ILS stands for Instrument Landing System and is a standard International Civil Aviation Organisation (ICAO) precision landing aid that is used to provide accurate azimuth and descent guidance signals for guidance to aircraft for landing on the runway under normal or adverse weather conditions. Instrument landing system (ILS) facility is a highly accurate and dependable means of navigating to the runway in IFR conditions. The ILS provides the lateral and vertical guidance necessary to fly a precision approach. When all components of the ILS system are available, including the approved approach procedure, the pilot may execute a precision approach. 



The ILS consists of: -

  1. Localizer:- The primary component of the ILS is the localizer, which provides lateral guidance. The transmitter and antenna (Shown above) are on the centreline at the opposite end of the runway from the approach threshold.
  2. Glide Path:- The glide path component of ILS �provides vertical guidance to the pilot during the approach. Glide path �is located 750 to 1,250 feet (ft) down the runway from the threshold (shown above), offset 400 to 600 ft from the runway centre line.
  3. Markers:-

    (i) Outer marker; (OM): The outer marker (if installed) is located 3 1/2 to 6 NM from the threshold within 250 ft of the extended runway centreline to provide the pilot with the ability to make a positive position fix on the localizer.



    (ii) MIDDLE MARKER (MM): The middle marker ( if installed) is located approximately 0.5 to 0.8 NM from the threshold on the extended runway centerline. The middle marker crosses the glide slope at approximately 200 to 250 ft above the runway elevation.
  4. DME : Distance Measuring Equipment (DME) is normally collocated with glide path and provides slant distance to the aircraft with respect to touch down point.
  5. The approach lighting system:-Various runway lighting systems serve as integral parts of the ILS system to aid the pilot in landing. Any or all of the following lighting systems may be provided at a given facility: approach light system (ALS), sequenced flashing light (SFL), touchdown zone lights (TDZ) and centerline lights (CLL-required for Category II & III operations.)�
  6. RUNAWAY VISUAL RANGE (RVR) : In order to land, the pilot must be able to see appropriate visual aids not later than the arrival at the decision height (DH) or the missed approach point (MAP).
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