Definition
A non-precision instrument approach that combines a Simplified Directional Facility (SDF) for lateral course guidance with Distance Measuring Equipment (DME) for slant-range distance information from a designated ground station. The SDF provides a fixed final approach course similar to a localizer but with a wider course width (6° or 12°) and is not always aligned with the runway. DME provides continuous distance readouts that allow the pilot to identify fixes, step-down points, and the missed approach point along the approach.
Plain English
An approach that uses one signal to keep you on the correct path toward the runway and a second signal that tells you exactly how far you are from the airport. Together they let you fly down to a safe minimum altitude even though there is no glide slope.
Context Anchor
Seen on instrument approach charts, avionics equipment lists, and in discussions of what approach equipment must be working before an instrument approach can be flown.
Why Pilots Care
Loss of either the SDF or DME signal raises approach minimums or may render the procedure unusable, directly affecting whether a pilot can legally continue an instrument approach.
Intuition Check
Do not treat SDF/DME as the same thing as an ILS. SDF/DME can guide you left and right and show distance, but it does not provide a precision glidepath.
Example Sentence 1
The crew briefed the SDF/DME approach into the airport, noting that the final course was offset 3° from the runway centerline.
Example Sentence 2
With the SDF/DME available, the crew could confirm distance to the missed approach point while maintaining the published course.