Just to take this a little bit further, FAA regulations restrict thrust reductions to 25% of the rated engine, (I presume that the JARs are the same.) This thrust reduction comes with strict limitations on its use, such as the runway surface condition, mels/cdls, periodic checks etc. Initially the crew performed this form of DERATING by fooling the engine into thinking that the outside temperature was hotter than it actually was, these were called "assumed temperature derates".
Although ATM Derates were sufficient for older engines they weren’t sufficient for modern day engines, which are generally overpowered, the engine manufacturers introduced a second method of further reducing the engine thrust, which were called fixed Derates. Fixed Derates are basically take an engine and pretend that its a smaller engine. The size of this size reduction are usually airline selectable and can be as much as 30-35% less than the installed engines thrust.
Fixed Derates have a number of operational advantages:
1: Less restrictive than the assumed temperature method.
2: Permits a greater thrust reduction when combined with assumed temperature thrust reductions.
3: The lower Thrust/VMCG may allow greater limited takeoff weights on certain contaminated runway lengths.
This decrease in takeoff/climb thrust is reflected in lower engine temperatures that increases engine life and reduces maintenance costs. These maintenance cost savings are considered more beneficial than the extra amount of fuel required for the slower climb associated with the reduced thrust.
Theoretically this all sounds great, but I’m still waiting for certain engine manufacturers to financially justify derating engines above 10%.
But you should be able to see that Derates are a totally different kettle of fish to Flat Rating………
Mutt.
Last edited by mutt; 16th September 2002 at 21:04.