Wake turbulence is primarily proportional to the aircraft's weight, since this is generally directly related to the size of the aircraft and the amount of air the wings need to move to provide enough lift. Aircraft are categorized with respect to wake turbulence based on their maximum certificated takeoff weights. FAA AC 90-23G describes classes for ATC purposes as follows:
- Super - Interim class for the A380 and AN225
- Heavy - 300,000 lb or greater
- Large - 41,000 - 300,000 lb
- Small - 41,000 lb or less
Heavy and Super category aircraft are required to add their category to their callsign when contacting ATC, e.g., "Speedbird 12 Super".
Aircraft in higher categories have larger separation requirements when in front on aircraft in smaller categories, such as between takeoffs or landings, and ATC will issue caution messages cautionary advisories when required (Section 2-1-20).
The AC linked above discusses further on wake turbulence, such as how it propagates and how it should be avoided or escaped.
The FAA is also working on a 6 category system (A to F), which is in effect at MEM, SDF, CVG, and ATL. There are plans to expand this to the Metroplexes of Houston, New York, Charlotte, Chicago, and Northern California in 2015.
This approval was based on years of joint research and development by
the FAA, Eurocontrol, scientific experts in wake, and experts in
safety and risk analysis. Categories are now based on weight,
certificated approach speeds, wing characteristics, along with special
consideration given to aircraft with limited ability to counteract