B757 performance
With regards to flap setting for take off at differing temp:
Most of our performance charts are now optimum flap. In many airports, and taking one as an example - at 32C and hotter, the charts stipulate flap 5 which is a field limit. Between 23C and 30C, they stipulate flap 15 which is a 5 min thrust limit. At 18C and cooler, they revert to flap 5 which again is a field limit. All these examples are zero wind.
So, my question is, does anyone know why the flap and limit type change around 28C, whereas BOTH hotter and cooler requires the use of in this case flap 5? This scenario is very common at many runways.
Hope I've explained this adequately.
Thanks