Definition
A precision instrument approach and landing classification with a decision height not lower than 200 feet above the touchdown zone elevation, and either a visibility not less than 1/2 statute mile or a runway visual range not less than 1,800 feet. CAT I is the most commonly available precision approach category and is flown by pilots holding a standard instrument rating without special authorization.
Plain English
CAT I is the basic level of precision instrument approach. It lets a pilot descend on instruments down to 200 feet above the runway, where they must see the runway environment to continue and land. If they can't see it, they must go around.
Context Anchor
You will see CAT I in instrument approach discussions, ILS minimums, and alternate airport planning for commercial operations.
Derivation
The roman numeral I marks this as the first and least demanding tier of precision approach categories (CAT I, II, III). Lower categories allow lower decision heights and visibility, but require additional aircraft equipment, crew training, and authorization.
Why Pilots Care
It defines the lowest weather conditions under which a pilot may legally continue a precision approach to land or must execute a missed approach.
Intuition Check
Do not read CAT I as meaning a type of aircraft. Here, CAT means Category, and CAT I describes the approach operation and its weather minimums.
Example Sentence 1
The ILS to Runway 27 is a CAT I approach with a 200-foot decision height and 1/2 mile visibility minimum.
Example Sentence 2
Alternate planning for a CAT I airport requires the forecast weather to be at or above those published limits.