In article ,
Robin wrote:
When I fly on UAL I sometimes listen to the
cockpit radio on channel 9 from a general interest. Usually the pilot
will identify the flight by the airline and flight number (e.g. United
282), but sometimes (only on 747s in my experience) they add the word
"Heavy" (e.g. United 192 Heavy).

It refers to the wake turbulence category. 'Heavy' aircraft generate
the strongest wake vorticies (a byproduct of the wing producing lift)
and have to be separated more from other airplanes during certain
operations. The strength of the vortices is generally proportional to
the amount of lift (which, in order to fly level, is equal to the weight).
Heavy starts at 136,000KG, but certain aircraft (the 757 comes to mind)
generate strong wake vortices due to wing design and are called 'heavy'
without weighing that much.

--
Ben Jackson

http://www.ben.com/