This is not limited to below FL180. This region of space includes altitudes where most student pilots and cowboy pilots fly. Stratosphere pilots (e.g. SR-71 and U-2 pilots) and Exosphere pilots (e.g. Apollo, Soyuz, and Space Shuttle) experience only brief passages through the idiosphere before continuing to spheres of higher existence.
by BocaAVMemes December 09, 2013