ILS - Instrument landing system

ILS - Instrument landing system

An ILS is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway. The ILS can be used in situations such as in low ceilings or reduced visibility due to fog, rain, or blowing snow.

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, including the radio frequencies used by the ILS components or navaids and the minimum visibility requirements prescribed for the specific approach.

Radio-navigation aids must keep a certain degree of accuracy set by international standards of ICAO and periodically check critical parameters with properly equipped aircraft to calibrate and certify ILS precision.


ILS categories
There are three categories of ILS which support similarly named categories of operation.
Information below is based on ICAO, FAA and JAA; certain states may have filed differences.

Category I (CAT I)
– A precision instrument approach and landing with a decision height not lower than 200 feet (61 m) above touchdown zone elevation and,
with either a visibility not less than 800 meters or 2400 ft or a runway visual range not less than 550 meters (1,800 ft) on a runway with touchdown zone and runway centerline lighting.

Category II (CAT II)
– A precision instrument approach and landing with a decision height not lower than 100 feet (30 m), and a runway visual range not less than 350 meters (1,150 ft) (ICAO and FAA - USA) or 300 meters (980 ft) (JAA EASA - Europe).

Category III (CAT III) is subdivided into three sections:

Category III A – A precision instrument approach and landing with: 
a) a decision height lower than 100 feet (30 m) above touchdown zone elevation, or no decision height (alert height); and
b) a runway visual range not less than 200 meters (660 ft).
       
Category III B – A precision instrument approach and landing with:   
a) a decision height lower than 50 feet (15 m) above touchdown zone elevation, or no decision height (alert height); and
b) a runway visual range not less than 50 meters (160 ft) (ICAO and FAA) or 75 meters (246 ft) (JAA).

ILS History
Tests of the ILS system began in 1929, and the Civil Aeronautics Administration (CAA) authorized installation of the system in 1941 at six locations.
The first landing of a scheduled U.S. passenger airliner using ILS was on January 26, 1938, as a Pennsylvania Central Airlines Boeing 247-D flew from Washington, D.C., to Pittsburgh and landed in a snowstorm using only the Instrument Landing System.

The first fully automatic landing using ILS occurred at Bedford Airport UK in March 1964.

Source: Modified after Wikipedia