ILS – Instrument Landing System

[Ref ACE The Technical Pilot Interview 101/348]

(Last Update : 2020 Apr 28 1555Z)

Instrument Landing System (ILS)

ILS is a ICAO standard,  ground-based precision approach radio aid that give non-visual aid, both track guidance (lateral guidance) and slope (vertical guidance) to facilitate the aircraft to land. ILS is particular useful in Instrument Meteorological Conditions (IMC).

The ILS consists of :

  1. two highly directional transmitters [Localizer (LLZ) and Glide slope (G/P)]
  2. marker beacons

An ILS consists of two separate highly directional transmitters:

Be noticed that Localizer transmitters is the horizontal antenna array located at the opposite end of the runway while the Glide slop transmitters is located near the side of the runway almost abeam the “aiming point” of approaching side.


Localizer (lateral) – the azimuth beam works in VHF band (108-112 MHz) – is to provide tracking guidance along the extended runway centerline; and

Glide slope (vertical) – the elevation beam uses UHF frequencies (329.3-335 MHz) – is to provide vertical guidance toward the runway touchdown point. The guide path projection angle is normally adjust to 3 degrees above the horizontal plane

The glide path frequency is selected automatically when its paired VHF localizer channel is selected.

DME is usually paired with the ILS frequency so that it is selected automatically with the ILS.


Also, an ILS includes two or three marker beacons aligned along the approach:

  1. Outer marker
  2. Middle marker
  3. Inner marker

For aircraft using ILS in approach, last phase of flight will require approach lights, touch down and centre line lights, runway lights.



ILS system is subject to signal interference by surface vehicles and/or aircraft, hence “critical area” is established in front of the equipment for protection of signal.

ILS approach chart is one of the important components for ILS operations. Those charts provide a lot of useful information:

  • ILS frequency & identifier code
  • ILS inbound course & glideslope angle
  • Final approach point
  • Minimum Descent Altitude/Height & ILS Categories
  • Go-around/Missed-approach procedure





How does an ILS work?

Localizer, is a set of directional antennas, which located beyond the end of the runway. Localizer sends out radio signals in the horizontal axis of the runway. And the signals are two distinguish modulated frequency on either side of the centerline of the runway.

Glideslope, sends out signals to the vertical axis towards your runway, and is located perpendicular to the touchdown zone. In most case, the angle between glideslope signals and the runway is in a three degrees.

DME gives pilots a slant range towards the runway. DME makes it a lot easier to monitor the distance. The reason why pilots need to know the distance because localizer receiver (the plane) must be within range to receive signals. Also, there are different levels of accuracy in different distance between the planes and the runway threshold.



Read More –

How to fly an ILS?

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