By Tom Turner

Flying a Baron 58P at Flight Level 250 some time ago, it occurred to me that there is no federal requirement for training in this pressurized, turbocharged twin beyond the Multiengine-Instrument ticket I earned years before in a 150-hp Piper Apache. Later, in unpressurized, turbocharged airplanes I was regularly flying at 20,000 feet using supplemental oxygen, and giving the only checkout new owners of similar airplanes would likely get in flight "up there."

Eventually it came to me that operations near and in the flight levels have several significant and subtle differences from altitudes where most of us fly, differences that can range from nuisance to deadly hazard. What are those…