Definition
A precision instrument approach using an Instrument Landing System certified to Category I standards, providing lateral guidance from the localizer and vertical guidance from the glide slope down to a decision height (DH) no lower than 200 feet above the touchdown zone elevation, with a visibility requirement typically no less than 1,800 feet runway visual range (RVR) or 1/2 statute mile.
Plain English
An ILS approach that lets a pilot fly down to 200 feet above the runway in low visibility. At that point the pilot must see the runway environment to continue; if not, they go around.
Context Anchor
Seen when briefing or flying an ILS approach on an instrument approach chart, especially in the minimums section and approach briefing.
Derivation
"Category" comes from Latin categoria meaning a class or grouping. ILS approaches are sorted into categories (I, II, III) based on how low the pilot can descend before needing to see the runway. CAT I is the least demanding category and the standard ILS most pilots fly.
Why Pilots Care
It sets the exact lowest altitude and visibility you may use to continue the approach and land, directly affecting whether you can complete the landing or must go around in marginal weather.
Grounding Statement
On this approach, the instruments guide you toward the runway, but you still need enough outside visibility at the decision height to continue to land.
Intuition Check
Precision does not mean the approach guarantees a perfect landing. Here, precision means the procedure provides both horizontal and vertical electronic guidance. CAT I does not mean the easiest or safest approach; it means a specific ILS category with specific minimum visibility and decision-height limits.
Example Sentence 1
The weather was reported at 300 feet overcast and 1 mile visibility, which was above the minimums for the CAT I ILS to Runway 27.
Example Sentence 2
With reported visibility at three-quarters of a mile, the crew was legal to fly the ILS CAT I precision approach.