Definition
A weight classification suffix added to an aircraft's call sign in radio communications when that aircraft is capable of takeoff weights of 300,000 pounds or more, regardless of whether it is operating at that weight during a particular flight.
Plain English
A word added after the call sign on the radio to tell controllers and other pilots that this is a very large aircraft. Pilots of these aircraft must say 'Heavy' so everyone knows a big jet is in the area and extra spacing is needed behind it.
Context Anchor
You may see this term in discussions of ATC radar weather displays or hear it when a controller describes the intensity of precipitation near your route.
Derivation
Heavy comes from an Old English word meaning weighty. In weather use, it shifted from physical weight to the idea of a large amount or strong intensity, as in heavy rain.
Why Pilots Care
Controllers apply greater separation minima behind HEAVY aircraft on approach and departure to prevent wake turbulence encounters that can cause loss of control.
Grounding Statement
On a radar weather display, heavy means the reflected signal is strong enough to mark that area as significant precipitation.
Intuition Check
Do not read HEAVY here as aircraft weight or cloud weight. In this context, it means the intensity of precipitation shown on radar.
Example Sentence 1
On initial contact with approach, the crew called, 'Boston Approach, United 42 Heavy, level flight level two-four-zero.'
Example Sentence 2
United 247 heavy, fly heading 180, climb and maintain one zero thousand.