Definition
An instrument approach procedure that uses a Localizer-Type Directional Aid for lateral guidance. An LDA transmits a signal comparable in accuracy to an ILS localizer but is not aligned with the runway centerline, typically because terrain or other obstacles make a straight-in localizer installation impractical. An LDA approach may be published with or without glideslope guidance; when a glideslope is provided, the approach is designated LDA/Glideslope. If the final approach course differs from the runway heading by more than 30 degrees, the approach is published as a circling-only procedure.
Plain English
An instrument approach that uses a localizer-style signal for left-right guidance, but the signal is not lined up with the runway. The pilot follows it down toward the airport and then either lines up with the runway visually for landing, or circles to land if the approach course points well off to one side.
Context Anchor
Seen on instrument approach charts and in air traffic clearances, such as when a controller clears a pilot for an LDA approach.
Derivation
Localizer-Type Directional Aid: 'Localizer-Type' because the signal works like an ILS localizer, and 'Directional Aid' because it provides directional guidance toward the airport rather than a precision runway-aligned course. The 'type' wording signals that it behaves like a localizer but is not officially one, mainly because it is not aligned with the runway.
Why Pilots Care
Provides a safe, published path for landing in low visibility when a full ILS is unavailable and the runway alignment requires an angled approach.
Grounding Statement
On an LDA approach, the course needle can be centered while the airplane is still approaching the runway area at an angle.
Intuition Check
Do not assume “localizer-type” means the same thing as a normal localizer or ILS approach. In an LDA approach, the guidance is similar, but the course may be offset from the runway centerline.
Example Sentence 1
We briefed the LDA approach into Roanoke and reminded ourselves that the final course was offset from the runway, so we'd need the runway in sight before maneuvering to land.
Example Sentence 2
Because the LDA course was offset by 12 degrees, the crew began the circling maneuver at the missed approach point.