Wake turbulence
During flight, lift is generated by low pressure above the wings and high pressure below the wings. This is how an aircraft is able to fly. Air, however, wants to circulate from the region of high pressure underneath the wing to the area of low pressure above the wing. This happens at the end of the wings as this is where the pressure differential ends. The air circulation is shed from the tips of the wings. The forward movement of the aircraft means that the air circulation evolves into a vortex in the wake of the aircraft. Wake turbulence exists in this vortex, or to be precise in the two counter-rotating vortices – one vortex from each wing. The severity of the wake turbulence depends on the aircraft involved.

The aircraft's size, mass, speed and the form of the wings are decisive factors. So the wake turbulence from a Jumbo jet will be significantly larger, and therefore more dangerous, than that from a smaller aircraft. That means that an air traffic controller needs to keep the size of the aircraft in mind when establishing separation between aircraft. If a smaller aircraft encounters the wake turbulence of a Jumbo jet, it can be sent into a roll around the length of the aeroplane or the pilot might lose control of the aircraft. At a high enough altitude, an experienced pilot might be able to correct this situation but near the ground an accident is much more likely.