Definition
Category III (CAT III) is the most demanding classification of precision instrument approach, used at airports equipped to support landings in extremely low visibility conditions where the pilot may have little or no visual reference to the runway environment. CAT III approaches are subdivided into CAT IIIa, IIIb, and IIIc, each defined by progressively lower decision height (or no decision height) and lower runway visual range minimums. They require specially certified aircraft, autoland or fail-operational flight control systems, qualified crews, and a CAT III-certified ILS facility and runway.
Plain English
The lowest-visibility instrument approach category. It allows landings when the pilot can barely see, or sometimes cannot see, the runway until the aircraft is on or very near it. Both the airplane and the airport must be specially equipped and approved for it.
Context Anchor
Seen in instrument approach, low-visibility landing, and radio altimeter discussions. Radio altimeter height information is especially important in Category II and Category III operations.
Derivation
Category means a class or group. III is the Roman numeral for three. In instrument approach use, Category III is the third and most demanding low-visibility landing category, beyond Category I and Category II.
Why Pilots Care
It enables safe landings at airports when weather would otherwise close the field, greatly improving schedule reliability and access.
Intuition Check
Category III does not mean the aircraft speed category, such as Category A, B, or C. Here it means a low-visibility instrument landing approval level.
Example Sentence 1
The crew briefed a CAT III approach into Heathrow because the fog had reduced runway visual range below CAT II minimums.
Example Sentence 2
Category III authorization requires both aircraft autoland capability and crew training for zero-visibility touchdown.