Many, if not most, FAA Part 25 (Transport) aircraft want to get certified for high altitude airport operations. These operations put challenges on the aircraft in various ways that need to be tested and certified. This includes mainly pressurization systems and engine operations (starting and thermal issues). The FAA allows for 3000’ extrapolation of flight test data. The highest commercial airport is approximately 14,500’ so testing is desired at 11,500 or above. A commonly used major airport test location is at La Paz, Bolivia (13,325’). Testing at high altitudes presents some unique safety issues, mainly physiological, that I will discuss below.