Basically the jet engine is a heat engine - the hotter it gets the more thrust it produces. The trade off is that the hotter it gets the shorter its life.The more you open the trottle, the more thrust you get, but this is balance against useful life. The RR Spey in the BAC 1-11 had a much better life than the Spey installed in the Phantom! . .The aircraft manufacturer is looking for a thrust to meet his certification requirements.. .As much of the marketing is aimed at operations above ISA the perfomance is generally rated to +15C although early in the engine developement this can be lower.. .The certified thrust of a particular mark of engine is found in the Type Data Sheet with the maximum ISA temperature at which that thrust can be maintained ( flat rating), above that temperature the thrust falls off. As an example the Type Data sheet for the Rolls Royce RB211-22B-02 shows the engine as producing 41,030 thrust pounds up to ISA+13.9o. This is the flat rating for the engine. Anything from 0 to ISA + 13.9 produces 41,030 lbs and above that the thrust falls off. The -524D4 engine produces 51,980 lbs up to ISA +15o. .Originally, different thrusts meant a different model of engine but it is now common for the same model and variant of an engine to have different thrust ratings even for engines with no physical changes. These are refered to as -A thrust or -B thrust etc. The significant difference is that engines with the same internal parts (e.g. turbine disc's) have a different life on these parts, depending on the thrust level that the engine is operated to.. . <img src="wink.gif" border="0"> <img src="wink.gif" border="0">