Mcdhu,
Boeing also use the FLEX method of reducing thrust by assuming that the actual temperature is higher than it actually is, they call it the Assumed Temperature Method (ATM) of thrust reduction.
This method of reducing power comes with a number of restrictions, such as the requirement to demonstrate that the engine can provide maximum takeoff thrust, cant be used on contaminated runways or with MELS/CDL’s and are Limited to a maximum thrust reduction of 25% of the installed engine.
These restrictions severely penalized the amount of times that reduced thrust could be used so the concept of DERATED engines was initiated. This is basically treating the engine as if it’s a smaller completely different engine, complete with its own AFM data; this removes most of the above limitations and also allows for thrust reductions greater than the ATM limit of -25%.
Take the GE-90-90B engine which is rated at 90,000 lbs of thrust, this engine has two fixed DERATES picked by the airline, TO1 and TO2, we basically limit the installed engine thrust by –10% and –20% and treat this aircraft as if it has 3 different types of engine.
Approximately 90% of our takeoffs are at TO1, the vast majority of these are then combined with Assumed Temperature Method thrust reductions and an increased environmental envelope up to 69°C, so thrust reductions of approx 35% (-10% DERATE and then -25% ATM) are achieved and the takeoff is planned using approx 65% of the installed engine thrust.
The purpose behind all this is to increase engine life and decrease maintenance costs. It can also provide greater takeoff weights on contaminated runways due to the lower VMCG as discussed above.
There is also a “feeling” that a large engine working at less than full grunt is less likely to fail during the takeoff.
Cheers
Mutt.
Last edited by mutt; 25th Jun 2002 at 04:47.