ICAO defines three wake-turbulence categories of aircraft (individual countries may have additional categories):
- LIGHT = max certificated takeoff weight less than 7000 kg (15,500 lb)
- MEDIUM = between 7000 kg and 136,000 kg
- HEAVY = higher than 136,000 kg (300,000 lb)
The word "heavy" is stated after the call sign of heavy aircraft, because this category of aircraft poses the greatest risk to other aircraft in terms of wake turbulence. Emphasizing the presence of a heavy aircraft helps ATC to optimize arrival/departure planning, and gives operators of other aircraft increased situational awareness.