ILS Errors and Limitations

ILS Accuracy and Required Tracking

To be considered established on the ILS localizer, the aircraft must remain within half full-scale deflection of the localizer needle. Before starting descent on the glidepath, the aircraft must be within half-scale localizer deflection. While following the glidepath, pilots must not exceed half-scale fly-up deflection to ensure the aircraft stays within protected airspace.

Beam Bends and Signal Distortion

ILS beams can be distorted by terrain, buildings, or vehicles near the antennas, causing 'beam bends'—deviations from the nominal course or glidepath. These are assessed by flight testing and can lead to misleading indications if not recognized.

Multipath Interference

Multipath interference occurs when ILS signals reflect off objects within the coverage area, such as hangars, vehicles, or uneven terrain. These reflections can cause fluctuating or erroneous indications, especially during approach or when aircraft/vehicles move in critical or sensitive areas near the antennas.

False Glidepaths and Localizer Errors

The ILS glidepath transmitter produces multiple vertical lobes. Above the correct glidepath, the aircraft may intercept a 'false glidepath'—an erroneous signal typically at multiples of the true glide angle. Pilots must intercept the glidepath from below to avoid these false signals. Localizer errors can also occur outside the published coverage area, leading to incorrect course indications.

Facility Categories and Coverage

ILS approaches are divided into Categories I, II, and III (with IIIA, IIIB, and IIIC subcategories), each with increasing capability for lower decision heights and visibility. Category III guidance extends from the coverage limits to the runway surface, supporting autoland operations. Each category has strict monitoring and signal integrity requirements.

Critical and Sensitive Areas

The areas around ILS antennas are protected to prevent signal disruption. The critical area must be kept clear when the ILS is in use, and the sensitive area is protected during low-visibility procedures to prevent multipath and signal distortion.

System Monitoring and Failures

ILS ground stations are equipped with automatic monitors. If signal integrity is lost or parameters exceed limits, the system automatically deactivates the navigation signal (within 6 seconds for Cat I, 2 seconds for Cat II/III) and notifies air traffic control.

Summary Table: ILS Errors and Limitations

• Multipath reflections: cause fluctuating or false indications
• False glidepaths: result from multiple lobes above the main beam
• Beam bends: deviations due to environmental or structural interference
• Localizer errors: occur outside coverage or due to interference
• Strict monitoring: ensures signal integrity and rapid shutdown on failure