Definition
A non-precision instrument approach that uses a Simplified Directional Facility to provide lateral (left/right) guidance to a runway. The SDF transmits a single course signal similar to a localizer, but with a wider course width (typically 6° or 12°) and a course alignment that may not be exactly aligned with the runway centerline. It provides no vertical guidance; descent is flown using published step-down altitudes to a Minimum Descent Altitude (MDA).
Plain English
An instrument approach that gives you a left/right needle to follow toward the runway, but no glide path. The signal is less precise than a localizer and may point slightly off the runway centerline, so you fly down using published altitudes rather than a smooth glide.
Context Anchor
Seen on instrument approach charts and during instrument flying when an airport has an SDF procedure instead of, or in addition to, other approach types.
Derivation
"Simplified" because the facility is a stripped-down, lower-cost version of a localizer — fewer requirements for course width and centerline alignment. "Directional" because it provides direction (lateral guidance) only, not vertical.
Why Pilots Care
It provides a usable instrument approach option at airports that lack a full ILS, allowing continued operations in lower weather minima than a visual approach.
Grounding Statement
On an SDF approach, the instrument can help you stay left or right of the published course, but you must manage the descent yourself using the chart.
Intuition Check
Do not read “simplified” as “easy” or “less strict.” An SDF approach still requires exact instrument flying and strict use of the published procedure.
Example Sentence 1
With the ILS out of service, the crew briefed the SDF approach and noted that the final course was offset 3° from the runway centerline.
Example Sentence 2
Because the SDF course is wider than a localizer, the pilot began correcting for wind drift earlier in the approach.