Localizer type directional aid

Type of instrument approach

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 minima 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.

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