Definition
The vertical displacement of the aircraft above or below the published glideslope path during an ILS approach, displayed on the glideslope indicator as movement of the needle or pointer away from center.
Plain English
How far the aircraft is sitting above or below the correct descent path on an ILS approach, shown by how far the glideslope needle has moved off center.
Context Anchor
Seen on an instrument approach display and in automatic flight control system discussions, especially when the aircraft is following vertical guidance toward a runway.
Derivation
Deviation' comes from the Latin 'deviare,' meaning 'to turn off the road.' Here it means the aircraft has drifted off the correct vertical path, not that anything is broken or wrong with the equipment.
Why Pilots Care
Excessive glideslope deviation can result in an unstable approach, requiring a missed approach or risking an unsafe landing.
Intuition Check
Do not read “deviation” here as any sideways drift or general error. Glideslope deviation specifically means being above or below the intended vertical path to the runway.
Example Sentence 1
The autopilot captured the glideslope and corrected the small glideslope deviation that had developed in the descent.
Example Sentence 2
A full-scale glideslope deviation prompted the crew to execute a missed approach.