Definition
The maximum displacement of a course or glideslope needle from its centered position on a navigation indicator. On a localizer, full-scale deflection represents an angular displacement of approximately 2.5° from the centerline; on a glideslope, approximately 0.7° from the on-path angle. When the needle is pinned against the edge of the scale, the aircraft is at or beyond this maximum displayed deviation and the actual deviation may be greater than the instrument can show.
Plain English
The needle has moved as far as it can go to one side of center. Once it reaches the edge of the scale, the instrument cannot show how much further off course the aircraft really is.
Context Anchor
Seen while intercepting or tracking the localizer during an ILS approach.
Derivation
"Deflection" comes from the Latin deflectere, meaning to bend or turn aside. The needle is being turned aside from center to show the aircraft is off course. "Full-scale" simply means the needle has reached the end of the scale it moves across.
Why Pilots Care
Signals that the aircraft has left the reliable localizer signal area and requires prompt correction or a missed approach.
Analogy
It is like a car’s fuel gauge sitting on empty: the gauge has reached its limit, so it does not tell you whether there is a little fuel left or almost none.
Intuition Check
Do not read full-scale deflection as a precise distance from the runway path. It means the indicator has reached its display limit, so the aircraft is at least that far off course.
Example Sentence 1
As the aircraft drifted right of the localizer, the course needle moved to full-scale deflection, prompting the pilot to execute a missed approach.
Example Sentence 2
Maintaining the needle inside half-scale deflection keeps the aircraft within the protected ILS course.