Yesterday on the 747 flight from JFK to AMS the flight map showed speeds of 1135kph and 707mph as we flew over England. Is that even possible? I was waiting for a sonic boom. Captain did mention 'extreme winds".

Likely IAS - 'Indicated Airspeed', which is not corrected for instrument deviation and added winds. It's not the true airspeed or groundspeed, so in the end you likely weren't traveling at transonic speeds

Nope, that's not IAS, but ground speed. IAS is indeed indicated airspeed based on your present lift in standard conditions (sea level, temperature etc.) and does not exceed 340 knots. True airspeed is IAS corrected by the altitude and temperature. Ground speed is true airspeed + wind. In order to see sonic booms your true airspeed (not the displayed ground speed) would need to exceed mach number. On high altitudes, the upper speed limit is bounded primarily by mach number (ratio of your speed to speed of sound), because a conventional aircraft would stall (high speed stall) if they come too close to the speed of sound.

Many years ago I was on a flight from EDI-LTN and the captain made an announcement to the effect of "we have a very strong tail wind that we could break the sound barrier, but we aren't allowed to do it over land"

didnt think much much of it at the time, but anyone know if that's correct ?

My understanding is that it is not correct. The speed to break the sound barrier does not include the wind.

interesting.

Many thanks.

Depends on what speed is shown. Speed in Mach only really make sense with regard to the altitude. Indications in kph or mph usually refer to groundspeed.

When looking at IAS you need to consider the environment the aircraft is flying in. E.g. Take moving walkways:

• If you stand on them. Your speed compared to the the moving walkway is 0. Yet your acutal groundspeed is the speed of the moving walkway.
• If you walk on the moving walkway, you walk walking pace compared to the moving walkway. Your groundspeed is the combination of your walking speed and the speed the moving walkway is operating at.
• You walk backwards on the walkway at the exact speed the moving walkway is operating. Your ground speed (walking pace + speed of the walkway) is 0.

Yes, it's possible. No, there won't be a sonic boom.

The speed of sound does not include wind -- if the air in a certain place is moving in a certain direction at 200 km/h, then the speed of sound in that place will be 200km/h faster as well. A plane flying there can exceed "the speed of sound" (in still air) easily by flying within 200km/h of that value. It's not moving faster than the speed of sound in the air around it, but its ground speed is faster than that.

(This is quite unlike the speed of light which is absolute.)

The airshow always shows the ground speed.

Also, a sonic boom wouldn't be heard if you were in the plane even if it did cross the sound barrier -- the boom is heard only outside the plane when the shockwave passes.

FlightAware seems to concur...

http://flightaware.com/live/flight/K.../EHAM/tracklog

I always assumed that with a high tailwind the ground speed would be higher, as the air speed would always be the same irrelevant of winds.

Some time ago on a KLM 737-700 we surpassed 1000km/h at FL390 which I thought was cool. Don't think I ever flew that high or fast short haul since. It cut quite some travel time off the AMS-ATH flight too!
http://upload.xandrios.net/1489136788_IMG_0289.JPG