Definition
An autopilot/flight director annunciation indicating that the altitude reference signal has been lost or is invalid, causing the system to drop the altitude hold or altitude capture function. When ALTITUDE FAIL is annunciated, the autopilot can no longer reliably maintain or capture the selected altitude and the pilot must monitor altitude manually or take control.
Plain English
A warning from the autopilot that it has lost the information it needs to hold your altitude. It is telling you it can no longer be trusted to keep the airplane at the height you set.
Context Anchor
Seen on an autopilot, flight guidance display, or alert panel during instrument flying or autopilot use.
Derivation
Altitude comes from a Latin word meaning “height.” Fail means to stop working as expected. Together, the phrase points to a height-related function that the system can no longer perform reliably.
Why Pilots Care
The pilot must immediately assume manual control of pitch to prevent altitude deviation until the fault is resolved.
Intuition Check
Do not read ALTITUDE FAIL as a small altitude error or a warning that you personally failed to hold altitude. It means the aircraft’s automatic system cannot reliably use or control altitude.
Example Sentence 1
When ALTITUDE FAIL appeared on the mode annunciator, the pilot disengaged the autopilot and hand-flew the airplane back to the assigned altitude.
Example Sentence 2
After the static system malfunction, the autopilot annunciated ALTITUDE FAIL and reverted to pitch hold.