Definition
A precision Instrument Landing System approach with the lowest minimums of any ILS category, permitting landings in conditions of very low or zero ceiling and runway visual range. Category III is subdivided into IIIa, IIIb, and IIIc, with progressively lower visibility requirements down to no decision height and no visibility minimum. It requires specially certified aircraft, autoland or fail-passive/fail-operational flight control systems, qualified crews, and an airport with the supporting ground equipment, lighting, and surface movement procedures.
Plain English
The most capable type of ILS approach. It lets properly equipped aircraft and trained crews land safely when the cloud base is on the ground and visibility is almost zero — conditions where a normal ILS would not be legal or safe to fly.
Context Anchor
Seen in low-visibility approach procedures, airline or operator manuals, airport low-visibility plans, and SMGCS discussions about moving safely on the airport surface in poor visibility.
Derivation
‘Category’ comes from the Greek katēgoria, meaning ‘a class or grouping.’ ICAO and the FAA use numbered categories (I, II, III) to group ILS approaches by how low the weather can be and still allow a safe landing. Higher category number means lower allowable weather minimums and more demanding equipment.
Why Pilots Care
It keeps runways open for arrivals when weather would otherwise force diversions, directly affecting schedule reliability and safety margins.
Intuition Check
Do not read “Category III ILS” as just a stronger or newer ILS. It is an approved low-visibility landing category, and it does not mean every pilot may land in near-zero visibility.
Example Sentence 1
The crew briefed a Category III ILS to Runway 27L because the reported visibility was below Cat II minimums.
Example Sentence 2
During the preflight briefing the captain reviewed the Category III ILS minima because the forecast called for dense fog at arrival.