Originally Posted by
F-16GUY
One more question: When punching all the numbers into the FMS, to get the flex power setting, doesn't the system then also display the required distance from brakes release to rotation or T/O, so that the crew can do a sanity check whether the runway a head will suffice?
The calculation of the reduced thrust level to be used is done by the Flt Ops Performance Engineering department. They will use the manufacturer's certified data taken from the Aeroplane Flight Manual and run them against a set of unique parameters of the day. These would include typically, for any single takeoff
- runway physical characteristic and state: dry / wet
- obstacles under the pre-determined OEI lateral path
- wind, temperature and QFE
- TOW, flap setting and even CG
- perhaps some airframe deviations that may have a degrading effect.
- bleed and anti-ice configuration of the engine.
Somehow this is delivered to pilots, these days mostly run directly on the flightdeck by them. Results are the V-speeds (1, R, 2) and a temperature.
Speeds are bugged, and the temperature fed (via the FMS keypad) to the engine control units.
That resulting temperature is an assumed OAT, under which the take-off would be limiting.
To answer what was asked: Some of the modern software does have an option to display the margins of TODA / ASDA / etc. which would be there
if it really was as hot outside as the resulting temp. Normally though, a sanity check is not done because the calc is run with a given runway as one of the inputs.