Definition
A ground-based instrument approach navigation aid that transmits a localizer-type signal of comparable accuracy to an ILS localizer, but is not aligned with the runway centerline. Because of its offset alignment, an LDA approach is not classified as a precision approach and typically does not provide electronic glide slope guidance.
Plain English
It is a navigation signal that works like the side-to-side guidance part of an ILS, but the signal is not lined up straight down the runway. So it gets you close to the airport on a known track, but you usually have to maneuver visually to land.
Context Anchor
You will see LDA on instrument approach charts and in approach clearances when flying in clouds or low visibility.
Derivation
“Localizer” comes from “localize” — to fix a position. The word “directional” signals that the aid provides a course (direction) to fly, while “aid” simply means a navigation helper. Together: a localizer-style signal used as a directional helper, but not a full ILS.
Why Pilots Care
Pilots must recognize the offset alignment so they can plan the visual segment to the runway and maintain situational awareness during the approach.
Intuition Check
Do not assume an LDA is the same as a full ILS. It gives localizer-like left-right guidance, but it may not be aligned with the runway and may not include glidepath guidance.
Example Sentence 1
We briefed the LDA approach into the valley airport, noting the final course was offset about 15 degrees from the runway.
Example Sentence 2
After intercepting the LDA course, the crew began the descent while monitoring the offset to the runway.