What are critical areas on an airfield?
The zones around the ILS localizer and glide path antennas that aircraft and vehicles must stay out of, or move through only under control, so the landing signal stays clean — and the holding positions, stop bars and signs that keep traffic outside them.
An instrument landing system guides an aircraft by radio signals from the localizer and glide path antennas. A large metal object moving into those signals — a taxiing aircraft or a vehicle — can distort them. Critical and sensitive areas are the ground zones defined to prevent that. The areas themselves are defined in ICAO Annex 10; the airfield lighting, marking and signs that stop traffic at their edge come from Annex 14.
1Critical area and sensitive area
The critical area is an area of defined dimensions about the localizer and glide path antennas where vehicles, including aircraft, are excluded during all ILS operations. The sensitive area extends beyond it: parking and movement of vehicles, including aircraft, is controlled there to prevent unacceptable interference during ILS operations, mainly from large moving objects outside the critical area but still within the airfield. Neither has a single fixed size. The dimensions depend on the antenna, the aircraft and vehicles that could interfere, and the category of operation, so they are defined for each installation.
2How the airfield enforces them
Where a taxiing aircraft or vehicle could interfere with a radio navigation aid such as the ILS, Annex 14 calls for a runway-holding position on the taxiway. In general a CAT I runway-holding position protects the critical area, and a CAT II/III holding position, set farther out, protects the larger sensitive area. Each is shown on the ground by the yellow holding-position marking and a red mandatory sign, and Annex 14 calls for a stop bar at runway-holding positions serving a runway intended for use when runway visual range is below 550 m, with limited exceptions (appropriate aids and procedures, or, between 350 m and 550 m, procedures that limit movements to one at a time). Aircraft hold outside the area until the tower clears them. The sensitive-area protection is normally applied only while low-visibility procedures are in force (the CAT II/III position is used then); in good weather aircraft may use the CAT I position. Each aerodrome sets its own trigger.
| Feature | Critical area | Sensitive area |
|---|---|---|
| Rule | Vehicles and aircraft excluded during all ILS operations | Parking and movement controlled during ILS operations; locally enforced mainly in low visibility |
| Extent | Defined dimensions about the localizer and glide path antennas | Extends beyond the critical area |
| Protected by | In general, the CAT I runway-holding position | In general, the CAT II/III holding position, farther out |
| Visual cues | Yellow holding-position marking, red mandatory sign, red stop bar where installed | |
Frequently Asked Questions
Q1What is an ILS critical area?
An area of defined dimensions about the localizer and glide path antennas where vehicles, including aircraft, are excluded during all ILS operations, because their presence would cause unacceptable disturbance to the ILS signal in space.
Q2How is an ILS sensitive area different?
A sensitive area extends beyond the critical area. Parking and movement of vehicles, including aircraft, is controlled there to prevent unacceptable interference with the ILS signal during ILS operations, mainly from large moving objects outside the critical area but still within the airfield.
Q3How do critical and sensitive areas affect taxiway lighting and marking?
Taxiways that enter these areas are protected by holding positions: yellow marking across the taxiway, a red mandatory sign, and in low visibility a red stop bar. In general a CAT I runway-holding position protects the critical area and a CAT II/III holding position, farther out, protects the sensitive area.
Q4Are critical area dimensions fixed?
No. They depend on the antenna type and position, the aircraft and vehicles that could interfere, and the category of operation, so they are defined for each installation. Typical examples are given in ICAO Annex 10, Volume I, Attachment C.
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