Instrument Landing System

What Is an ILS?

The Instrument Landing System (ILS) is the international standard precision approach and landing aid, providing both lateral (localizer) and vertical (glideslope) guidance to pilots approaching a runway in instrument meteorological conditions. It enables aircraft to execute approaches down to a Decision Altitude (DA) as low as 50 feet above the runway threshold, or to land fully automatically with no visibility restriction at all.

How It Works

The ILS has two primary radio components:

• Localizer (LOC): A VHF antenna array at the far end of the runway transmitting on 108–111.95 MHz. It produces two overlapping beams (modulated at 90 Hz and 150 Hz) — the aircraft's receiver compares the signals to determine left/right deviation from runway centerline, displayed on the CDI needle.
• Glideslope (GS): A UHF antenna system beside the runway transmitting on 329–335 MHz, producing a 3° descent angle (typically). The aircraft receiver compares 90/150 Hz signals to display above/below glidepath deviation.

Marker beacons (outer, middle, inner) provide distance references. Modern ILS installations often replace markers with DME or GPS distance readouts. The autopilot can couple directly to ILS signals for autoland. The FMS sequences ILS frequencies automatically when an ILS approach is active in the flight plan.

Evolution and Modern Systems

ILS was standardized by ICAO in 1949 and has remained the primary precision approach system ever since. Category I ILS requires 200 ft decision height and 550 m visibility. Category II allows 100 ft DH and 350 m RVR. Category III (IIIa/IIIb/IIIc) enables approaches to 50 ft or zero DH with RVR as low as 75 m — effectively zero visibility landings. The Airbus A380 and Boeing 777 are certified for Cat IIIb autoland. Competitors include GBAS (Ground-Based Augmentation System) and SBAS (satellite-based) approaches, but ILS remains dominant globally. A HUD is often required for lower-category Cat III operations.

Regulatory Requirements

ICAO Annex 10 defines ILS specifications globally. Airports must conduct regular ILS flight inspections (using calibrated aircraft) and maintain critical and sensitive areas clear of vehicles and aircraft during low-visibility operations. Airlines must be approved for specific ILS categories; crews require special training, recent experience currency, and the aircraft must have certified avionics for the applicable category.