Definition
A precision instrument approach using an Instrument Landing System certified to Category I standards, permitting an approach to a decision height (DH) of not less than 200 feet above the touchdown zone elevation and a visibility of not less than 1/2 statute mile (or runway visual range of not less than 1,800 feet, reducible to 1,400 feet with operative touchdown zone and centerline lighting).
Plain English
The most common type of ILS approach. The pilot can fly down to 200 feet above the runway on instruments alone. By that point they must see the runway environment to land; if they don't, they go around.
Context Anchor
Seen on instrument approach procedures and in discussions of ILS approach minimums.
Derivation
Category' here is a certification class. ILS approaches are divided into Categories I, II, and III based on how low the pilot is allowed to descend on instruments before needing to see the runway. Category I is the least demanding tier and is what most ILS-equipped airports and aircraft support.
Why Pilots Care
It sets the minimum ceiling and visibility allowed for a normal ILS landing, directly affecting whether a flight can complete its approach or must divert.
Grounding Statement
On a CAT I ILS, the instruments guide you close to the runway, but the final decision to land still requires seeing enough of the runway area at the published minimums.
Intuition Check
CAT I does not mean the first step of learning ILS approaches. It means a specific approved approach category with defined low-visibility landing minimums.
Example Sentence 1
The weather was reported at 300-foot ceilings and 3/4 mile visibility, well above CAT I minimums, so the crew briefed the ILS approach to runway 27.
Example Sentence 2
With reported visibility above minimums, the flight was cleared for the ILS CAT I to runway 27.