All aircraft create disturbances in the air, known as wake vortex, when they fly. If a following aircraft flies in this disturbed area it can cause potentially dangerous control problems. The amount of disturbance depends on a number of factors, including aircraft speed, configuration (if the wheels are up or down, whether the flaps are deployed etc) and weight. Controllers need to take account of the wake vortex when separating aircraft, and since the amount of disturbance varies with aircraft weight, aircraft are split into groups depending on their maximum take off weight. The 3 basic internationally recognised groups are light, medium and heavy. Although some countries such as the UK may have slightly different grouping such as the UK's light, small, medium and heavy. (The Airbus A380 is subject to review and at the moment is being treated as a special super-heavy category). On first contact with an air traffic control unit, heavy aircraft add the word heavy to their callsign as an extra reminder to controllers of the need for extra spacing due to their size.