NWSRG - yes, apologies, my initial post didn't really explain things all that well. (I did warn you my brain was in "sleep" mode after a night spent flogging across the ocean!
)
As mutt and fantom have explained, before each take off we consult the performance charts for the specific runway we will be using. Because these charts are specific to the runway, they take into account it's length, it's height above sea level and any obstacles that will affect the take off path in the event of an engine failure. The other 2 variables that affect the T/O are ambient temperature and aircraft weight. Knowing what the temperature is allows us to work out how much weight we can lift off the runway - seeing as we know how much the aeroplane weighs when it's empty, we can then work out the total amount of fuel and payload that can be loaded. (And despite what you may read in the press, in most companies the Captain ALWAYS wins the argument if it comes down to trading payload and fuel in order to meet the weight limit - it's always the freight that stays behind.
).
Once we know how much the aeroplane is going to weigh at T/O we can then consult the charts to see whether it's possible to use Flex power for T/O, or whether we will need to use TOGA. In other words, let's say that the temperature allows us to T/O at 250 tons, but the aeroplane will only weigh 230 tons, we will then be able to Flex the T/O - use less than full power and still safley get airborne, or stop on the runway, even if we lose an engine at the critical moment. The chart will also give us the V speeds to use during the T/O - V1, VR and V2.
As far as the actual power that is set, this also depends on the ambient conditions - basically it depends on the air density, which is affected by temperature and height above sea level, and also by humidty etc. The Airbus is a very clever machine (which is why I fly them - designed for dumb pilots
) and the FADEC will sense all these factors via built in sensors and automatically compute just how much thrust the engine can develop. When we put the T/Ls into the appropriate detent, the FADEC will set the corresponding thrust by itself, unlike the Boing where the pilots have to set the thrust manually - that's so much hard work.
I hope this makes things a bit clearer?
t