Hi, I was asked this question the other day, and could'nt give an answer, so, On my yearly trips to the USA, I notice the flight time is usually about one hour longer LHR/JFK than the return, I know it's to do with the wind direction, but does it mean the aircraft (B747-400) can't attain it's cruise speed of M.85 on the westward leg, or do we exceed it ( unlikely, I would think) on the east bound. Any explanation of how it works would be appreciated.Fegards,Russell.

This should solve the question - the difference in flight times is affected by the flight level assigned and the length of time in the jet stream.http://squall.sfsu.edu/crws/jetstream.htmlAllcott

The aircraft does attain its cruise speed, but the point is that this speed is an airspeed, whereas to determine the flight time you need to consider the ground speed (because your destination is a fixed distance away on the ground). So if you are flying into a 50 knot headwind flying westbound for example, then all else being equal your ground speed is 50 knots lower than your (true) airspeed. Likewise if you have a 50 knot tailwind going eastbound then the ground speed is 50 knots higher than the airspeed.The combined effect of this is a 100 knot differential in ground speed between flying west and flying east which is why the eastbound flight is shorter in duration.An analogous situation is to think about running up an escalator to reach the next floor of a building. If you are running up the "up" escalator then you reach the top quicker than if you are trying to run up the "down" escalator because it is moving in the direction you want to go, even though you are running at the same speed and the distance from the bottom of the escalator to the top is the same.I hope that makes sense!

Thanks Allcott and Matthew for the excellent explanations.It now makes perfect sense to me.Russell.