Chapter
9
Instrument Landing System (ILS)
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ILS Components |
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ILS Frequencies . . . . . . . . . . . . |
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DME Paired with ILS Channels . . . . . . |
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ILS Identification . . . . . . . . . . . |
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Marker Beacons |
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Ground Monitoring of ILS Transmissions . . |
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ILS Coverage |
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ILS Principle of Operation . . . . . . . . |
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ILS Presentation and Interpretation . . . . |
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ILS Categories (ICAO) |
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Errors and Accuracy |
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Factors Affecting Range and Accuracy . . . |
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ILS Approach Chart |
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ILS Calculations . . . . . . . . . . . . |
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ILS Summary |
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Questions . . . . . . . . . . . . . . |
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Answers . . . . . . . . . . . . . . |
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Instrument Landing System (ILS) |
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(ILS) System Landing Instrument 9
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Instrument Landing System (ILS) |
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Introduction
The Instrument Landing System (ILS) has been in existence for over 40 years and is still the most accurate approach and landing aid in current use. The system provides pilots with an accurate means of carrying out an instrument approach to a runway, giving guidance both in the horizontal and the vertical planes. It even enables aircraft to carry out automatic landings. ILS is a precision approach system because it gives guidance in both the horizontal and the vertical plane.
ILS provides the pilot with visual instructions in the cockpit to enable him to fly the aircraft down a predetermined glide path and extended runway centre line (localizer) to his Decision Height (DH). At decision height the pilot decides to land (if he has the required visual references and sufficient room to manoeuvre the aircraft for a safe touchdown) or he goes around (overshoots) and carries out the published missed approach procedure.
Instrument Landing System (ILS) 9
Figure 9.1 The Instrument Landing System (ILS)
ILS Components
The system requires a suitable ground installation and airborne equipment. The ground installation has three distinct components as shown in Figure 9.1, namely localizer, glide path and marker beacons; in some installations a back course may also be available.
The localizer (LLZ) transmits in the VHF band and is located about 300 m from the up-wind end of the runway.
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Instrument Landing System (ILS) |
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(ILS) System Landing Instrument 9
The glide path (GP) transmitter operates in the UHF band, and is frequency paired with the localizer. It is located 300 m in from the threshold and about 200 m from the runway edge abeam the touchdown point.
Marker beacons transmit at 75 MHz in the VHF band. These include the outer marker (OM), the middle marker (MM) and possibly an inner marker (IM). They are provided to enable the pilot to cross-check the aircraft’s height against ranges and timing to the runway threshold.
Back course approaches are allowed in some countries. This enables aircraft to make a nonprecision approach on the back beam of the localizer transmitter.
Some ILS installations also have a co-located low powered NDB, called a locator (L), at the site of the OM beacon.
Distance Measuring Equipment (DME) that is frequency paired with the ILS frequencies are now increasingly provided to supplement or replace the range information provided by marker beacons.
ILS Frequencies
Localizer
The Localizer operates in the VHF band between 108 and 111.975 MHz to provide 40 channels, e.g. 108.1 108.15; 108.3 108.35; 108.5 108.55 -111.95 MHz. This part of the frequency band is shared with VOR: the frequencies allocated are odd decimals and odd decimals + 0.05 MHz.
Glide Path
The glide path operates in the UHF band between 329.15 and 335 MHz to provide 40 complementary channels. e.g. 329.15, 329.3, 329.45, 329.6 - 335 MHz.
Markers
All markers transmit at 75 MHz. There is no interference problem as the radiation pattern is a narrow fan-shaped vertical beam.
Frequency Pairing
The GP frequency is paired with the localizer and selection of the frequency is automatic. The localizer and glide path transmissions are frequency paired in accordance with the list published at ICAO e.g. 108.1 MHz is paired with 334.7 MHz, and 111.95 MHz is paired with 330.95 MHz. The advantages of this are:
•One switch activates both receivers - this reduces the pilot’s workload.
•Frequency selection is made easier and quicker as there is only one to consider.
•The potential for a wrong frequency selection is reduced.
•Only one identifier is needed.
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