Definition
The published minimum altitude at which an aircraft on an ILS or similar precision approach is expected to intercept the glideslope from below while flying inbound on the localizer. It is depicted on the instrument approach chart by a lightning-bolt symbol on the profile view and represents the lowest altitude the pilot may descend to before glideslope capture.
Plain English
The altitude shown on the approach chart where you should be flying level when the glideslope needle comes alive and starts guiding you down toward the runway.
Context Anchor
Seen on instrument approach charts during approaches that provide vertical descent guidance to a runway.
Derivation
From 'glideslope' (the angled radio beam that guides descent to the runway) plus 'intercept' (Latin intercipere, 'to catch between'). The altitude is the level you fly until you 'catch' the descending beam.
Why Pilots Care
Reaching this altitude at the right time keeps the aircraft on the correct vertical path; arriving too high or too low can result in an unstable approach or missed approach.
Grounding Statement
Picture flying level at the charted altitude until the descent path comes down to meet you, then following that path toward the runway.
Intuition Check
Do not assume this is the altitude where the airplane automatically starts down. It is the altitude where you are expected to meet the descent path; you still must fly or monitor the descent.
Example Sentence 1
Cleared for the ILS, the pilot leveled off at the glideslope intercept altitude and waited for the needle to center before beginning the descent.
Example Sentence 2
ATC cleared the flight to the glideslope intercept altitude before handing off to the tower.