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lightoutandarmed
3rd Jun 2002, 04:10
just wondering which corresponds to what...

max range cruise to endurance and long range cruise to range or is it the other way around?

thanks

mutt
3rd Jun 2002, 04:36
MRC is the highest Specific Air Range (SAR) for a given amount of fuel, LRC is 1% less.


Mutt.

john_tullamarine
3rd Jun 2002, 08:20
... on the faster side of MRC ..

Alex Whittingham
3rd Jun 2002, 09:27
Where did you get the 1% figure from mutt? I've been trying to reference that all week!

Bus14
3rd Jun 2002, 10:30
The 1% figure is taught on the Airbus Perf Course in Toulouse, but I don't have a direct reference.

The graph for an A340-314 at Fl330 (don't have the mass) gives a max SR of 62.1 NM per tonne of fuel at .783 mach, and a LRC SR of 61.5 NM per tonne at .803 mach. A significant improvement in speed for a small decrement in SR.

Bus14
3rd Jun 2002, 10:54
Or,....in reply to the original question!

As has already been said, mrc is for max range.

For max endurance you fly at the best lift/drag speed (min clean speed?). On an Airbus the nearest displayed speed to min drag is green dot, but that is the single engine min drag speed. The managed holding speed is the correct one for two engines, but (on the A320 family) the two are very close on most engine/airframe combos. Some pilots hold at green dot speed, but the purists hold at the managed speed, which may be a few knots higher.

LRC, the speed which gives a specific range 1% less than max SR, is a commercial compromise between range and speed. For what it's worth, on the military fast jets 95% range speed is used for LRC rather than the civil 1%.

Sorry about the Airbus biased reply, but it's the only info I have.

In summary, for my simple mind, I use the following philosophy:

Normal Ops - LRC

Predicted low fuel at destination - MRC (cost index 0 on an FMC/FMGC). It will only give you 1% more range, but every little helps.

Predicted early at destination (airfield closed/ poor weather, etc) - max endurance (green dot-ish on an Airbus)

(edited to acknowledge that Bik made his post while I was writing mine)

Alex Whittingham
3rd Jun 2002, 11:56
Thanks for that info guys. One thing still puzzles me, the cost index in the FMS already controls the commercial trade off between speed and fuel consumption. I understood the FMS also modifies MRC to give the best range speed for the entered winds. If it is capable of providing all the benefits of LRC in the MRC selection what then is the point of having a separate LRC speed?

lightoutandarmed
3rd Jun 2002, 14:39
thanks BIK...
do correct me please if i'm wrong. so dp davies is referring to MRC when he talks about range on p.66 of HBJ and the endurance he talks about is simply endurance. perhaps the concept of LRC wasn't around yet when he published the book???

thanks

Intruder
3rd Jun 2002, 21:51
In the 747-400, MRC takes into account both current winds and Cost Index. LRC does neither.

mutt
4th Jun 2002, 17:17
Alex,

I'm using Boeing documentation for the 1% reference, let me know your email and I will send it to you.

The use of MRC and LRC was around a long time before FMS's, you also have airlines which have this wonderful computer but cant ascertain correct costings to make it work properly :)

Cheers

Mutt.

[email protected]

Alex Whittingham
4th Jun 2002, 17:21
Thanks. e-mail is [email protected]

I'm starting to suspect that LRC is a hangover from pre FMS days with no practical application (other than planning, thanks BIK!) Am I being unduly harsh?

mutt
4th Jun 2002, 17:31
Alex remember that a badly planned flight is one where the crews have to use MRC! :)



Mutt.

(2 .jpgs @ 1 meg on the way to you. Check your email.)

quid
4th Jun 2002, 19:13
.....and, it's important to note that as you get lighter, the speeds get slower. It's never a constant indicated M or IAS.