Definition
The angular width of the usable signal that defines the localizer course on an Instrument Landing System (ILS). It is the total angle, measured horizontally, between the points where full-scale left and full-scale right needle deflection occur. Course width is set during installation so that the linear width at the runway threshold is approximately 700 feet, which typically results in a total angular width of between 3 and 6 degrees depending on the runway length.
Plain English
It's how wide the localizer beam is from one edge to the other. Inside that wedge of signal, your cockpit needle tells you how far left or right of the runway centerline you are. Outside it, the needle is pegged and gives you no useful guidance.
Context Anchor
Seen in instrument landing system discussions, especially when learning how the localizer provides left-right guidance during an approach to a runway.
Derivation
Localizer comes from localize, meaning to fix or identify a position. Course means the path to be followed, and width means how broad something is. Together, the phrase points to how broad the localizer’s intended guidance path is.
Why Pilots Care
Sets the sensitivity of the localizer needle, so the pilot knows exactly how much correction is needed to stay on the precise path.
Grounding Statement
Far from the runway, a small angle covers a large sideways distance; close to the runway, that same angle covers much less distance, so the guidance feels tighter.
Intuition Check
Do not think of localizer course width as the physical width of the runway. It is the angular width of the radio guidance path that tells the pilot left or right of the desired approach path.
Example Sentence 1
Because the localizer course width is narrower than a VOR's, even a small needle deflection on final means the aircraft is well off centerline.
Example Sentence 2
On the ILS approach the pilot noted the localizer course width gave steady needle movement even in light turbulence.