How Fast Do Commercial Airliners Fly?
Just how quickly do passenger aircraft fly?
Commercial jet aircraft cruise at about 400 – 500 knots (460 – 575 mph / 740 – 930 kph) – read more to understand the background behind it.
Speed can get a bit confusing when talking about an object moving through the air. You have a few types of speed; airspeed (and there a quite a lot of variations of airspeed) and ground speed.
Ground speed is self explanatory, it’s the time it takes you to cover a certain distance over the ground. When at their cruise altitude, aeroplanes might have a ground speed anywhere between 300 – 600 nautical miles per hour. Whilst they usually cruise at the same airspeed, the wind can make a big difference to the speed at which the aircraft passes over the ground. A tailwind pushes the aircraft along whilst a headwind slows the aircraft down.
Airspeed has a few different variations. If an aircraft is sat still on the runway and has a 20 mph headwind, the aircraft already has an airspeed of 20 mph, despite the fact it isn’t actually moving. This is because airspeed is a measure of the speed of the air over the wing. The speed of the air travelling over the wing dictates how much lift the wing is producing, and it’s this lift that allows the aircraft to support its own weight and allows it to fly.
If an aircraft has a take off speed of 140 mph, but has a 20 mph headwind, the aircraft will only need to achieve a 120 mph ground speed before it is able to take off. Conversely, if an aircraft has a 20 mph tail wind, it would need to achieve a 160 mph ground speed in order to lift off the ground.
Pilots always make reference their airspeed rather than ground speed as it is the airspeed that keeps the aircraft flying. The groundspeed is a byproduct. In principle, if you had about a 140 mph headwind the aircraft could lift off the ground without moving forward!
The Speed of Sound
When aircraft get to between 25,000 – 30,000 ft, they reference their speed to a “Mach Number”. This is simply a percentage of the speed of sound. For example, a Mach Number of 0.80 is 80% of the speed of sound. This is not a fixed speed, as the speed of sound varies with the temperature of the air.
The speed of sound at sea level with an air temperature of 15 degrees celsius is 761 Miles Per Hours. This reduces to about 660 miles an hour at -57 degrees celsius when at 36,000ft.
When aircraft approach the speed of sound, shockwaves start to form which causes aerodynamic issues. Aircraft therefore have a maximum mach number they can fly at, which is why this becomes the reference speed.