Air Density Change with Altitude
The atmosphere extends from the ground up into space, and is held in place by Earth's gravity. The ground sets a definite bottom to the atmosphere, but its top is not so well defined. Instead, the atmosphere gradually fades away into nothing.
Because air has weight and is compressible, the air is relatively thick at low altitudes and becomes less dense the higher you go. This is because the weight of the air above compresses the air below.
Even though the atmosphere extends to over 300,000 feet, it is so compressed by gravity that more than half of the air in the atmosphere is below 18,000 feet. This means, although you won't be approaching outer space in your single engine airplane by any means, these air density differences still have a great effect on airplane performance.
Pressure Lapse Rate
As you increase in altitude, you move into thinner and thinner air, resulting in a pressure decrease. In the standard atmosphere, pressure decreases at a rate of 1 inch every 1,000 feet of altitude gain from sea level to 10,000 feet. As altitude increases, aircraft performance degrades as a result of this decreasing pressure.
Engine Power and Air Density
A primary limitation affecting airplane performance in a piston driven airplane is engine power. The amount of power a piston engine can produce is dependent on air pressure. The engine produces power by burning a fuel/air mixture. When lower density air is taken into the engine, the engine is actually taking in a smaller quantity of air at one time. As a result, fuel flow must be lessened to maintain a combustible fuel/air mixture. Reduced engine power results.
Since air density decreases with altitude, piston engines lose power with altitude. The propeller also becomes less efficient with altitude, since the thinner air is harder for the propeller to grip. When the airplane's powerplant is no longer able to produce an excess of thrust, the airplane will no longer be able to climb.
Note: Turbochargers and superchargers are sometimes added to an airplane. They compress the outside air, increasing the air pressure so the engine can produce more power. A piston engine without a turbocharger or supercharger is referred to as normally aspirated.
Types of Altitude
True altitude is MSL altitude, the actual height of the airplane above mean sea level. Airport terrain and obstacle elevations found on aeronautical charts are true altitudes.
Absolute altitude is AGL altitude, the actual height of the airplane above the ground. Indicated altitude is shown to the pilot on the altimeter.
Actual atmospheric pressure and temperature vary from the standard. Pressure altitude is altitude corrected for non-standard pressure. Finally, density altitude is pressure altitude corrected for non-standard temperature. On a standard day, pressure, density, indicated, and true altitude will all be the same.