Definition
A category of ILS (Instrument Landing System) precision approach that permits landing with a runway visual range (RVR) of not less than 700 feet and no decision height, or a decision height below 100 feet. It is one of the lowest-minimums approach categories, used in very poor visibility.
Plain English
A type of instrument approach that lets a properly equipped aircraft and trained crew land in extremely low visibility — down to about 700 feet of runway visibility — with little or no decision height before touchdown.
Context Anchor
Seen in instrument approach charts, approach minimums, airport operating rules, and aircraft approval discussions for very low-visibility landings.
Derivation
The Roman numeral 'III' marks the third and most capable tier of ILS categories (Cat I, Cat II, Cat III). Within Cat III, lowercase letters 'a', 'b', and 'c' subdivide it by how low the visibility minimums go — 'a' being the highest of the three sub-minimums.
Why Pilots Care
Sets the exact visibility and height limits that determine whether a pilot may legally complete a Category III ILS landing.
Intuition Check
Do not read Iiia as a general label for “bad weather.” It is a specific landing approval category with exact visibility and equipment requirements.
Example Sentence 1
With the fog rolling in, only crews and aircraft certified for Category IIIa or lower could continue the approach.
Example Sentence 2
Only crews qualified for low-visibility approaches may use IIIA or lower minima.