Definition
Backup display configurations on a glass cockpit that automatically reroute essential flight information to a remaining functional screen when a primary display fails. In a typical helicopter installation, if the Primary Flight Display (PFD) or Multi-Function Display (MFD) fails, the surviving display reconfigures to show the critical flight instruments needed to keep flying safely.
Plain English
If one of the cockpit screens fails, the other screen automatically rearranges itself to show the most important flight information so the pilot can keep flying.
Context Anchor
Seen in helicopter IFR certification and equipment discussions, especially when checking whether a helicopter can safely continue flight after certain system failures.
Derivation
From 'revert,' meaning to fall back or return to an earlier state. A reversionary mode is the system 'falling back' to a simpler backup arrangement when normal operation is lost.
Why Pilots Care
Allows continued instrument flight after a display failure instead of losing all reference information at once.
Intuition Check
Do not read reversionary mode as meaning the system has repaired itself. It means the system has changed to a backup or reduced way of operating so the pilot can keep the essential function available.
Example Sentence 1
When the MFD failed during the approach, the system entered reversionary mode and the airspeed, attitude, altitude, and heading appeared on the remaining display.
Example Sentence 2
The pilot practiced flying in reversionary mode during the IFR checkride to demonstrate continued safe operation after a display failure.