WHAT IS ILS? It is a ground-based instrument approach system which provides precision guidance to an aircraft approaching a runway. installed on each end of a runway. It was accepted as a standard system by the ICAO, (International Civil Aviation Organization) in 1947. Uses radio signals and sometimes coupled with high-intensity lights. Enable a safe landing during Instrument meteorological conditions (IMC), such as low ceilings or reduced visibility. Instrument Approach Procedure charts (or "approach plates") are published for each ILS approach, providing pilots with the needed information to fly an ILS approach during Instrument flight rules (IFR) operations.
HOW IT WORKS?• BASIC PRINCIPLE: ILS works on basic principle of bearing by lobe comparison.• MAIN COMPONENTS REQUIRED BY ILS:• Guidance information: the localizer and glide slope.• Range information: the outer marker (OM) and the middle marker (MM) beacons.• Visual information: approach lights, touchdown and centerline lights, runway lights.
ILS SUBSYSTEM-11. LOCALIZER SUBSYSTEM It is used to provide lateral guidance to the aircraft and thus allows for tracking the extended runway centerline. Localizer information is typically displayed on a course deviation indicator (CDI) which is used by the pilot until visual contact is made and the landing completed. It consists of:i. Localizer antenna array (ground equipment)ii. Localizer signal receiver (onboard equipment)
LOCALIZER ANTENNA ARRAY• A VHF transmitter emitting highly directional lobes is located typically 1,000ft (300 m) beyond the stopping end of the runway.• The two lobes are amplitude modulated; the one to the right at 150 Hz and the other to the left at 90Hz on one of the carrier frequency between 108.10 MHz and 111.95 MHz.• Only odd frequencies are for the localizer. There are 40 channels available for ILS localizer.• The localizer radiation patterns are normally arranged.• The dual-frequency localizers are extremely precise and can be used for the ILS categories II/III.
LOCALIZER SIGNAL RECEIVER• This equipment enables to receive the localizer signal, process it and to display the aircraft’s position on an onboard indicator .• If the aircraft on approach is aligned with the runway centerline, the CDI will display no difference in the depth of modulation (DDM) between the 90 Hz and 150 Hz audio tones; therefore, the CDI needle is centered.• If the aircraft is to the right of the centerline, the 150 Hz modulation will exceed that of the 90 Hz and produce a Needle indicates direction of deflection on the CDI towards the left. runway.• If the aircraft is to the left of the centerline, Centered Needle = Correct the 90 Hz modulation will exceed that of Alignment the 150 Hz and produce a similar but opposite deflection. (FULL SCALE DEFLECTION = 2.5• When the aircraft is outside this course DEG FROM THE CENTRE LINE) guidance sector, the CDI is required to provide full scale deflection.
ILS SUBSYSTEM-21. GLIDE SLOPE SUBSYSTEM• Guidance to touch down zone in elevation is provided by two overlapping lobes producing on inclined plane of equi- signal. It consists of:i. Glide slope antenna array (ground equipment)ii. Glide slope signal receiver (onboard equipment)
GLIDE SLOPE ANTENNA ARRAY• The transmission of the lobes for vertical guidance is in UHF band between frequencies 329.30 and 335.0 MHz with 150 KHz spacing, providing 40 channels.• The radiation is arranged such that 150 Hz modulated signal lobe is below the 90 Hz modulated lobe and the plane of equi-signal thus formed, normally defines a slope of 3 to the horizontal.• A slope of approximately 3 intersects the runway at approximately 300 mtrs (1,000 ft) from its beginning and provides a descent of 300 feet for every one nm of forward travel.
GLIDE SLOPE RECEIVER• Similar to the localizer it receives, processes and displays the signal from the glide slope array. Glide- slope path display is identical to the localizer indication.• If the aircraft is on 3 degree glide path, equal amounts of the 90 Hz and 150 Hz are received and the CDI will be centered. Needle indicates above/below• If the aircraft is above the glide glidepath. path, the 90 Hz modulation exceeds Centered Needle = Correct Glidepath that of the 150 Hz and produces a deflection on the CDI downwards. The sensitivity is set so that the• If the aircraft is below the full-scale indications occur at approx established glide path, the 150 Hz 2.3 and 3.7 degrees elevation. modulation predominates and produces a similar but opposite deflection.
MARKER BEACONS • The purpose of marker beacons is to inform the pilot about the horizontal distance from the runway touchdown zone. • All beacon types operate at a carrier frequency of 75.0 MHz and operate in such a way that they vertically transmit a cone of radio waves. • The receiver onboard an aircraft is fixed to 75 Hz and will catch the signal during antenna flyover. OM 4 to 7 NM from the runway threshold normally indicates where an aircraft intercepts the glide path when at the published altitude. MM 3500 feet from the runway threshold,is the Decision Height point for a normal ILS approach. On glide path at the MM an aircraft will be approximately 200 feet above the runway. IM 1000 feet from the runway threshold, is the Decision Height point for a Category II approach.
RUNWAY LIGHTING SYSTEM• The instrument designed to provide visibility information is called a transmissometer.• It is normally located adjacent to a runway. The light source is separated from the photo-electric cell receiver by 500 to, 700 ft.• The receiver, connected to the instrument readout in the airport tower, senses the reduction in the light level between it and the light source caused by increasing amounts of particulate matter in the air.• In this way the receiver measures the relative transparency or opacity of the air. The readout is calibrated in feet of visibility and is called runway visual range (RVR).
ILS CATEGORIESCategory I Category III A• A minimal height of resolution at 200 ft • A minimal decision height lower than 100 (60,96 m). ft (30,48 m)• The visibility of the runway is at the • The visibility of the runway is at the minimum 1800 ft (548,64 m) minimum 700 ft (213,36 m)• The plane has to be equipped apart from • The aircraft has to be equipped with an the devices for flying in IFR (Instrument autopilot with a passive malfunction Flight Rules) conditions also with the ILS monitor or a HUD (Head-up display). system and a marker beacon receiver. Category III BCategory II • A minimal decision height lower than 50• A minimal decision height at 100 ft (30,48 ft (15,24 m) m) • The visibility of the runway is at the• The visibility of the runway is at the minimum 150 ft (45,72 m) minimum 1200 ft (365,76 m) • A device for alteration of a rolling speed• The plane has to be equipped with a radio to travel speed. altimeter or an inner marker receiver, an autopilot link, a raindrops remover and Category III C also a system for the automatic draught control of the engine can be required. • Zero visibility.• The crew consists of two pilots.
LIMITATIONS1. Installation of ILS can be costly due to the complexity of the antenna system and siting criteria.2. To avoid hazardous reflections that would affect the radiated signal, ILS critical areas and ILS sensitive areas are established. Positioning of these critical areas can prevent aircraft from using certain taxiways. This can cause additional delays in take offs due to increased hold times and increased spacing between aircraft.3. Localizer systems are sensitive to obstructions in the signal broadcast area like large buildings.4. If terrain is sloping or uneven, reflections can create an uneven glide path causing unwanted needle deflections.