Definition
A precision instrument approach category that permits an ILS approach and landing with a Runway Visual Range (RVR) of less than 700 feet but not less than 150 feet, with no decision height or a decision height below 50 feet.
Plain English
A very low-visibility ILS approach. The pilot can fly the approach and land when forward visibility down the runway is between 150 and 700 feet, using cockpit instruments almost the entire way down -- and sometimes all the way to touchdown without ever needing to see the runway by a set altitude.
Context Anchor
Seen on instrument approach information, company procedures, and low-visibility landing discussions for airports equipped and approved for very low-visibility ILS operations.
Derivation
ILS stands for Instrument Landing System. 'Category' here means a graded level of capability, ranked I through III, with III being the most demanding. The III-B subdivision is one step more capable than III-A and one step less than III-C, sorted by how little visibility is required.
Why Pilots Care
It sets the exact visibility and height limits that decide whether a flight can land safely or must divert, directly affecting schedule reliability and safety in fog or heavy rain.
Grounding Statement
This is an ILS landing category for conditions where the runway may not be clearly visible until the aircraft is almost on it.
Intuition Check
Do not read Category IIIB as simply meaning “more advanced” or “harder.” It is a specific low-visibility operating category with defined limits and required approvals.
Example Sentence 1
The crew briefed an ILS Category IIIB approach into Heathrow because the reported RVR was 400 feet.
Example Sentence 2
After touchdown in 150 feet visibility, the autopilot kept the aircraft on the runway centerline using ILS Category IIIB rollout mode.