Definition
A ground-based navigation facility that provides lateral course guidance to a runway, using equipment of the same type as an ILS localizer, but with the course not aligned with the runway centerline. Course alignment offset from the runway centerline is typically greater than three degrees, which prevents the facility from being designated as a localizer. An LDA may or may not be equipped with a glide slope; when it is, the approach is published as an LDA with glide slope.
Plain English
A landing-aid radio beam, like the one used in an ILS, but pointed at an angle to the runway instead of straight down it. It guides the pilot toward the airport, but a turn to line up with the runway is needed near the end of the approach.
Context Anchor
Seen on instrument approach charts and in approach clearances when a published approach is named “LDA.”
Derivation
The name describes its function. It uses the same hardware as a localizer but is called a 'directional aid' rather than a localizer because its course is not aligned with the runway, so it cannot meet the precision-approach definition of a localizer.
Why Pilots Care
It allows safe instrument approaches at airports where terrain or layout prevents a full instrument landing system aligned to the runway.
Intuition Check
Do not assume an LDA is the same as a normal ILS localizer. An LDA gives similar side-to-side guidance, but its course is offset from the runway and must be flown exactly as published.
Example Sentence 1
We briefed the LDA Runway 6 approach and noted that the final course is offset, so we'd need to maneuver visually to land once the runway was in sight.
Example Sentence 2
After intercepting the LDA signal, the crew maintained the displayed course until the runway became visible.