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How A Localizer Directional Aid (LDA) Approach Works

It's like a localizer, except it's not aligned with the runway. Here's what you should know about flying LDA approaches...

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LDA Approaches Are Similar To LOC Approaches

LDAs are used in places where terrain or other factors prevent the localizer antenna from being aligned with the runway that it serves.

A Localizer Directional Aid Approach (LDA) uses the same equipment as a standard localizer. This usually results in an LDA course width very similar to a localizer, although it may differ based on terrain and obstacle considerations. Click here to learn more about how localizers work.

The only major difference between LOC approaches LDAs is that LDAs are not aligned with the landing runway.

Even though the LDA isn't aligned with the runway, straight-in minimums can be published when course alignment doesn't exceed 30 degrees between the course and runway.

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However, circling only minimums are published if the LDA's alignment exceeds 30 degrees.

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LDA + Glideslope Approaches

A limited number of LDA approaches also incorporate a glideslope. When an LDA has a glideslope, you'll see the note "LDA/GS" in the landing minimums section of the approach. You may also find "LDA/GS" in the planview of the approach, like the LDA to Runway 25 at Eagle, CO.

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Example: Honolulu (PHNL) LDA RWY 26L

When you're flying into Honolulu, most of the time you'll land on east-facing runways due to the trade winds. On rare occasions, when a strong wind comes out of the west during a frontal passage, you'll find yourself landing on Runways 26R/L.

This creates a problem for the Honolulu Airport. Terrain east of the airport, including the Diamond Head volcanic crater, prevents any straight-in ILS or RNAV approaches to Runways 26R/L. You can't make an instrument approach over downtown Honolulu either, which is also east of the airport.

Because of this, the only option was to create an LDA approach to Runway 26L, which avoids terrain altogether. Note the sharp left turn you need to make to land on Runway 26L from this LDA approach.

Transitioning To A Visual Landing

The most challenging part of an IFR-weather LDA isn't the approach itself, but the transition to visual flying. If you pop out of the clouds around minimums, you need to have a firm picture in your mind of where the runway will be, because you won't be perfectly aligned with the runway centerline. How you plan to maneuver to align with the runway, and at the same time remain stabilized, is something you should plan and brief before you start the approach.

Here's a video example of an airliner fly the LDA to Runway 26L in Honolulu. This crew did a great job flying a stable approach. It may not look like much, but turning final around 500 feet above the ground is unusual for a large aircraft.

Have You Flown An LDA?

Have you ever flown an LDA approach? Tell us about your experiences in the comments below.

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and an Embraer 145 First Officer for a regional airline. He graduated as an aviation major from the University of North Dakota in 2018, holds a PIC Type Rating for Cessna Citation Jets (CE-525), and is a former pilot for Mokulele Airlines. He's the author of articles, quizzes and lists on Boldmethod every week. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures on his YouTube Channel.

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