Definition
The vertical displacement of the aircraft above or below the published glideslope on an Instrument Landing System (ILS) approach, shown on the cockpit display as the position of the glideslope needle relative to its centered, on-path reference. Deviation indicates how far off the correct descent path the aircraft is, with full-scale deflection representing the edge of the usable glidepath signal.
Plain English
It is how far above or below the correct ILS descent path the aircraft is. The needle on the cockpit display moves up if you are below the path and down if you are above it, so you can correct back onto the proper glide angle.
Context Anchor
You encounter this during an ILS approach and in terrain alerting system discussions, where the system may warn if the aircraft gets too far below the proper descent path.
Derivation
Deviation comes from the Latin 'deviare,' meaning 'to turn aside from the way.' Here it literally means turning aside from the published descent path — a useful reminder that the term measures departure from a correct reference, not just movement in general.
Why Pilots Care
Large or persistent deviation indicates an unstable approach that may require a go-around.
Grounding Statement
Picture the ILS glideslope as a steady downhill line to the runway; deviation is how far the airplane has moved above or below that line.
Intuition Check
Deviation does not mean a general flying mistake here. It means a measured position above or below the ILS descent path.
Example Sentence 1
The crew corrected a half-dot glideslope deviation by reducing the descent rate and re-centering the needle before reaching minimums.
Example Sentence 2
Strong crosswinds caused noticeable ILS glideslope deviation on short final.