Piston engines do not operate well in thin air, and this is one of the reasons why most small planes fly at altitudes of below 15,000 feet. But in fact, small aircraft generally fly much lower than that for a variety of other reasons.

Small general aviation planes fly at lower altitudes (typically between 2,000 and 10,000 feet) primarily because they are often unpressurized and use piston-driven engines that operate more efficiently in denser air. Unlike large commercial jets, which have pressurized cabins and jet engines that thrive in the thin, low-drag air of the stratosphere (30,000+ feet), small planes would require oxygen systems for the pilot and passengers if they flew much higher than 12,000 feet. Additionally, many small aircraft are flying shorter distances where climbing to high altitudes would be inefficient; the fuel and time spent climbing would outweigh the benefits of thinner air. Flying at lower altitudes also allows pilots to navigate using Visual Flight Rules (VFR), where they can maintain sight of the ground and landmarks. Furthermore, air traffic control often reserves higher altitudes for faster, turbine-powered commercial traffic to maintain safe separation. While flying low makes the plane more susceptible to weather and turbulence found in the lower atmosphere, it is the natural "sweet spot" for the performance capabilities and mission profiles of most small private aircraft.