Woofer,
If as I suspect, you are studying for the JAR ATPL exams, this explanation might help.
Jet engine specific fuel consumption (SFC) is the amount of fuel that is consumed per hour to produce each pound of thrust. SFC is lowest when the engines are operating within the 85% to 95% RPM range. As RPM increases above or decreases below this range SFC increases.
Maximum jet aircraft range in still air is achieved when flying such that the TAS:drag ratio is maximised and the SFC is minimised. The best TAS
rag ratio occurs at 1.32 of the minimum drag speed (VIMD). This is the maximum range cruise speed (MRC). So maximum range cruise in a jet aircraft is achieved by flying at MRC at the altitude at which the drag at this speed is equal to the thrust produced at 85% to 95% RPM.
Because of the shape of the TAS:drag curve it is possible to increase speed within a narrow band, without incurring significant drag increases. At 1.15 MRC, for example, the range is still close to 99% of its maximum value. Similarly. Within the 85% to 95% RPM band, changes in RPM cause very little change in SFC. So there is a narrow band of speeds around the MRC, in which the range is very close to maximum.
But the total cost of operating an aircraft is not just the fuel costs. It also includes other factors such as equipment running costs and crew costs. Getting passengers to their destinations more quickly than your competitors do, also has a cash value. 1.32 VIMD is a comparatively low speed, so non-fuel costs can be reduced by flying faster and reducing flight duration. For greatest overall efficiency (lowest total costs) it is necessary to fly faster in order to trade off increasing fuel costs against decreasing non-fuel costs
So aircraft usually fly at a speed slightly higher than the MRC to reduce flight time whilst retaining a good level of efficiency. This is achieved by flying at the long range cruise speed (LRC). The actual value of the best speed for a given flight depends upon the distance to be flown and the relationship between fuel costs and non-fuel costs.
Exam questions tend to concentrate on things like "is mach number at LRC higher or lower than at MRC?"
[ 18 December 2001: Message edited by: Keith Williams. ]