Definition
The point during an ILS approach at which the aircraft, while tracking the localizer at the published intermediate altitude, intersects the glide slope signal from below and begins a stabilized descent along it toward the runway.
Plain English
The moment on an ILS approach when the aircraft flies into the angled descent beam coming down from near the runway, and starts following it down.
Context Anchor
Seen during ILS approaches, usually after the aircraft is lined up with the runway course and before the final descent to landing.
Derivation
Interception comes from Latin intercipere, meaning to catch or seize between. The aircraft, flying level, catches the descending glide slope beam as it crosses through the aircraft's altitude.
Why Pilots Care
Capturing the glide slope at the proper location and altitude produces a stable descent; intercepting too high or fast can result in an unstable approach or missed approach.
Grounding Statement
Picture the airplane flying toward the runway until the glide slope indicator centers, then beginning the descent.
Intuition Check
GS does not mean groundspeed here. In this ILS context, GS means glide slope, the vertical descent path.
Example Sentence 1
At GS interception, the pilot lowered the landing gear, reduced power, and began the descent down the glide path.
Example Sentence 2
Maintaining the assigned airspeed until GS interception helps prevent an overspeed condition on final.