No hawk, it's a little difficult to do a comparison of weights because of the way airbus have set up their charts. When we calculate t/o speeds and flex temp for given conditions, we always use the speeds computed for a given runway length, pressure alt and temp, regardless of our weight because we assume the highest permissible RTOW and use those speeds, the speeds would never change, only the flex temp would. Yes MD83FO has it right.
For example with the numbers I have used above at -20 degrees the max RTOW is 83.8. And the calculated V speeds are based off the assumption of a 3000 meter runway, -20 degrees and 0ft PA and 83.8t. Now suppose the aircraft was only loaded to 70t TOW. We would find that the max ambient temp at which a 70t a/c could takeoff given the runway length and PA. In this case it would be ~67 degrees. So we would use a flex of 67 but the V speeds would not change. If on the other hand we had a a/c loaded to 77t the max temp given the runway and PA is ~51. So we would use flex 51 BUT THE SAME SPEEDS AS BEFORE.
Now I understand this is a quirk of Airbus as they base their speeds off the max RTOW, so all aircraft using those numbers are safe, If its any help the max RTOW in the first instance (-20 degrees) is 83.8t, whereas the max RTOW in the second instance is 77.6t (50 degrees). So even with a LOWER TOW (77.6t) the V speeds when expressed in TAS are higher due to the lower temp. The temperature is dictating the speeds in this instance not the weights.
BTW this is not to say weight doesn't have an effect, it's just that airbus don't let it by always assuming the max permissible T/O weight. I think that if someone had charts from something smaller, a piper or Cessna we could find the answer.
Last edited by Airmann; 31st May 2012 at 14:59.