Formal instructions from air traffic control authorizing a specific route or action ((climb or descend, entry into controlled airspace)). Pilots may deviate from an ATC clearance in an emergency or when compliance would threaten safety of flight
EESiFlo North America, Terminology and Glossary: Aviation Terms General Aviation, Pennsylvania, USA 2002. flowmeterdirectory.com/flowmeter_terminology, (Avn - Amer) {550}