The landing is one of the complex phases in flying especially during low visibility conditions and the Pilot needs accuracy in identifying the runway to make smoother landings.
Instrument Landing System (ILS) allows pilots to conduct precision landings with the help of ground-based instrument guidance. ILS is designed to provide both lateral and vertical guidance to aircraft to achieve the precise way of flying with Lower landing minimums as possible.
History
In the early, 1930s the airports installed the earliest form of approach lighting for pilots to provide descent guidance during low visibility conditions and this approach was called Glide-slope / Glide-path.
The Radio Navigation aids were under development around the same time to provide lateral guidance. The first radio navigation called the Four-course radio range was introduced in 1929 to guide the pilots with help of Morse code signals. But these methods proved difficult in terms of usability and reliability.
ILS Invention
The Instrument Landing System (ILS) was designed and began its testing in 1929. The ILS system incorporated the best features of both approach lighting and Radio beacons with higher frequency transmissions.
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 ILS system.
After 1941, The United States Civil Aeronautics Administration (CAA) approved the installation of ILS systems in six locations across the United States.
The ILS adopted continuous improvements over the period of time with an improved course indicator (Localizer) which assists lateral assignment of the aircraft an Improved glide path or landing beam to show if the plane was above or below the glide slope, and two marker beacons for showing the progress of approach to the landing field.
On January 15, 1945, the U.S. Army introduced an ILS with a higher frequency transmitter to reduce static and create straighter courses, called the Army Air Forces Instrument Approach System Signal Set 51
In 1949, the International Civil Aviation Organization (ICAO) adopted this army standard for all member countries. In 1964, the first ILS equipment for fully automated landings occurred at Bedford Airport in the UK.
How does ILS work?
The Instrument Landing System is common and most used precision runway aid consisting of two radio beacons (Localizer & Glide-slope) to provide lateral and vertical guidance to achieve safer landings.
ILS LOC (Localizer) aerials are located at end of the runway which transmits two narrow intersecting beams (Slightly left and right of the runway centre line).
The intersection of these two beams assists aircraft to identify the centreline of the runway by aligning laterally (left and right alignment with help of airborne equipment).
ILS GS (Glideslope) located on the aerodrome also transmits two radio beams (slightly above and below the required altitude) and creates a vertical profile.
ILS GS (Glideslope) located on the aerodrome also transmits two radio beams (slightly above and below the required altitude) and creates a vertical profile.
The intersection of these beams defines the GS and it provides vertical guidance (assist aircraft to align higher or lower according to GS) to the aircraft.
Distance Measurement
The distance measurement between the aircraft and the runway touch-down is important for the pilots to make decisions and conduct missed approach (Go Around) when unable to establish visual contact, especially during low visibility conditions.
In the year 1930, the Marker Beacons (Outer Marker, Middle Marker and Inner Marker) were specifically designed to use along with Instrument Landing System to determine the position along an established route to the runway.
The Marker Beacons were actively used up to 1960 and then ILS integrates with upgraded modern equipment such as DME (Distant Measurement Equipment) and the latest Satellite-based systems.
ILS Approaches
ILS approaches are specifically designed for suitable / ILS-equipped runways in an airport which provides better IFR guidance. The pilot follows ILS guidance until the specified decision height of the ILS approach and will only make the decision to continue to landing if a visual reference of the runway is available else missed approach (Go Around) will be conducted.
In Addition, ILS Approach Plate / Chart explains ILS frequency, different ILS categories and relevant weather minimums (DH and RVR).
The different ILS categories were designed with different landing minimums based on aircraft suitability and pilot qualification.
These weather minimums are used as part of the Pre-flight planning process to validate whether the landing is legal at the planned destination based on forecasted weather data at the time of landing.
Future Systems
The traditional ILS aids were slowly replaced by the help of satellite-based navigation aids to reduce the infrastructure costs of the airport and as well as to meet the future air traffic demands. There are some alternative systems are designed to replace the ILS,
There are some alternative systems are designed to replace the ILS,
Microwave Landing System (MLS)
Transponder Landing System (TLS)
Localizer Performance with Vertical guidance (LPV)
Ground-Based Augmentation Systems (GBAS)
These ground-based approach aids were slowly replaced by Satellite-Based Navigation and Approach Aids (GNSS) to meet the future aviation demands.