Aircraft Classes: For the purposes of Wake Turbulence Separation Minima, ATC classifies aircraft as Heavy, Large, and Small as follows:
a. Heavy. Aircraft capable of takeoff weights of more than 255,000lbs whether or not they are operating at this weight during a particular phase of flight.
b. Large. Aircraft of more than 41,000lbs, maximum certificated takeoff weight, up to 255,000lbs.
c. Small. Aircraft of 41,000lbs or less maximum certificated takeoff weight.
I would like to add that many airlines get a waiver or exemtion. For example I flew the Saab 340 at Amer Eagle and our op specs granted us authority to operate as a large airplane--even though we were only 29300 gross.
I'm curious, what is the purpose of the exemption? I guess to allow less separation between you and the guy in front of you, but if so, what is it about the Saab (or any other plane granted such an exemption) that makes it less susceptable to wake turbulence? Higher wing loading?
IMHO the exemption on the Saab and ATR(and probably many other models- but can on vouch for saab) had nothing to do with safety but rather economics. The FAA allowed this as they probably felt it was safe enough and did not want to congest the busy airspace(JFK,LGA,BOS,DCA,DFW,Lax, ....)and create any more delays by adding the additional mile seperation and 1 minute under certain circumstances. * Probably no different than allowing land and hold short operations in order to better utilize our resources--I think anyone can see that landing more than one a.c. at a time greatly increases the chance of incidents but are willing to look the other way