Definition
An Instrument Landing System approach category that permits operations down to a runway visual range (RVR) of not less than 700 feet, with no decision height limitation or with a decision height below 100 feet. It requires specially certified airborne equipment, a qualified flight crew, and a Category III-certified ILS ground installation.
Plain English
A precision ILS approach that lets a properly equipped and trained crew land in very low visibility — as little as 700 feet of forward visibility along the runway — and either descend below 100 feet before deciding to land, or use no fixed decision height at all.
Context Anchor
Seen on instrument approach charts, low-visibility landing procedures, and airline or operator approval documents.
Derivation
"Category" comes from Greek katēgoria meaning "a class or grouping." ILS approaches are sorted into categories (I, II, III) by how low the visibility and decision height can be. Category III is then split into A, B, and C — with A being the least demanding of the three.
Why Pilots Care
Allows continued operations and safe landings at airports when weather would otherwise close the runway, provided the aircraft and crew hold the required certification.
Intuition Check
Do not read Category IIIA as just a harder version of a normal ILS. It is a specific low-visibility landing approval with exact height and runway-visibility limits.
Example Sentence 1
With the fog rolling in and RVR reported at 800 feet, only aircraft and crews authorized for ILS Category IIIA could continue the approach.
Example Sentence 2
Only crews and aircraft certified for Category IIIA may conduct the approach when ceiling and visibility fall below standard minima.