Definition
Category III (CAT III) approaches are precision instrument approaches conducted to runways equipped with an Instrument Landing System (ILS) certified for the lowest landing minimums. They allow operations in conditions where the runway visual range (RVR) is below the limits of CAT I and CAT II, and in some cases permit landings with no decision height and no requirement for the pilot to see the runway environment before touchdown. CAT III is divided into three subcategories — IIIa, IIIb, and IIIc — based on RVR and decision height limits, with IIIc allowing operations down to zero visibility. CAT III operations require specially certified aircraft, autoland or fail-operational autopilot systems, qualified flight crews, and an approved ILS facility with the appropriate ground equipment and lighting.
Plain English
A type of instrument approach used in the very worst visibility, where the airplane and its automatic systems do most of the landing because the pilot may not be able to see the runway until the aircraft is on the ground — or, in some cases, not at all.
Context Anchor
Seen in Instrument Landing System (ILS) procedures, airline and transport-category operations, and discussions of low-visibility landing minimums.
Derivation
Category' comes from the Greek katēgoria, meaning a class or division. CAT I, II, and III are simply progressively lower visibility classes of ILS approaches — the higher the number, the worse the weather the system is approved to handle.
Why Pilots Care
Allows scheduled operations to continue in fog and very low visibility that would otherwise close the airport, provided the aircraft, crew, and runway meet strict certification standards.
Grounding Statement
Picture a landing in dense fog where the pilot may not clearly see the runway until the aircraft is almost on it, so the approved guidance systems must carry the approach nearly to touchdown.
Intuition Check
Do not read Category III as simply meaning “third in a list” or “more difficult.” In this context it means a specific approved class of very-low-visibility precision approach with special equipment and authorization requirements.
Example Sentence 1
Heavy fog dropped the visibility below CAT II minimums, so the crew briefed and flew a Category III approach with autoland coupled to the ILS.
Example Sentence 2
After completing the required simulator training, the captain was authorized to fly Category III approaches in revenue service.