Definition
An automatic change by an aircraft's flight control or autoflight system from its normal operating mode to a simpler, more basic mode when the system detects a failure, loss of input data, or a condition outside its normal operating envelope. The system reverts to a degraded mode that the remaining sensors and computers can still support safely.
Plain English
When part of the aircraft's automated system fails or loses the data it needs, the system drops back to a simpler way of working that it can still handle reliably. It doesn't quit -- it falls back to a basic level the pilot then has to manage more directly.
Context Anchor
Seen in autopilot and flight director operation, especially during instrument flying or when a selected guidance mode drops out.
Derivation
From Latin 'reverti', meaning 'to turn back' or 'return.' The system is literally turning back to an earlier, simpler way of operating when it can no longer support the full mode.
Why Pilots Care
Mode reversion maintains safety by preventing the autopilot from continuing with invalid guidance, but the pilot must recognize the change and be ready to intervene or select an appropriate new mode.
Intuition Check
Mode reversion is not just the pilot choosing a different setting. In this use, the aircraft system may change modes automatically, and the pilot must catch that change.
Example Sentence 1
After the airspeed sensor iced over, the flight control system performed a mode reversion to a basic law that no longer provided automatic envelope protection.
Example Sentence 2
The crew monitored the mode reversion from approach mode to altitude hold after the glideslope became unusable.