pez man,
The airbus FMGS (made by Sextant) will calculate many different factors, to ensure that flight predictions, such as fuel burn and so on are highly accurate. The airport dependant charts are used to ensure that the aircraft will actually get off the ground at the predicted take off weight.
Aircraft instruments, and aircraft performance are always calibrated against a datum called the International Standard Atmosphere, (ISA) or Jet Standard Atmosphere(JSA). The standard day is as follows
Temp 15°C at mean sea level
Pressure 1013.25Mb (hPa)
Density 1.225Kg/m3
Lapse rate (loss of temperature with altitude) 1.98°C/1000 feet
Tropopause at 36,090 feet (11,000m)
The aircraft manufacturer will calculate the weights that the aeroplane will lift and how much runway it requires to do so assuming the standard conditions. They will also publish a set of correction figures to the weights and published speeds to be applied when the true conditions deviate from the ISA Standard Day.
To illustrate this, if the take off is to be made, from, say, Mexico City, which is thousands of feet above sea lavel, and in summer more than 15°C then the air will be less dense, so the aircraft wing becomes less efficient and so will the engine. Therefore the aircraft will lift less weight. This means that a longer runway may be needed to get off the ground.
Most major airlines will publish their own performance manuals that have a simple table format to enable crews to calculate their take ff performance, including V speeds, and weights for any given runway.
It is interesting to note that the B777 (as operated by my company) has a data uplink available to transmit the performance information directly into the FMS.
Our Airbuses may well have the ability to do an uplink too, but I believe that Airbus philosophy is to keep the crew in the loop.
I hope that helps. If not, send me a Private Message.
Blue skies!
