The term 'HEAVY' is actually used to warn other aircraft of possible
hazards because of wake turbulence. The term is used to remind
controllers and pilots to allow for adequate separation of aircraft
using the same routes. FAA definitions are:
Term: Take-off weight: Examples:
----- ------------------------- ---------------------
HEAVY greater than 300,000 lbs. 747, DC-10, B-52, C-5
LARGE 12,500 - 300,000 lbs. C-130, 727, 737, 757
SMALL less than 12,500 lbs T-38, C-12
There are complex rules about the amount of separation based on what
type is following what type (HEAVY following HEAVY, LIGHT following
HEAVY, etc.) and in what environment (ground taxiing, IFR final
approach, straight VFR flight, etc.). Basically other planes follow
HEAVYs with more separation than SMALLs.
A Boeing 727-231 has a take-off weight of 142,000 lbs., in the LARGE
category.