Airplanes fly at an amazing speed, and it's hard to put into words how fast they can go. But it's also important to understand how fast a plane travels, because there are some speeds that a commercial airplane should never go near.
Most passenger aircraft, like Boeing 787s and CRJ 200s, fly around 488 knots at cruising altitude. This is well below Mach 1, and is safe for passenger aircraft. The fastest commercial jets, however, are capable of flying at much higher speeds. The SR-71 Blackbird, which was once the world's fastest plane, achieved speeds of over 2,000 mph.
How fast a plane flies depends on many factors. One is the size and weight of the aircraft. Another is the wind. An airplane will fly faster in tailwinds, and slower when flying into headwinds. It's also a factor of how high the aircraft is flying, because the air pressure decreases with altitude.
The most common way to measure an airplane's speed is indicated airspeed (IAS), which is displayed on a pilot's airspeed indicator, often called a "pitot tube" or PFD. But pilots rely on another type of airspeed for their flight, ground speed (GS). GS is calculated from IAS by adding in the aircraft's speed over the ground, and then correcting it for any positioning instrument errors.
GS is also what determines whether or not an airplane can break the sound barrier. If an airplane reaches the speed of sound, it creates shock waves that can damage the aircraft and cause dangerous aerodynamic problems. Only a handful of airplanes have ever reached supersonic speeds, including the Concorde and military fighter jets.
