ILS: Instrument Landing System

ILS: Instrument Landing System

Instrument Landing System - aviationnepal
Figure 1 – The description and placement of the individual parts of the ILS system

ILS: Instrument Landing System

The final part of the flight, landing is one of the most difficult phases of the flight and becomes very hard in case of poor visibility. Instrument Landing System (ILS) is a precision runway approach aid based on radio waves guidance system which provides assistance during landing in case of poor visibility. It provides pilot with both horizontal and vertical guidance during an approach to land. Ground equipment required by ILS is two directional transmitting systems: localizer and the glide scope, along with two or more marker beacons. The on board equipment comprises of a frequency selector, receiver and antennas for localizer, glide scope and marker transmissions. Received information is decoded and displayed to the pilot by an indicator which may be Course Deviation Indicator (CDI), Horizontal Situation Indicator (HSI) or Electronic Horizontal Situation Indicator (EHSI) depending on the airplane.

The localizer (LOC) provides the azimuth guidance using two narrow intersecting beams and is usually located at the end of runway. One beam is transmitted slightly towards the left of the runway centerline while other is transmitted towards the right. The point of intersection of these two beams define the “on LOC” indication. This can be seen in the figure below. Localizer works on one of the 40ILS channels within VHF ranging from 108 to 112 MHz, each localizer’s frequency first decimal is an odd number. The signal emitted has an angular width of 30 to 60 and assists the course guidance throughout the descent path to the runway threshold from a distance of 18NM from the antenna and between a height of 1000ft above the highest path along the approach terrain and 4500ft above the antenna site.


The ILS glide scope (GS) also works in a similar manner as the localizer. They are normally located on the aerodrome and transmit two narrow intersecting beams; one slightly below the vertical line another slightly above the vertical line. The point of intersection of these two signals give the “on GS” location. A simple representation is shown in the figure below. It operates on 40ILS channel of UHF band from 329.15 to 335 MHz. It radiates its signal only in the direction of localizer front course. It is usually located between 750ft to 1250ft from the approach end of the runway and offset between 250ft and 650ft from the runway central line. Normally it is usable to a distance of 10NM but can be extended based on request. The usual GS angle is 30 but exceptions may occur such as for terrain or noise abatement.


The marker beacons are located at notable distance from touchdown. The middle marker is located at a distance of 1000m to 2000m from runway threshold and the outer marker is located at a distance of 4500 to 7500m from the middle marker. The inner marker (only installed on airport with category III landing information certification) is installed 305m from the runway threshold. The distance between the airport and the airplane along the beam is unknown, except at these marker beacons. The indications pilot receive while passing over a marker beacon is shown in the table below:

Outer Blue 400 Hz, two dashes per second
Middle ... Amber 1300 Hz, alternate dash and dot
Inner …. White 3000Hz only dots

The navigation receiver is the control unit where pilot selects the ILS frequency and normally entered in a NAV 1 selector. NAV 1 selector may be a frequency selector (tuned by rotating knobs), band selector (heavier airplane) or mode selector (NAV, DME).

In the indicator (of any type) the localizer indicator is represented by a vertical line, bar or rectangle which can move from left to right inside the instrument with reference to the centered position. The glide path indicator is represented by either a horizontal line or a bar which can move top to bottom position or a small triangle or square next to the graduation bar on the left or right side of the instrument.

An ILS approach may not be normally continued unless the Runway Visual Range (RVR) is above the specified minimum. ILS is used by pilot until the decision height (DH) is reached. At the DH, the approach may not be continued if the specified visual reference is met, otherwise, a go-around must be flown.

Categories of ILS are defined by precision approach categories. The three categories are: CAT I, CAT II and CATIII. CAT III is further subdivided into three categories: CAT III A, CAT III B and CAT III C

Category of Operation Decision Height (DH) Runway Visual Range (RVR)
CAT I DH ≥ 200ft RVR ≥ 1750ft or VIS ≥ 2400ft
CAT II 100ft ≤ DH < 200ft RVR ≥ 750ft
CAT III A No DH or DH <100ft RVR ≥ 700ft
CAT III B No DH or DH <50ft 150ft ≤ RVR< 700
CAT III C No DH No RVR limitation

Most of the ILS are certified with CAT I, about two hundred are categorized CAT II and about 50 ILS are certified CAT III, mostly in Western Europe which has the worst flying weather conditions in the developed world. Some special regulations are applied on CAT II and CAT III ILS like aircraft equipment accuracy shall be compatible with the ILS category flown, Pilot shall be trained and qualified for ILS category flown and Airfield shall be compatible with ILS category selected.


Article By;
Saurabh Agrawal Undergraduate Student (Aeronautics Elective) Pulchowk Campus, IOE