Definition
A non-precision instrument approach aid that provides lateral (left/right) 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 a specific runway length as a localizer is. SDF approaches provide no vertical guidance.
Plain English
A radio signal that tells the pilot whether they are left or right of a path leading to the runway. It works like a simpler, less precise version of a localizer, and it does not tell the pilot anything about how high or low they should be.
Context Anchor
Seen on instrument approach charts, especially in the plan view where the approach course and navigation aid are shown.
Derivation
Simplified — because it is a stripped-down version of the localizer system. Directional — because it gives direction (left/right) toward the runway. Facility — the ground-based equipment that transmits the signal. The name reflects that it was designed as a cheaper, less precise alternative to a full ILS localizer.
Why Pilots Care
It enables instrument approaches at airports that lack a full instrument landing system, expanding safe landing options in low visibility.
Intuition Check
Do not read “simplified” as “easier” or “less important.” Here it means the facility provides a simpler form of directional guidance than a localizer, with less precision.
Example Sentence 1
The approach chart showed an SDF approach to runway 18, so the pilot briefed the offset course and planned to maneuver visually for landing once the runway was in sight.
Example Sentence 2
During the missed approach briefing the crew noted the SDF provided only lateral guidance with no vertical path information.