Definition
A precision instrument approach and landing classification with a decision height lower than 50 feet (or no decision height at all) and a runway visual range less than 600 feet but not less than 150 feet. CAT IIIb approaches require specially certified aircraft, qualified flight crews, an approved airborne system (typically autoland), and a runway with the necessary lighting, markings, and protected ILS critical/sensitive areas.
Plain English
A type of low-visibility approach where the pilot can land with almost no forward visibility, using the aircraft's automatic landing system. The runway visual range can be as low as 150 feet, and there may be no decision height at all.
Context Anchor
Seen in instrument approach procedures, airline and advanced instrument operations, and FAA discussions of CAT II and CAT III approaches.
Derivation
The category numbering system for ILS approaches was set up by ICAO. Category III is the lowest-visibility tier, and the letters a, b, and c subdivide it by how little visibility is allowed. CAT IIIb sits between IIIa (which still requires more visibility) and IIIc (which allows zero visibility but is not currently authorized operationally).
Why Pilots Care
Enables commercial and general aviation operations to continue in fog and poor weather instead of diverting, improving schedule reliability and safety when properly equipped.
Grounding Statement
In practice, CAT IIIb is used when fog or very low visibility makes the runway hard to see until the aircraft is almost on it.
Intuition Check
CAT IIIb does not mean simply “a more difficult approach.” It means a specific approved category of approach with very low visibility limits and special equipment, training, and authorization requirements.
Example Sentence 1
The crew briefed a CAT IIIb approach into Heathrow with the autoland engaged and an RVR of 300 feet.
Example Sentence 2
Airport operations were restricted to CAT IIIb qualified aircraft due to the reported runway visual range of 200 feet.