Pressure altitude is an imaginary altitude that is used for vertical separation. Since it is not practical for aircraft that fly long distances at high altitude to constantly change their altimeters to a local setting, it is agreed that above a certain altitude all aircraft use pressure altitude as a uniform altimeter setting. Each country publishes its own transition altitude. A transition altitude is the altitude above sea level where pilots must set their altimeters to standard pressure. When flying with the altimeter set to standard pressure, the altitude is expressed in flight level.
Flight level is obtained by removing the altitude's three rightmost digits.
For example and altitude of 21,000 feet will be expressed as FL210. Air density is a major factor in aerodynamic performance and engine efficiency. An increase in air temperature, altitude and humidity causes a decrease in air density. Since lift and drag are inversely proportional to the air density, a loss of lift is caused by reduced air density. The propeller efficiency is decreased for the same reason.