Definition
A selectable mode on a flight director or autopilot that commands the aircraft to capture and track a published instrument approach, typically following both the localizer (lateral guidance) and glideslope (vertical guidance) of an ILS, or the lateral and vertical paths of another approach type the system supports.
Plain English
A button or switch setting that tells the autopilot or flight director, "fly the approach." Once armed, it locks onto the approach guidance signals and follows them down toward the runway.
Context Anchor
Seen on a flight director or autopilot mode panel during instrument approach setup and final approach.
Derivation
Approach comes from an old word meaning to come nearer. In aviation, an approach is the planned final part of getting nearer to a runway for landing; APP is the shortened cockpit label for that mode.
Why Pilots Care
It reduces pilot workload and improves accuracy when flying an approach in low visibility or at night.
Intuition Check
Do not read “approach” here as simply getting closer to the airport. In this context, APP is a specific flight guidance mode selected for the final instrument approach path.
Example Sentence 1
After being cleared for the ILS, the pilot tuned the localizer frequency, set the inbound course, and selected APP on the flight director.
Example Sentence 2
Once APP mode captured the signals, the autopilot flew the aircraft down the correct path without further input.