Definition
For the purposes of Wake Turbulence Separation Minima, ATC classifies aircraft as Heavy, Large, and Small based on maximum certificated takeoff weight. Heavy aircraft are those capable of takeoff weights of 300,000 pounds or more, whether or not they are operating at that weight during a particular phase of flight. Large aircraft are those of more than 41,000 pounds maximum certificated takeoff weight up to but not including 300,000 pounds. Small aircraft are those of 41,000 pounds or less maximum certificated takeoff weight.
Plain English
Air traffic control sorts aircraft into three size groups -- Heavy, Large, and Small -- using the most an aircraft is certified to weigh at takeoff. The groups are used to decide how much spacing to keep between aircraft so smaller ones don't fly into the wake of bigger ones.
Context Anchor
Seen in AIM glossary material, ATC separation discussions, and wake turbulence guidance for takeoffs, landings, and aircraft following another aircraft.
Derivation
“Class” comes from a Latin word meaning a group or division. That helps here because aircraft classes are not individual aircraft types; they are groups used by ATC for a specific purpose.
Why Pilots Care
A pilot cannot legally operate an aircraft without holding the correct class rating for its type, directly affecting training path, checkride requirements, and operating privileges.
Intuition Check
Do not confuse aircraft classes here with pilot certificate class ratings, such as single-engine land or multiengine sea. In this AIM glossary context, aircraft classes are ATC wake-turbulence groupings.
Example Sentence 1
Because the airliner ahead was a Heavy, the tower applied extra spacing before clearing the small trainer to take off behind it.
Example Sentence 2
A private pilot with only the single-engine land class may not operate a seaplane without the additional sea class endorsement.