Sistema de aterrizaje por instrumentos (ILS)

Descripción general

The Instrument Landing System (ILS) is the worldwide standard precision approach and landing aid, providing independent lateral (localizer) and vertical (glideslope) guidance that allows aircraft to descend through cloud and low visibility to land safely. ILS has been the dominant precision approach technology since the 1940s and remains the most widely installed system at major commercial airports globally. ICAO classifies ILS approaches into three categories based on visibility minima: Category I (decision height 200 ft, RVR 550 m), Category II (100 ft, 300 m), and Category III (no decision height / RVR down to zero).

The system consists of ground transmitters at the airport and airborne receivers on the aircraft. The aircraft's ILS receiver feeds lateral and vertical deviation signals to the flight director and autopilot, enabling fully coupled automatic approaches. With a suitably certified aircraft and crew, Category IIIb ILS approaches can be flown to essentially zero visibility, allowing operations in dense fog that would ground VFR traffic. This capability is critical for hub airports in northern climates where low-visibility conditions can occur for days at a time.

Cómo funciona

The ILS localizer antenna array, located at the far end of the runway, transmits two overlapping VHF lobes (90 Hz modulation on the left, 150 Hz on the right) centered on the runway centerline. The airborne receiver measures the difference in modulation depth (DDM) to determine whether the aircraft is left or right of centerline. At the centerline, DDM is zero. The localizer provides guidance within approximately ±35 degrees of the runway heading and ±2.5 degrees of the centerline within the approach sector.

The glideslope antenna, located approximately 300 m from the threshold on one side of the runway, transmits two UHF lobes (90 Hz above, 150 Hz below) centered on the approach path, typically at 3 degrees above the horizontal. The airborne receiver detects the DDM to determine if the aircraft is above or below the glidepath. When both localizer and glideslope needles are centered on the course deviation indicator (CDI), the aircraft is on the ideal approach path. The outer marker (OM), middle marker (MM), and inner marker (IM) — often supplemented by DME or GPS checkpoints — provide along-track position checkpoints.

Componentes principales

• Localizer (LOC): Ground-based VHF transmitter (108.10–111.95 MHz, odd tenths only) providing azimuth guidance on the runway centerline. The airborne receiver is a standard nav radio tuned to the ILS frequency.
• Glideslope (GS): Ground-based UHF transmitter (329.15–335.00 MHz) providing vertical guidance on a 3-degree descent path. Paired automatically with the localizer frequency.
• Marker Beacons: 75 MHz fan-beam transmitters at fixed distances from the threshold. The outer marker (OM, approximately 4–7 nm out) triggers a blue light and low-pitch audio tone; the middle marker (MM, approximately 0.5 nm) triggers an amber light and medium tone.
• Airborne ILS Receiver: Combined LOC/GS receiver driving deviation needles on the CDI, HSI, or PFD. Modern avionics integrate the ILS signal with the flight director and autopilot for coupled approaches.
• Approach Lighting System (ALS): Although not part of the ILS electronics, the associated high-intensity approach lighting system provides the visual transition reference at decision height.

Aplicaciones en aeronaves

• Boeing 737-800: Dual ILS receivers connected to dual autopilot channels; CAT IIIa capability with autoland. Many operators are CAT IIIb certified with fail-operational autopilot systems.
• Airbus A320-200: Dual Thales or Honeywell ILS receivers integrated with the FMGC for FLS (FMS Landing System) augmentation; dual autopilot autoland to CAT IIIb with zero decision height on approved runways.
• Boeing 777-300ER: Triple ILS receivers and triple autopilot providing fail-operational autoland; certified for CAT IIIb with RVR as low as 50 m at approved airports.
• Boeing 787-9: Dual ILS with GPS/FMS integration; Head-Up Display option augments the approach with flight path vector for enhanced crew awareness during CAT II/III operations.

Ventajas y limitaciones

ILS is the most operationally proven precision approach system, with over seven decades of service and an excellent safety record. Its independent ground and airborne components, standardized frequencies, and compatibility with autopilot autoland systems make it the backbone of low-visibility operations at major airports. The worldwide availability of ILS at more than 2,000 airports (compared to fewer than 100 with GLS/GBAS) ensures universal applicability for commercial operations.

The primary limitation of ILS is its susceptibility to interference and signal bending caused by terrain, large aircraft on parallel taxiways, or reflective structures near the antenna arrays. This forces complex critical area restrictions — aircraft and vehicles must vacate areas near ILS antennas when an aircraft is on a Cat II/III approach, reducing airport throughput. ILS also requires a straight final approach segment and cannot support curved or segmented approaches to terrain-constrained airports. GPS-based approach systems (LPV, SBAS, GBAS/GLS) can provide equivalent precision in curved approach geometries and are beginning to supplement ILS at some airports.