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Instrument Landing System (ILS)
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Instrument Landing System (ILS) is defined as Instrument Landing System, a precision runway approach aid employing two radio beams to provide pilots with vertical and horizontal guidance during the landing approach.
An Instrument Landing System is a precision runway approach aid employing two radio beams to provide pilots with vertical and horizontal guidance during the landing approach. The localiser provides azimuth guidance, while the glide-slope defines the correct vertical descent profile. Marker beacons and high intensity runways lights are also part of the ILS.
The ILS localiser aerials are normally located at the end of the runway; they transmit two narrow beams, one slightly to the right of the runway centreline, the other slightly to the left (see Figure 1). Airborne equipment provides information to the pilot showing the aircraft’s displacement from the runway centreline.
The ILS glide-slope aerials are normally located on the airfield; they transmit two narrow beams, one slightly below the ideal glide-slope, the other slightly above (see Figure 2). Airborne equipment provides information to the pilot showing the aircraft’s displacement above or below the ideal glide-slope. The aerials are so located that the glide-slope provides a threshold crossing height of about 50 ft. Typically, the approach slope is set at 3 degrees, although it may be lower or higher to suit particular airfield conditions.
Two or more marker beacons may be located on the ILS approach path to indicate range from touch-down (see Figure 2). Typically, the first marker beacon (the Outer Marker) is located about 5 NM from touch-down while the second marker beacon (the Middle Marker) is located about 1 NM from touch-down. Other navigation aids (e.g. an NDB, VOR and/or DME) sometimes replace the Outer Marker or are co-located with it.
ILS information is usually fed to the autopilot to allow an automatic procedure to be flown.
An approach may not be commenced unless the runway visual range (RVR) is above the specified minimum. When an approach is flown, the pilot follows the ILS guidance until the decision height (DH) is reached. At the DH, the approach may only be continued if the runway is in sight; otherwise, a go-around must be flown.
Different categories of ILS approach are defined which allow suitably qualified pilots flying suitably equipped aircraft onto suitably equipped runways to continue the approach to a lower DH: and continue an approach at a lower reported RVR:
- Category I permits a DH not normally below 200 ft above touch down. This is usually coupled with a minimum RVR for the approach to be made of 550 m. A Category I approach may be flown manually or using the autopilot;
- Category II permits a DH of not lower than 100 ft and an RVR not less than 350 m;
- Category IIIA permits a DH below 100 ft and an RVR not below 200 m;
- Category IIIB permits a DH below 50 ft and an RVR not less than 50 m;
- Category IIIC is a full auto-land with roll out guidance along the runway centreline and no DH or RVR limitations apply. Taxi in after a landing in such conditions can be problematic and few aircraft/crew/ airport combinations are so qualified.
The output of ILS equipment is continuously monitored for signal quality and accuracy and the installation is automatically switched off if any anomaly is discovered. The reliability of this monitoring function and is progressively increased where approaches to minima lower than Cat 1 are allowed.
The special conditions which apply for Category II and III ILS operation cover aircraft equipment; pilot training and the airfield installations. In the latter case, both function, reliability and operating procedures are involved. An example of the latter is the designation of runway holding points displaced further back from the runway so as to ensure that aircraft on the ground do not interfere with signal propagation.
The Localiser (LLZ) element of an ILS installation is frequently available for a LLZ only approach to cover unavailability of the ILS GS element. This would provide only sufficient guidance for a Non-Precision Approach.