Definition
On a Flight Director System (FDS) mode selector, APP is the mode that arms or engages the system to capture and track a published instrument approach guidance signal, typically an ILS localizer and glideslope or a VOR/LOC course, so the flight director commands the pitch and roll needed to fly the approach.
Plain English
APP is the button or switch position that tells the flight director to follow an instrument approach. Once the aircraft is in range of the approach signal, the system captures it and shows the pilot how to steer down to the runway.
Context Anchor
Seen as APP on a flight director or autopilot mode selector, or on the mode display, while setting up and flying an instrument approach.
Derivation
APP is simply the shortened form of Approach. Cockpit panels use three-letter labels to save space, so Approach becomes APP.
Why Pilots Care
It supplies precise steering cues that let the pilot or autopilot follow the approach path accurately in low visibility.
Intuition Check
APP does not mean a phone application here. In this cockpit context, APP means approach mode for the flight director or autopilot.
Example Sentence 1
Once cleared for the ILS, the pilot pressed APP on the flight director to arm the localizer and glideslope capture.
Example Sentence 2
Once captured in APP mode, the flight director provided pitch and bank commands to stay on the glideslope.