The Speed of Sound

It’s important to note that even when a passenger jet has a ground speed of 700 mph at 36,000ft it is still not flying faster the than speed of sound. The speed of sound relates to the mach number of the aircraft and not the ground speed – the ground speed is irrelevant to mach number. No current commercial passenger jet can fly faster than the speed of sound.

When aircraft get to between 25,000 – 30,000 ft, they reference their speed to a “Mach Number” rather than knots. 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 on the wing which causes aerodynamic issues and can lead to a high speed stall. Aircraft therefore have a maximum mach number they can fly at, which is why this becomes the reference speed.

Airspeed Examples

Lets have a look at IAS in more detail. If an aircraft is sat still on the runway but there is a 20mph wind blow straight onto the aircraft’s nose, the aircraft already has an indicated airspeed of 20 mph, despite the fact it isn’t actually moving. This is because indicated 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 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.