Definition
A non-precision instrument approach aid that provides lateral (left/right) guidance to a runway, similar to an Instrument Landing System localizer but with looser tolerances. An SDF transmits a course signal that may be offset from the runway centerline by up to 3 degrees and uses a wider course width (typically 6 or 12 degrees) than a standard ILS localizer. It provides no glideslope or vertical guidance.
Plain English
A radio signal that helps a pilot line up with the runway during an instrument approach. It only tells you whether you're left or right of course, not whether you're high or low. It's less precise than a full ILS and the course may not point exactly down the runway.
Context Anchor
Seen on instrument approach charts and used when flying an approach by reference to cockpit navigation instruments instead of outside visual cues.
Derivation
The name describes its purpose. 'Simplified' because it's a stripped-down version of an ILS localizer. 'Directional' because it gives direction (left/right) toward the runway. 'Facility' is the standard FAA word for a ground-based navigation installation.
Why Pilots Care
Allows instrument approaches to smaller airports that lack a full ILS, though with higher minimums due to reduced accuracy.
Intuition Check
“Simplified” does not mean casual or easier to fly. Here it names a specific type of radio navigation facility with limited guidance compared with a full precision landing system.
Example Sentence 1
The approach plate showed an SDF approach to Runway 18, so we briefed the higher minimums and the offset final course.
Example Sentence 2
Because the airport offered only an SDF approach, the pilot flew a higher minimum descent altitude than at airports with an ILS.