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Home > Flying the Plane > Does sonic booms arise from a strong tail wind?
Does sonic booms arise from a strong tail wind?
Flying - Flying the Plane
Saturday, 17 March 2012 00:59

Supersonic Flight, Sonic Booms
 
Hi Capt Lim,

A Boeing 777 cruising at 450 knots is being blown by a jet stream of 180 knots; the resultant ground speed is 630 knots (450 + 180). This is 48 knots (630 - 582) greater than the speed of sound at 12000 meters...

Does the sonic boom reach the ground? Does the pilot have to throttle back to avoid this?

Lee Ferguson,

Hi Ferguson,

I wrote about this in the Travel 3Sixty magazine here under the heading, 'From Subsonic to Supersonic'

Passengers have often asked me if an aircraft encounters a strong tail wind in a jet stream, can it break the sound barrier.

The answer is ‘No’. Airbus and Boeing planes flying currently are all subsonic planes. It means they are designed to fly below the speed of sound. Even if there is a strong tail wind pushing the plane forward and the ground speed has exceeded the theoretical speed of sound - in reality, the plane has not gone supersonic.

This confusion arises from the misunderstanding of ground speed and the plane’s speed. Ground speed is the speed at which an object travels relative to a fixed point on the Earth’s surface. The difference between ground speed and airspeed is caused by the influence of winds on the overall speed of the aircraft.

For example, if you were walking at a speed of 2 mph on a moving walkway (walkalator or travelator), which is also moving at 2 mph, your actual speed as observed by a person NOT on the moving walkway would be fairly speedy – 4 mph (2 + 2). However, as far as you are concerned, you are still walking at 2 mph!

So the question of sonic boom does not arise and the pilot need not have to throttle back! Hope that answers your question.

PS. To check for any latest updates or postings, you can follow my new Twitter at @CaptKHLim


Ground Speed Versus Airspeed


Sonic Boom


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