In the UK we differ slightly from the ICAO classification. Here we use four categories still based on their maximum total weight at take-off as follows:
* Heavy: 136 000 kg or greater
* Medium: less than 136 000 kg and more than 40 000 kg
* Small: 40 000 kg or less and more than 17 000 kg
* Light: 17 000 kg or less
But there is also a note that says 'It will be noted that several aircraft types have been grouped in vortex wake categories which do not conform to those listed above. For example, the B707, DC8, VC10 and IL62 have been classified as MEDIUM as experience has shown that the characteristics of these types conform more to that group.'. In the reference table the B757 is classed as a Medium but the detailed rules say to use 4 miles instead of 3 when there's another Medium behind a B757.