Definition
A non-precision instrument approach navigation aid that provides lateral (left/right) course guidance to a runway, similar to a localizer but with reduced accuracy. The SDF course may be offset from the runway centerline by up to 3 degrees, and the course width is fixed at either 6 or 12 degrees, rather than being tailored to produce a standard width at the runway threshold. SDF provides no glide path (vertical guidance).
Plain English
A ground-based radio aid that gives pilots a needle showing whether they are left or right of the path to a runway. It works like a localizer but is less precise, and it does not tell you anything about how high you should be.
Context Anchor
Seen on instrument approach charts and in instrument approach briefing when a procedure uses an SDF course for final approach guidance.
Derivation
Simplified' because it is a stripped-down version of a localizer — easier and cheaper to install, with looser tolerances. 'Directional' because it provides direction (lateral guidance) toward the runway. 'Facility' is the standard FAA word for a ground-based navigation installation.
Why Pilots Care
Provides a usable instrument approach option at airports that lack an ILS or other more accurate systems.
Grounding Statement
On an SDF approach, the radio signal helps you stay on the published left-right path toward the airport, but altitude control still comes from the procedure and your instruments.
Intuition Check
Do not read “facility” as an airport building here. In this term, it means a ground-based radio aid that provides approach guidance.
Example Sentence 1
The approach plate showed an SDF approach to Runway 18, so the pilot briefed the wider course tolerance and the 3-degree offset from the runway.
Example Sentence 2
During the SDF procedure the aircraft maintained the published minimums until the runway environment was in sight.