hi,
leaving packs on engines for t/o causes you to lose 1900 kgs or 4 degrees on the flex depending on the system you are using.
therefore selecting packs off or on the APU at high weights will increase the flex temp, reducing thermal and mechanical stresses in the engines.
The IAE 2527 A5 engine is flat rated up to ISA +31 at sealevel,therefore it requires the same EPR setting at all temp below Tref. The engine is limited for mechanical reasons in this reason. When above Tref the engine is limited by the heat generated and the EPR is reduced so as not to overheat the engine. The main reason for engine wear is excessive heat and the best way of saving the engine is to reduce the heat. This is done by reducing EPR using a Flexible take off.
bye guys