Definition
Definition 1 — Localizer-type Directional Aid: A non-precision instrument approach navigation aid that provides lateral course guidance similar to an ILS localizer, but is not aligned with the runway centerline (typically offset by more than three degrees). It does not normally include glideslope (vertical) guidance. Definition 2 — Landing Distance Available: The length of runway declared available and suitable for the ground run of an aircraft landing, measured from the threshold to the end of the runway (or to the end of any declared landing distance, whichever is shorter).
Plain English
LDA has two unrelated meanings. As Localizer-type Directional Aid, it is a runway approach signal that guides you left and right toward the airport, but is angled off from the runway, so you must maneuver visually to land. As Landing Distance Available, it is simply how much runway you have to roll out on after touchdown.
Context Anchor
Seen in instrument approach procedure descriptions and in airport or runway distance information.
Derivation
Localizer-type Directional Aid is descriptive: it acts like a localizer (lateral course signal) and serves as a directional aid, but it is not a full ILS. Landing Distance Available is also descriptive: the runway length declared as available for landing.
Why Pilots Care
It allows safe landings at airports without a full ILS by providing precise directional guidance during the approach.
Intuition Check
Do not assume LDA always means one thing. In approach charts, it usually means Localizer-type Directional Aid; in runway distance data, it usually means Landing Distance Available.
Example Sentence 1
The LDA approach to Runway 6 is offset by twelve degrees, so once we break out we'll need to sidestep visually to line up with the runway.
Example Sentence 2
LDA procedures often require additional training because the final approach course is offset from the runway.