When you compute your takeoff performance margins, with a Flex temperature (Airbus) or Assumed Temperature (Boeing), margins are calculated as if OAT is equal to your Flex temperature. Actually with real OAT, your true speed will be slower than calculated so your real margin is greater than displayed margin.
Enter the data used for performance calculation :
- RWY length (ASDA)
- Stop Margin calculated
- FLEX (Assumed temp)
it will display your true margin. In this example, calculated margin=4m, real margin=433m.
There is an empirical method to calculate (3% per 10°C difference) but here is the physical calculation based on energy.
BUT DON’T THINK YOU CAN STOP 400m AFTER V1 ! If you stop at V1 you’ll stop 400m before the end. BUT if you stops 400m after V1, you’ll get out of runway… (energy increases as speed^2)