localizer type directional aid

A localizer type directional aid (LDA) or Instrument Guidance System (IGS) is a type of localizer-based instrument approach to an airport. It is used in places where, due to terrain and other factors, the localizer antenna array is not aligned with the runway it serves. In these cases, the localizer antenna array may be offset (i.e. pointed or aimed) in such a way that the approach course it projects no longer lies along the extended runway centerline (which is the norm for non-offset and non-LDA localizer systems). If the angle of offset is three degrees or less, the facility is classified as an offset localizer. If the offset angle is greater than three degrees, the facility is classified as a localizer-type directional aid (LDA). Straight-in approaches may be published if the offset angle does not exceed 30 degrees. Only circling minimums are published for offset angles greater than 30 degrees. As a "directional aid", and only a Category I (CAT I) approach, rather than a full-fledged instrument landing system (ILS), the LDA is more commonly used to help the pilot safely reach a point near the runway environs, where he or she hopefully can see the runway, at which point he or she will proceed and land visually, as opposed to (for example) full Category III (CAT III) ILS systems that allow a pilot to fly, without visual references, very close to the runway surface (usually about 100 ft) depending on the exact equipment in the aircraft and on the ground.

An LDA uses exactly the same equipment to create the course as a standard localizer used in ILS. An LDA approach also is designed with a normal course width, which is typically 3 to 6 degrees. (At each "edge-of-course", commonly 1.5 or 3 degrees left and right of course, the transmitted signal is created in such a way as to ensure full-scale CDI needle deflection at and beyond these edges, so the pilot will never falsely believe they are intercepting the course outside of the actual course area. The area between these full-scale needle deflections is what defines the course width.) An LDA approach (considered a non-precision approach) may have one or more marker beacons, perhaps a DME, and in rare instances a glide slope, just as other precision approaches have, such as ILS approaches.

If the offset is not greater than 30 degrees, straight-in approach minima may be published; circling minima only are published when offset exceeds 30 degrees.

List of LDA approaches in the United States

The following 28 LDA approaches are available in the United States (as of May 2021):

# PAJN, LDA X RWY 08, Juneau International Airport, Juneau, AK
# PAPG, LDA/DME-D, Petersburg James A. Johnson Airport, Petersburg, AK
# PASI, LDA/DME RWY 11, Sitka Rocky Gutierrez Airport, Sitka, AK
# PAVD, LDA/DME-H, Valdez Pioneer Field, Valdez, AK
# PAWG, LDA/DME-C and LDA/DME-D, Wrangell Airport, Wrangell, AK
# KFYV, LDA/DME RWY 34, Drake Field, Fayetteville, AR
# KBIH, LDA/DME RWY 17, Eastern Sierra Regional Airport, Bishop, CA
# KCCR, LDA RWY 19R, Buchanan Field, Concord, CA
# KSNA, LDA/DME RWY 20R, John Wayne Airport–Orange County, Santa Ana, CA
# KTVL, LDA RWY 18, Lake Tahoe Airport, South Lake Tahoe, CA
# KVNY, LDA-C, Van Nuys Airport, Van Nuys, CA
# KEGE, LDA/DME RWY 25, Eagle County Regional Airport, Eagle, CO
# KGJT, LDA/DME RWY 29, Grand Junction Regional Airport, Grand Junction, CO
# KHFD, LDA RWY 02, Hartford–Brainard Airport, Hartford, CT
# KDCA, LDA Y RWY 19, Ronald Reagan Washington National Airport, Washington, DC (No longer in use)Amendment 5 to DCA’s River Visual Runway 19 approach, June 5, 2015, by the FAA
# KDCA, LDA Z RWY 19, Ronald Reagan Washington National Airport, Washington, DC
# PHNL, LDA/DME RWY 26L, Honolulu International Airport

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