I believed as mentioned here, warm up times have
different meanings for different engines. For the
Trent engines, Rolls is a bit concerned about the
oil system (heat exchangers ect) when the oil is
excessively cold and viscous requiring a 50 deg C
oil temp prior to higher power. Their counterparts at General Electric are not quite so concerned,
-10 deg C for the CFM(if memory serves me right).
But have a look at the oil pressures involved,
normal idle pressure for the Trent is around 70
psi where for the GE (CFM 56, CF6-50, CF6-80)
its' around 20 psi. Viscous oil will have more of
an impact on the Trent when advancing power.

The other reason for warm up more common to all
engines, is to allow the turbine section to
thermally stabilize prior to high power. A good
example at the moment is the CFM56 on the A340,
a great engine but it works hard to get airborne.
For CFM's getting tired, EGT margin becomes
critical and a good warm up period can help
prolong life on wing.

N2

Yeah BMEP, particularly if Herman Nelson was on strike. Blow pot time!:p

EGT margin, what are the acceptable limits? Are you provided with the data and the OATL?