Hi guys!


One of my dads friends, who has no aviation knowlegde whatsoever, told me today that commercial jets often pass the speed of sound whilst cruising. He explained this was because of the high tailwinds they can get from the jetstream.

I know that commercial jets* cannot pass the speed of sound safely, but I don't know how to explain it to him.


Quite frankly, it's got me a bit confused too.. ???


The mach speed will go up when you get a tailwind, since the mach speed is related to the groundspeed, correct?
-Then how can you prevent an aircraft from going past mach 1 if you get a really really strong tailwind?

I know the design of modern day carriers greatly limits the speed near mach 1, but can it really prevent it?


I'm a little confused, help would be greatly appreciated. I don't need hour-long technical explanations, but a short, packed version would nice.

Think about it, if it was true, all that matters is the indicated airspeed. I don't know myself, but like you said, it would break the sound barrier, if talking about the relative ground speed.

What would rip an airliner apart from overstressing (in this case, from speed), would be because of the sheer air resistance, caused by great speeds. If there is a strong tailwind, this will, in effect, blow the air away from the aircraft, and thus decreasing air resistance (and also air flow over the wings - indicated airspeed).

Think of it this way:

If you had a 100mph tailwind, and the plane was travelling at a actual airspeed of 150mph (not talking about airliners specifically here), then the indicated airspeed would be 50mph, and therefore the aircraft would more than likely stall, due to the lack of airflow over the wings, in the direction of the tail.

In other words, I don't think that the Mach gauge in an airliner would pass Mach 1, even though they could be travelling faster than the speed of sound.

...I think! At least that's my take on all this scientific malarky. Please correct me if I'm wrong!

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I thnk you're a tad confused there Tweek. The tailwind could be 500 mph & the airspeed would still be 150. In this case the groundspeed would be 150 + 500 = 650 mph but the aircraft would not be supersonic.

In the same way a balloon has no airspeed at all. If there was no wind it would rise & fall vertically.

PS. The only supersonic commercial airliners to date were Concorde & the Tu 144.

Ah, I meant straight & level c/w passengers.
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We're in danger of getting too technical here. Going back to Ecko's original question I'll try to explain this as simply as possible. It might be easier to think of a boat on a fast flowing river. The water is the air & the river bank is the ground.

Let's say the river is flowing at 10 mph & our boat can do a maximum of 5 mph. If the boat is going flat out downstream it will be travelling at 15 mph (5 + 10) in relation to the river bank. Now turn the boat round & go upstream & it will actually be going backwards at 5 mph (5 - 10) in relation to the river bank - but it's still going forward at 5 mph through the water.

Doug's right folks, you can test this in FS if you want a practical demonstration. Set the wind to one hundred knots and then take off into the wind and go 100kts you are stationary in ground speed but your airspeed is still 100kts.

Vice versa if you go the other direction your ground speed is 200kts but your airspeed remains the same. If you want to complicate matters you could start talking about huge gusts and acceleration times but the basic princeble is sound.

I get it now, easy to explain really.


You're going faster than the speed of sound, but you don't "hit" the sound barrier cause the air around the aircraft is travelling fast also. That way the complications of supersonic flight doesn't affect the aircraft. The aircraft is over the speed of sound, but not over mach 1.

Thanks for the help guys!

Yeah, over the ground you're going faster than the speed of sound, but like several posters said, the airflow around the aircraft you're flying in does not.
Remember what Rotty said: Groundspeed is just a  number and it doesnt reflect wing performance at all  

Also remember speed of sound decreases with pressure and temperature.  

Edit: Chris F beat me to it