Air space between 18,000 and 60,000 feet MSL over the conterminous United States. IFR clearances are required for all aircraft operating in CLASS A airspace. Formerly called the Positive Control Area
EESiFlo North America, Terminology and Glossary: Aviation Terms General Aviation, Pennsylvania, USA 2002. flowmeterdirectory.com/flowmeter_terminology, (Avn - Amer) {550}