Two days ago the Atlantic crossing speed record was broken three times by two Virgin Atlantic A350s and a British Airways B747. As a result, there were two types of reactions:

• People wondering how one can fly so fast without breaking the sound barrier
• Some people said that it was just not possible because these aircrafts can not pass the sound barrier.

The sound barrier is 1 224 km/h…well almost

An aircraft breaks the sound barrier when its speed exceeds that of the propagation of sound in the air, i.e. 1 224 km/h (340.29 m/s). At this moment it causes a shock wave generating the famous supersonic “bang”. By the way, if one day you hear such a noise, there is no need to look up to see the aircraft, it is already far away. Because it moves faster than sound, it is no longer physically present at the location where it generates the noise when you hear it. Anyway.

So don’t worry when in the airplane the screen in front of you indicates that you are flying at more than 1000 km/h, the plane is not in danger of breaking up because the speed of sound is not 1000 km/h as some people think but 1224.

Well, almost…

The speed of sound depends among other things on the density of the air and the temperature. And in altitude it is colder, the air is less dense so the sound propagates less quickly. Moreover, if we push the logic to the extreme, where the air is so sparse that it is absent (space vacuum) the sound does not propagate at all and the most absolute silence prevails.

At an altitude of 10,670m, which is the cruising altitude of commercial aircraft, the speed of sound is 295m/s or 1060 km/h.

So they did pass the sound barrier…or not since it is impossible for such aircrafts. In fact, it all depends on what speed we are talking about!

Air speed and ground speed

For an aircraft, two types of speed are measured. Its air speed and ground speed.

The ground speed is the speed of the aircraft as it would be seen by an observer on the ground or the speed at which its shadow would go when projected on the ground.

The air speed is the “real” speed of the aircraft in its air stream, the one it reaches by its own means, its speed in the air mass in which it evolves and is independent of the meteorological factors.

To understand the difference let’s make an analogy.

Imagine that you are standing still on a treadmill moving at 5km/h. Your own speed is 0km/h because you are not moving under your own power but for anyone watching you you are moving at 5km/h.

Imagine that you start running in the same direction as the treadmill at 10 km/h. Your own speed will be 10 km/h but seen from the outside you are moving at 15 km/h.

Well, your airspeed is the airspeed of the aircraft, and the speed at which you are moving for an outside observer is your ground speed.

The impact of winds on the speed of the aircraft

In the air the plane moves thanks to its engines, which determines the air speed. But there are winds aloft, notably the jet stream which is an air current a few hundred kilometers wide and a few kilometers thick, blowing in a west-east direction.

There are two jet streams: the tropical jet stream and the polar jet stream.

The wind speed varies between 200 and 300 km/h but can reach 400 km/h!

When a plane flies from London to New York it faces the wind. The air speed remains the same but the ground speed is reduced. On the contrary, on a New York-London flight, it is pushed by the wind (tailwind) and its ground speed increases.

A plane flying at a normal air speed of 900km/h but with a tailwind of 360km/h as it happened this Sunday has a ground speed of 1200 km/h…but its air speed remains 900 km/h.

And as you may have guessed by now, it is the air speed that is taken into account for the sound barrier, not the ground speed which includes the impact of the winds!

So the British Airways and Virgin Atlantic planes during the Ciara storm did not of course break the sound barrier, nor did all the planes that exceed 1060 km/h thanks to the tailwinds.

Photo : sound barrier by NYC Russ via Shutterstock