The term heavy is used during radio transmissions between air traffic control and any aircraft which has been assigned a maximum takeoff weight (MTOW) rating of 136 tonnes (300,000 lb) or more. Aircraft with a MTOW rating between 7 t and 136 t are considered medium, and aircraft with a MTOW rating less than 7 t are considered light. In the US, the FAA uses a slightly different categorization, adding a block between medium and heavy, labeling aircraft capable of maximum takeoff weights more than 41,000 pounds (19 t) and less than 300,000 pounds (140 t) as large.
All aircraft produce wingtip vortices that create wake turbulence in flight. The vortex strength increases when the aircraft is heavier and when it flies slowly. Thus, the term "heavy" (unlike light, medium and large) is included by heavy-class aircraft in radio transmissions around airports during take-off and landing, incorporated into the call sign, to warn other aircraft that they should leave additional separation to avoid this wake turbulence. All wide-body aircraft are classified as Heavy, with the exception of the first two Airbus A300s produced (the A300B1, MTOW of “only” 291,000 pounds (132 t), both shorter and lighter than the mass-production A300s), while the Airbus A380 (MTOW of 575 t (1,268,000 lb)) and the single Antonov An-225 (MTOW of 640 t (1,410,000 lb)) are classified in the even larger category of super. Certain variants of the narrow-bodied Boeing 707 and Douglas DC-8 were considered "heavy" based on MTOW.
Of special note here is the narrow-bodied Boeing 757. Under current guidance, the 757 is considered large, having a MTOW of only 116,000 kilograms (256,000 lb). However, after a number of accidents where smaller aircraft following closely behind a 757 crashed, the rules were changed so that controllers are required to apply the special wake turbulence separation criteria specified in paragraph 5-5-4 in the FAA guidelines for aircraft separation, as though the 757 were heavy.