an,
> Surely if it take off with the wind blowing in the same direction this is more fuel economical or is this not the case?
I'm not sure about the fuel economy, good question. Is more fuel used from ground roll/tire friction, than low altitude flight?
Economics aside, the reason is more to do with safety, and the amount of rwy used, as well as a lower groundspeed at liftoff being safer.
When the plane is
airborne and actually flying, then yes, a tailwind will increase the
groundspeed and the flight will be more economical, than a constant headwind.
But as BZ explained, the aircraft flies as a result of the
airspeed, therefore the stronger the headwind, the lower the groundspeed, less runway used and better climb performance (relative to the ground). And a lower groundspeed while taking off or landing is also much safer, as the aircraft is easier to handle and to stop if necessary.
>What about landing?
Same as takeoff, if the headwind were ideally exactly opposite the rwy direction, the strongest possible
constant headwind would be ideal. Unfortunately in the real world, strong winds are a) usually not perfectly aligned with the rwy, causing crosswind/directional problems and b) are often very gusty, to the point of "windshear", which is extremely dangerous.
Here's a link to a great site with more details on how planes fly :
http://www.av8n.com/how/
Mike