Definition
A cockpit instrument display, typically a horizontal pointer on the course deviation indicator (CDI) or HSI, that shows the aircraft's vertical position relative to the published glide slope of an ILS or the vertical guidance path of an RNAV approach. When the pointer is centered, the aircraft is on the correct descent path; deflection above or below center shows the aircraft is below or above the path respectively.
Plain English
A small needle on the cockpit display that tells the pilot whether they are on, above, or below the proper descent path during an instrument approach.
Context Anchor
Seen on VOR/DME RNAV units and other navigation displays that show distance, time, and speed to a selected navigation point.
Derivation
From 'glide slope,' meaning the angled descent path an aircraft follows toward the runway. 'Indicator' simply means the instrument that shows it. The name reflects exactly what it does: it indicates where the aircraft is in relation to the glide slope.
Why Pilots Care
It provides precise vertical guidance so the aircraft reaches the runway at the correct height and angle for a safe landing.
Grounding Statement
If the airplane is flying into a wind from ahead, the GS indicator may show a lower number because the aircraft is covering ground more slowly.
Intuition Check
Do not read GS as “general speed” or as speed through the air. Here, GS means groundspeed: speed over the ground.
Example Sentence 1
As the aircraft intercepted the final approach course, the pilot watched the GS indicator come alive and began the descent.
Example Sentence 2
During the RNAV approach, the GS indicator remained centered all the way to the decision altitude.