Any aircraft CAPABLE of a takeoff weight of 255,000 lbs or more, is classified as "heavy". This applies to some, not all, B757-200s, and to all B757-300s. B707-320s, and 420s are Heavies. Some, not all, DC-8s are Heavies. The KC
-135s that are reengined with CFM-56 engines are "heavies".
The requirement is based on what takeoff weight the aircraft is capable of.
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