Definition
A movable pointer on the Horizontal Situation Indicator (HSI) that shows the aircraft's vertical position relative to the ILS glideslope. When the pointer is centered, the aircraft is on the glideslope. When the pointer is above center, the glideslope is above the aircraft (fly up to recapture). When the pointer is below center, the glideslope is below the aircraft (fly down to recapture).
Plain English
A small needle on the HSI that tells you whether you are on, above, or below the correct descent path during an ILS approach. If the needle moves up, the path is above you and you need to climb a little; if it moves down, the path is below you and you need to descend a little.
Context Anchor
Seen during instrument approaches that provide vertical guidance, especially when using a horizontal situation indicator or similar cockpit display.
Derivation
“Glide” means to descend smoothly without a sudden drop, and “slope” means an inclined path. Together, “glideslope” points to the smooth downward path an aircraft follows toward the runway. “Deviation” means being off that path, and the pointer shows that difference visually.
Why Pilots Care
Keeps the aircraft on the proper vertical path to the runway, preventing low or high approaches that could lead to unsafe landings.
Intuition Check
The pointer does not point at the runway itself. It shows where the proper descent path is in relation to the aircraft: centered means on path, above center usually means the aircraft is below the path, and below center usually means the aircraft is above the path.
Example Sentence 1
As the aircraft intercepted the ILS, the glideslope deviation pointer began moving down from the top of the scale and centered as the descent started.
Example Sentence 2
The pilot adjusted pitch attitude to keep the glideslope deviation pointer centered during the approach.