Uncontrolled airspace, generally the airspace from the surface up to 700 or 1,200 feet AGL in most of the U.S., but up to as high as 14,500 feet in some remote Western and sparsely populated areas
EESiFlo North America, Terminology and Glossary: Aviation Terms General Aviation, Pennsylvania, USA 2002. flowmeterdirectory.com/flowmeter_terminology, (Avn - Amer) {550}