Pinched from another forum. Could this possibly be true?
A UK based schedule carrier does the following on their Airbus fleet, they make the a/c think (prgram it) that its in certain atmospheric conditions (incl altimeter settings etc) when its really not at all- these programmed settings allow for a more fuel efficient take off burn, this can only be done if runway lenghth allows it. They carry a lap top in the flight deck which calculates all this for 'm, so they can program the a/c with most efficient settings