Definition
A classification system for Instrument Landing System (ILS) approaches based on the lowest decision height (DH) and runway visual range (RVR) authorized for the approach. The categories are CAT I, CAT II, and CAT III (subdivided into IIIa, IIIb, and IIIc). Each successive category permits operations to lower minimums but requires progressively more capable airborne equipment, more precise ground facilities, additional flight crew training and authorization, and stricter operational requirements.
Plain English
ILS approaches are sorted into levels based on how low the clouds and how poor the visibility can be before the pilot must see the runway. Lower minimums mean a higher category, and higher categories require better equipment on the aircraft, better equipment on the ground, and special pilot training and approval.
Context Anchor
Seen on instrument approach procedures, in instrument training, and in discussions of low-visibility landings using an ILS.
Derivation
Category comes from an old Greek word meaning a class or kind. That helps here because an ILS category is not a runway type or an aircraft type; it is a class of ILS approach minimums.
Why Pilots Care
Selecting the right category ensures legal minima and adequate obstacle clearance during the approach.
Intuition Check
Do not confuse ILS approach categories with aircraft approach categories based on approach speed. ILS approach categories classify the approach minimums, not the airplane’s speed group.
Example Sentence 1
Because the destination was reporting an RVR of 1,200 feet, the crew briefed a CAT II approach, which their aircraft and training authorized.
Example Sentence 2
Faster aircraft in higher categories often require higher decision altitudes on the same ILS runway.