U

Upper area control (UAC)
Upper airspace above flight level 250 is controlled by upper area control centres, such as  Karlsruhe, Maastricht, Geneva (see also "Upper airspace" and "Division level").

Uncontrolled airspace
Airspace outside the area of responsibility of the air traffic control services.

Upper airspace
According to the International Civil Aviation Organisation, the boundary between upper and lower airspace is at 24,500 ft (see also "Division level").

Universal Coordinated Time (UTC)
All air navigation services organisations in the world use UTC. It ensures that all data relating to flights refer to the same time standard.