Definition
The locus of points, in any given horizontal plane, at which the difference in depth of modulation (DDM) between the two localizer signal lobes is zero. In practical terms, it is the narrow line of equal signal strength extending along the runway centerline that defines the lateral guidance path of an ILS approach.
Plain English
The invisible line in the sky, lined up with the runway, that the localizer radio signal creates for the aircraft to follow on final approach.
Context Anchor
Seen in instrument approach procedures, ILS and localizer approach discussions, and cockpit navigation indications during final approach.
Derivation
From 'localizer,' the radio transmitter that 'localizes' or pinpoints the runway centerline laterally, and 'course,' meaning the path being flown. The ICAO label means this is the internationally standardized definition used in ICAO documents.
Why Pilots Care
It supplies the lateral guidance needed to keep the aircraft aligned with the runway centerline for a safe landing.
Grounding Statement
Picture the localizer course as an invisible runway centerline extended out through the air for the aircraft to follow on approach.
Intuition Check
Course does not mean a class or lesson here. In this context, course means the path or line the aircraft is intended to follow.
Example Sentence 1
Once established on the localizer course, the pilot reduced bank angles and concentrated on holding the centerline needle.
Example Sentence 2
A slight left correction was needed to stay centered on the localizer course in the crosswind.