An ILS (Instrument Landing System) is an instrument landing aid used in aviation to help land planes in both normal and low visibility conditions.

The system works through airports transmitting 2 sets of radio waves that provide directional and positional data to aircraft. Pilots use that data to safely land aircraft.

There are multiple categories of ILS for different conditions and altitudes.

ILS isn’t always used by pilots, and some airports even provide alternative navigational aids. Yet, ILS has been internationally used since it was first tested ILS in 1929, and the first operative ILS system was introduced at the Berlin-Tempelhof Central Airport in Germany in 1932.

When ILS is Used

ILS is used when a visual approach is not possible because of bad weather, a nighttime landing, or other conditions. ILS use is at a pilot’s discretion with ATC permission.

Pilots sometimes even use the ILS localizer and glideslope for non-ILS landings. 

Pilots will also sometimes use ILS during clear weather landings, but most prefer visual approach landings since they have better control and more reaction time during emergencies. 

How ILS in Aircraft Works

ILS has these 3 main ground facilities that provide these functions:

  • Localizer: The localizer creates a horizontal plane to display the center runway line. 
  • Glide Path: The Glide Path creates a vertical plane for approaching the landing point at a right angle.
  • Marker Beacons: Marker beacons line up along the approach line. 

ILS uses 2 radio signals sent from the airport transmitters to the aircraft. 

Localizer: The localizer laterally guides the aircraft. A transmitter placed on the runway’s end sends it to the aircraft. 

Glideslope: The glideslope vertically guides the aircraft. Transmitters usually placed on the runway’s sides send the signal to the aircraft. The aircraft’s receiving equipment interprets both signals using their relative strength to determine position. 

There Are 3 ILS Categories

There are 3 major ILS categories, with multiple sub-categories that specify at what altitude pilots should switch to visual guidance from ILS.

Category 1

Pilots should fly visually at 200 feet above the runway.

This category is primarily for small general aviation aircraft.

Category 2

Pilots should fly visually between an altitude of 100 to 200 feet. 

Category 3 

This category is for flying below an altitude of 100 feet, such as during automatic landings in low visibility by an aircraft’s autopilot.

Category 3 is also divided among the following 3 sub-categories 

Category 3 A Approach 

A Category 3 A Approach is a precision instrument approach with either no decision height or a decision height below 100 feet (30m) and a visual runway range greater than 700 feet (200m). 

Category 3 B Approach

A Category 3 B Approach is a precision approach landing with either no decision height or a decision height below 50 feet (15m) and a visual runway range between 150 feet (50m) and 700 feet (200m).

Category 3 C Approach

A Category 3 C Approach is a precision approach and landing without either a decision height or visual runway range limit.

Most Planes Have ILS

Most planes have ILS, but pilots don’t always use ILS. The use of ILS is at the discretion of pilots with ATC permission.

Smaller planes generally only fly a Category 1 ILS, but larger aircraft can fly all 3 categories.

Flight control systems mostly control ILS approaches, and flight crew provide supervision in larger aircraft. 

ILS is not available at all airports, and some airports provide other navigational aids, which are often less accurate than ILS, too.

In conclusion:

  • ILS is an instrument landing aid that helps pilots land planes, primarily during low visibility conditions.
  • ILS is most often used during nighttime landings or bad weather conditions.
  • There are different ILS categories for different altitudes and situations.
  • ILS is internationally used and recognized, but some airports provide alternative navigational aids.
  • ILS is also mostly used by large aircraft, with smaller aircraft only being equipped for certain ILS categories.