For example, the air density at 30,000 ft (9,100 m) is 1⁄3 of that at sea level, thus only 1⁄3 of the amount of air can be drawn into the cylinder, with enough oxygen to provide efficient combustion for only a third as much fuel. So, at 30,000 ft (9,100 m), only 1⁄3 of the fuel burnt at sea level can be burnt.[14] (An advantage of the decreased air density is that the airframe experiences only about 1/3 of the aerodynamic drag. Plus, there is decreased back pressure on the exhaust gases.[15] On the other hand, more energy is consumed holding an airplane up with less air in which to generate lift.)