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Old 17th Dec 2001, 21:58   #1 (permalink)
 
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Question Difference between Max Range Cruise (MRC) and Long Range Cruise(LRC)

Can anybody kindly explain the difference between MRC and LRC. When is MRC normally used.
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Old 17th Dec 2001, 23:19   #2 (permalink)
 
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The maxi-range is the angle of attack where you get the maximum distance for a unit of fuel (the best distance for fuel consumption). Cd = Ch/ Vs where CH is consumption per hour and Vs = ground speed.
It is easy to demonstrate that the MR is obtain for the best Cl/Cd ratio (Piston engines) and best ratio: square root of Cl divided by Cd for turbine engine.
But the cost per hour of flight is not only depending upon fuel. If you accept to increase the fuel consumption by 1% the speed will be increased by 3 or 4 %. This is the Long Range: an accepted increase of 1% in Cd in front of the MR giving an increase in speed of around 3%.
This was with no wind....
After you have to take into account the wind
Regards.
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Old 17th Dec 2001, 23:22   #3 (permalink)
 
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I forget... MR is used when you are short in fuel. Long Range is not directly in use nowadays, but the concept remains through a faster cruise monitored by the cost index... an other story..
Cheers
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Old 18th Dec 2001, 02:00   #4 (permalink)
 
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In simple terms MRC is the speed one would fly when desiring "max range" in relation to fuel consumption. In theory LRC provides a slightly less fuel efficient cruise but provides better speed stability in average conditions.
Most Flight Management Computers have the ability to select LRC as a cruise mode. Note that LRC will not change when the cost index is changed. To determine MRC on these aircraft simply input a cost index of zero into the FMC. Econ Cruise is then MRC. Note that LRC does not change. So far I have not used MRC operationally but it's nice to know how to find it and, or use it.
Cheers.

[ 17 December 2001: Message edited by: Clipper811 ]
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Old 18th Dec 2001, 07:18   #5 (permalink)
 
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MRC (or "Econ" in the 747-400) takes into account the Cost Index eneterd into the FMS as well as current winds. Therefore, the calculated Mach number will vary minute-by-minute.

LRC discounts both corrections. Therefore it will vary less, though change with Gross Weight and Altitude.
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Old 18th Dec 2001, 21:54   #6 (permalink)
 
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Just to clarify, MRC is a defined speed that provides maximum range in relation to a given amount of fuel. So "ECON" is only equal to MRC when the cost index entered is zero. When zero is entered one is telling the FMC that the only "cost" consideration for cruise is fuel. Hence "max range cruise." If something other than zero is entered (into the cost index) one is telling the FMC to consider "other" costs such as crew, mx, on time dependability, etc. Again, the reason there is a LRC mode available on the FMC vs. a MRC is that the engineers think it more advisable to fly LRC for better speed stability when fuel conservation is the priority.
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Old 18th Dec 2001, 23:50   #7 (permalink)
 
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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. ]
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Old 20th Dec 2001, 11:34   #8 (permalink)
 
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Guys thank you all for your replies. They have been very helpful
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