It's travelling at 801mph relative to the ground.

If I'm swimming down stream at 1 mph in water moving at 4mph, relative to the bank I'm doing 5mph relative to the water 1mph.

No sonic booms where involved.

I'm quite well aware of that. I'm not sure how you read my post as saying anything different.

boyter asked how anything but time from A to B could be relevant. Well, not damaging the plane is relevant. And therefore the airspeed not being 801 MPH matters.

Just an interesting fact that the 787 has been tested to m0.97 in a dive, pretty impressive! But not near 801 MPH airspeed.

https://web.archive.org/web/20110408073905/http://www.flight...

Yes, for those readers who may not be familiar - the speed of sound changes with altitude. At sea level, it is around 750mph but at 40,000 feet it is around 620mph IIRC. It is affected by the density of the air (which is a combination of the air pressure and temperature) - hence why they use Mach numbers to denote airspeed above certain altitudes as that takes into consideration the air density to give you a speed relative to the speed of sound at that altitude.

> [The speed of sound] is affected by the density of the air (which is a combination of the air pressure and temperature)...

This is incorrect. The speed of sound in an ideal gas is a function of temperature only, and not its density. Namely, it is sqrt(gamma x R x T), where R and gamma are unchanging properties of the gas.

Thank you. I stand corrected.