciator—not merely an illuminated button—is the reliable indication.
A common mistake is pressing ALT during a climb and expecting the aeroplane to continue to the altitude shown in the selector. On many systems, that action captures the altitude at the moment the button is pressed. Our worked KAP 140 altitude-preselect example shows how arming, capture and ALT hold can be separate stages.
Which vertical mode should you use?
Use altitude hold after levelling; use VS, FLC/IAS or VNAV to reach another altitude, subject to the aircraft’s approved procedures.
| Mode | What pitch controls | Main consideration |
|---|---|---|
| ALT or ALT HOLD | Barometric altitude | Power and drag determine the resulting airspeed unless autothrottle or autothrust is active. |
| VS | Selected climb or descent rate | A demanding climb rate can consume airspeed when power is insufficient. |
| FLC or IAS | Selected airspeed during the climb or descent | Vertical speed varies with available thrust, drag and aircraft performance. |
| VNAV | A computed vertical path or managed target | Altitude constraints and capture logic vary considerably between aircraft. |
Does altitude hold control airspeed or throttle?
Altitude hold normally controls neither throttle nor airspeed directly. In an aircraft without autothrottle, the pilot must set enough power to maintain a safe speed while the autopilot adjusts pitch to remain level.
If power is too low, the autopilot may progressively raise the nose to preserve altitude, causing airspeed to decay. If power is excessive, it may lower the nose and allow speed to increase. The eventual result can be a stall warning, overspeed, mode reversion or autopilot disconnect, depending on the aircraft and its protections.
With autothrottle or autothrust engaged, a separate system usually adjusts thrust towards a speed target. That still does not make altitude hold unlimited: engine performance, configuration, icing and flight-envelope limits can prevent the commanded altitude from being maintained.
Why will an autopilot not hold altitude?
An autopilot usually drifts, overshoots or oscillates because the wrong mode is active, the aircraft is outside a sensible speed and power condition, or another input is fighting the pitch channel.
| Symptom | Likely cause | What to check |
|---|---|---|
| Aircraft continues climbing or descending | ALT capture is armed but not active, or another vertical mode remains engaged | Read the active and armed flight-mode annunciations. |
| Aircraft overshoots the selected altitude | Excessive vertical speed, late manual capture or incorrect arming | Use an appropriate approach rate and confirm capture before the target. |
| Altitude is held but speed decays | Insufficient power or excessive drag | Correct power and configuration while monitoring pitch and airspeed. |
| Aircraft repeatedly climbs and sinks | Pitch oscillation caused by speed, trim, turbulence, control input or a system fault | Stabilise the aircraft and follow the approved autopilot or disconnect procedure. |
| Held altitude disagrees with expectations | Incorrect barometric setting, static-system error or misunderstanding of the selected reference | Cross-check the altimeters, pressure setting and other instruments. |
Pilots should not fight an engaged autopilot through the controls. If the aircraft does not respond as expected, disconnect it, maintain control manually and follow the aircraft flight manual or checklist. For a closer breakdown of repeated pitch corrections, see our guide to diagnosing altitude-hold hunting and oscillation.
Why can altitude hold misbehave in a flight simulator?
A simulator models the same mode logic, but unwanted controller inputs and aircraft-specific coding can interfere with it. Duplicate pitch-axis assignments, a noisy yoke, a moving trim wheel, time acceleration or an add-on’s custom autopilot can all make ALT hold appear faulty.
First verify the active vertical mode and selected altitude, then remove stray pitch or trim inputs and stabilise power and airspeed. Simmers using Microsoft Flight Simulator can also consult our practical explanation of ALT, VS, FLC and VNAV in MSFS 2024; cockpit labels and capture behaviour still vary by aircraft.