1 Answers
A localizer type directional aid or Instrument Guidance System 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 in such a way that the approach course it projects no longer lies along the extended runway centerline. 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. 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 approach, rather than a full-fledged instrument landing system , 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 full Category III ILS systems that allow a pilot to fly, without visual references, very close to the runway surface 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. An LDA 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.