LOC approaches, and the majority of LDA approaches, do not provide vertical guidance. In rare cases, LDA approaches may include vertical guidance, in the form of a glide slope, and may be identified as an Instrument Guidance System (IGS) approach. For C and D category aircraft, circling approach designation will occur at a lesser offset angle. Any procedure for which the offset angle exceeds 30° will be designated as a circling approach. The localiser offset angle will determine whether or not straight in approach limits can be published or if the procedure will be designated as a circling approach. To allow for manoeuvring during the visual segment from the localiser to the runway, the approach will be designed such that the localiser track intercepts the extended runway centreline at a distance based on the offset angle that is, the greater the offset, the further from the threshold the interception will occur. LDA approaches are localiser-based approaches which exceed the runway centreline offset limitations identified in the previous section. Beyond these tolerances, the instrument procedure will be designated as a Localiser Type Directional (LDA) approach. Under Federal Aviation Administration (FAA) regulations, as defined in Order 8260.3D - United States Standard for Terminal Instrument Procedures (TERPS) - the maximum localiser offset is limited to 3° from runway centreline heading. The International Civil Aviation Organisation (ICAO), in Doc 8168: Construction of Visual and Instrument Flight Procedures, limits the localiser offset to a maximum of 5° from the extended runway centreline. Localiser ApproachĪ localiser approach can be designed and implemented as a "stand alone" approach or it can be the "downgrade" option for an Instrument Landing System (ILS) that has an out of service glide slope.
Airborne equipment provides information to the pilot showing the aircraft’s displacement from the optimal course. Where they intersect, these beams define the "on LOC" indication which is displayed on the aircraft instrumentation. These antennae transmit two narrow intersecting beams, one slightly to the right of the intended course centreline, the other slightly to the left. However, where obstacles or noise sensitive areas preclude aligning the approach with the extended runway centreline, they will be otherwise located. The LOC antennae are normally located at the end of the runway. In recent years, the beacons have been progressively replaced by a Distance Measuring Equipment (DME) source either paired to the localiser antenna or otherwise located on the aerodrome, for example with a VOR. The localiser provides azimuth guidance, while non-directional beacons, marker beacons and high intensity runways lights may also be provided as positioning aids for the approach. Vertical guidance is not provided in the vast majority of cases. Localiser (LOC) and Localiser Type Directional Aid (LDA) are ground based, non-precision, runway approach aids employing radio beams to provide pilots with horizontal guidance during an instrument approach. Localiser (LOC) and Localiser Type Directional Aid (LDA) Approaches are defined as non-precision runway approach aids based on a single ground based radio beam which provides pilots with horizontal guidance during an approach to land.
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