Definition
A Localizer-type Directional Aid is an instrument approach navigation facility comparable in accuracy to a localizer but not aligned with the runway centerline. Course alignment offsets greater than 3 degrees prevent it from being designated as a localizer-based approach, and straight-in minimums may or may not be published depending on the alignment and descent angle.
Plain English
An LDA is a runway approach guidance signal that works like a localizer but points slightly off to the side of the runway instead of straight down it. Pilots use it to get close to the airport on instruments, then line up with the runway visually for landing.
Context Anchor
Seen on instrument approach charts and in IFR alternate planning when deciding whether an airport has a usable instrument approach.
Derivation
The name describes its function: it provides directional guidance using localizer-type equipment, but because it isn't aligned with the runway it's called an 'aid' rather than a localizer. The distinction in name reflects that it cannot be used the same way as a runway-aligned localizer.
Why Pilots Care
Determines whether an airport qualifies as a suitable IFR alternate based on available approach types and required weather minimums.
Intuition Check
LDA can also mean Landing Distance Available in runway performance discussions. In this instrument-approach context, LDA means Localizer-type Directional Aid.
Example Sentence 1
The LDA approach to Runway 6 has a 15-degree offset, so we'll need the runway in sight before we can maneuver to land.
Example Sentence 2
For alternate requirements, an LDA with straight-in minimums can satisfy the regulation when weather permits.