Definition
A category of ILS (Instrument Landing System) approach that permits landings with a runway visual range (RVR) as low as 150 feet but no decision height, or a decision height below 50 feet. Category IIIb is one of the lowest-minimums precision approach classifications, requiring specially certified aircraft, equipment, crew training, and airport infrastructure.
Plain English
A type of instrument approach that lets a properly equipped aircraft and trained crew land in extremely poor visibility — down to about 150 feet of forward visibility on the runway.
Context Anchor
Seen in instrument approach, low-visibility landing, airport equipment, and operating-approval discussions.
Derivation
The 'III' (three) indicates the highest tier of ILS approach categories, where Category I has the highest minimums and Category III the lowest. The lowercase 'b' is a sub-tier within Category III, sitting between IIIa (slightly higher minimums) and IIIc (no visibility minimum at all).
Why Pilots Care
CAT IIIb operations require specific aircraft certification, autoland capability, crew qualification, and ground equipment. A pilot cannot legally fly a CAT IIIb approach without all of these in place — knowing the category determines whether the approach is even available to you.
Intuition Check
Do not read Iiib as a runway name, chart note, or equipment model. It is a landing-approval category for extremely low visibility.
Example Sentence 1
The crew briefed a CAT IIIb autoland because the RVR had dropped to 600 feet in dense fog.
Example Sentence 2
Only pilots with the proper authorization may fly a IIIb approach to the published minima.