ILS Approach Categories (CAT I/II/III)

What the Categories Classify

CAT I, II, and III are not different landing systems — they are operational categories describing how low an aircraft may descend, and how little visibility is required, during an ILS-guided approach. Each category pairs a Decision Height (DH), the altitude at which the crew must see the runway environment or go around, with a minimum Runway Visual Range (RVR) or visibility value.

The Categories

Three-Part Requirement

Flying a given category legally requires all three elements to line up: the ground infrastructure (ILS signal quality, approach and runway lighting, RVR reporting), the aircraft's certification and autoland or autopilot capability, and the flight crew's training and currency. An airport can be CAT III-equipped while a particular aircraft or crew is only approved to fly CAT I minima there.

Why Categories Exist

Low-visibility conditions — fog, heavy precipitation, low cloud — are a major cause of missed approaches and diversions. Standardizing exactly how low and how limited-visibility an approach can safely go lets airlines dispatch flights with confidence into marginal weather, and lets air traffic control apply consistent separation and runway protection procedures, such as ILS critical area protection, for the lower categories.

Regulatory Basis

The category framework originates in ICAO Annex 6 (Operation of Aircraft) and the detailed procedures in ICAO Doc 8168 (PANS-OPS), with national authorities such as the FAA and EASA publishing the certification and operational approval requirements airlines must meet to use each category.