Definition
A vertical scale on the Horizontal Situation Indicator (HSI) marked with reference dots above and below a center index, used with a moving pointer to show the aircraft's vertical position relative to the ILS glideslope. The center represents being exactly on the glideslope; each dot represents a fixed angular displacement above or below it.
Plain English
It's the row of dots on the side of the HSI that tells you how far above or below the correct descent path you are during an ILS approach. If the pointer sits in the middle, you're on the path. If it's above center, the path is above you and you need to climb a bit; if it's below center, you're high and need to descend.
Context Anchor
Seen on an HSI during an instrument approach that provides vertical guidance to the runway.
Derivation
"Glideslope" is the descending path you fly to the runway on an ILS. "Deviation" comes from Latin deviare, meaning "to wander off the way." The scale, then, simply shows how far you've wandered off the descent path.
Why Pilots Care
Accurate interpretation prevents flying above or below the safe descent path, reducing the risk of a high or low approach that could lead to an unstable landing or runway excursion.
Grounding Statement
As you descend toward the runway, this scale shows whether your airplane is tracking the intended path down, not just whether it is pointed in the right direction.
Intuition Check
“Scale” does not mean the size of the instrument here; it means the reference marks you read. “Deviation” does not mean a general mistake; it means how far you are from the intended descent path.
Example Sentence 1
As she intercepted the glideslope, the pointer centered on the glideslope deviation scale and she began her descent.
Example Sentence 2
A full-scale deflection on the glideslope deviation scale indicated the aircraft was well above the glide path and required an immediate descent adjustment.