Definition
An operating mode of an IFR-certified GPS receiver that automatically activates within 30 nautical miles of the destination airport, tightening the navigation accuracy and course deviation indicator (CDI) sensitivity from the en route value (typically 5 NM full-scale deflection) to 1 NM full-scale deflection in preparation for the approach phase.
Plain English
A setting your GPS switches into on its own as you get close to your destination airport. It makes the GPS more precise and makes the course needle more sensitive, so small drifts off course show up clearly before you start the approach.
Context Anchor
Seen during IFR arrivals and departures when using GPS or area navigation equipment near the destination or departure airport.
Derivation
Terminal' comes from the Latin 'terminus,' meaning 'end' or 'boundary.' In aviation, the 'terminal area' is the airspace and procedures around the end of a flight — the destination airport environment. 'Terminal mode' is simply the GPS mode used in that terminal area.
Why Pilots Care
Proper sensitivity prevents over- or under-correction on course; flying in the wrong mode can produce misleading guidance near the airport.
Analogy
It is like changing from a wide map view to a closer street-level map view as you near your destination. The closer view shows smaller position errors more clearly.
Intuition Check
Terminal mode does not mean the airplane is at the passenger terminal building. Here, terminal means the airport area, and mode means the navigation system’s operating setting for that area.
Example Sentence 1
About 30 miles out, the GPS switched from en route to terminal mode, and the course needle became noticeably more sensitive.
Example Sentence 2
Before starting the arrival the pilot confirmed the navigation system had entered terminal mode.