Definition
A non-precision instrument approach navigation aid that uses a localizer transmitter comparable in accuracy to an ILS localizer, but which is not aligned with the runway centerline. Because of this offset, the LDA provides lateral guidance only and is not part of a full ILS system. Some LDA approaches are paired with a glide slope, in which case vertical guidance is also available; without a glide slope, the LDA supports a non-precision approach.
Plain English
A radio signal that guides the airplane toward an airport along a path that does not line up with the runway, so the pilot must maneuver to line up with the runway near the end of the approach.
Context Anchor
Seen on instrument approach charts and during approach briefings when the selected approach is labeled LDA.
Derivation
‘Localizer’ comes from ‘localize’ — to pinpoint a location — referring to the radio beam that pinpoints a lateral path through space. ‘Directional aid’ simply means a navigation tool that gives direction. The name signals that it works like a localizer but only as a directional aid, not as a runway-aligned final approach guide.
Why Pilots Care
An LDA allows an instrument approach to a runway that lacks a straight-in localizer, but the offset requires the pilot to plan for a circling or offset maneuver at low altitude.
Intuition Check
Do not assume an LDA is a full ILS. An LDA gives precise left-right guidance, but it may not provide full vertical guidance and may not point straight down the runway.
Example Sentence 1
The LDA approach into Roanoke is offset from the runway, so after breaking out we had to maneuver visually to land.
Example Sentence 2
After the ILS localizer became inoperative, the crew switched to the published LDA procedure for runway 18.