Definition
A precision instrument approach using an Instrument Landing System (ILS) that permits descent to a decision height (DH) no lower than 200 feet above the touchdown zone elevation, with a runway visual range (RVR) no less than 1,800 feet (with touchdown zone and centerline lighting, otherwise 2,400 feet) or a visibility no less than 1/2 statute mile. It is the least demanding of the three ILS categories in terms of equipment, crew qualification, and weather minimums.
Plain English
A standard ILS approach where the pilot must be able to see the runway environment by 200 feet above the runway, with at least about half a mile of visibility. It is the most common ILS approach and has the highest weather minimums of the three ILS categories.
Context Anchor
Seen on instrument approach charts, in instrument training, and when briefing or flying an ILS approach to a runway.
Derivation
The 'Category' system comes from ICAO and FAA classification of ILS approaches by how low and in how poor visibility a pilot may legally fly them. CAT I is the entry-level category; CAT II and CAT III allow lower minimums but require more sophisticated equipment and training.
Why Pilots Care
Sets the minimum weather conditions under which a pilot may legally continue an ILS approach to a landing.
Intuition Check
Do not read CAT I as meaning the approach is automatically safe to land from. It means the approach has Category I minimums; you still must meet the published visibility requirement and see the required runway references before continuing below decision altitude.
Example Sentence 1
The forecast called for a 400-foot ceiling and one mile visibility, so a CAT I ILS approach was well within minimums.
Example Sentence 2
With the reported visibility at three-quarters of a mile, the flight was authorized to conduct the CAT I ILS approach.