# How Airplanes Work

Science | Modern

## Aircraft Speed

An F/A-18 Hornet emerges from a cloud created when it broke the sound barrier.
John Gay/US Navy/Getty Images

Once fueled, an airplane's minimum flight speed depends on the movement of the air around it. Maximum airspeed, on the other hand, is limited largely by technology. We use the speed of sound as the ultimate measuring stick for airplane velocity, and this is quite simply the rate at which a sound wave moves through a gas.

The exact speed of sound depends on the elasticity and density of the gas medium it's traveling through -- which means varying air pressure and air temperature prevent the existence of a global speed of sound. At 32 degrees Fahrenheit (0 degrees Celsius), the speed of sound in air is 1,087 feet per second (331 meters per second). Raise the temperature to 68 degrees Fahrenheit (20 degrees Celsius), and the speed climbs to 1,127 feet per second (343 meters per second).

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Whatever the details of the medium, we refer to the speed of sound as Mach 1, named after physicist Ernst Mach. If an airplane reaches the speed of sound, its speed is Mach 1. If the airplane reaches double the speed of sound, its speed is Mach 2.

Airplanes speeds that are less than Mach 1 are considered subsonic speeds, while those very close to Mach 1 are said to be transonic. Velocities surpassing the speed of sound are divided into high supersonic (Mach 3 through Mach 5) and hypersonic (Mach 5 through Mach 10). Speeds swifter than Mach 10 are considered high hypersonic.

If you've ever heard a supersonic aircraft fly overhead, then you've probably heard a sonic boom. Once an airplane attains Mach 1, the sound waves emitted by the plane can't speed ahead of it. Instead, these waves accumulate in a cone of sound behind the plane. When this cone passes overhead, you hear all that accumulated sound at once.

We'll head inside the airplane next to investigate which cabin systems work to keep us healthy at high altitudes.