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View Full Version : What is "long range" (sounds simple)?


MasterBates
19th Sep 2003, 07:30
One thing has often puzzled me.
In the FMC (757 w/ or w/o Pegasus), when LRC is selected there is a very little difference in speed and predicted arrival fuel, apart from ECON CRZ. When, however, cost index is set zero (fuel most expensive part of the formula) the range increases significally in ECON.
I always thought LRC meant long range! Why could this be, and what is LRC according to the programmers of our software?

MasterBates.

:confused:

LEM
19th Sep 2003, 17:09
It would be great if we had a "Maximum range" Cruise page in the FMC.

LRC is defined as the Maximum Range reduced by 1%.
That's because we must go faster when flying into a headwind to keep the best fuel mileage.
LRC will give the maximum range with a 100kts headwind, with a very low penalty in case of a tailwind.
Typically LRC is Maximum Range speed plus 10-25kts.

In your example, a very low cost index gives a very low speed, which gets closer to Maximum Range (remember LRC is faster, and is optimized for a strong headwind.)
In other words, with a tailwind the slower you fly the longer distance you can cover with your fuel.


:\ hope I didn't say too many b******t!

;)

slice
19th Sep 2003, 17:39
I am surprised that FMCs don't include the ability to calculate for max range allowing for head/tailwind - after all it's just max range speed, GS and a bit of calculus. But if as you say there is not much in it maybe they just didn't bother ?

MasterBates
19th Sep 2003, 21:51
Thanks LEM
This makes sense. I hadnīt figured the headwind effect!
Wouldnīt it be nice though if the LRC would be calculated for the head or tailwind put in the FMC so as to give the max range.
:ok:
MB

GlueBall
25th Sep 2003, 00:47
In most RVSM airspace and on the Tracks LRC can rarely be used as airplanes are required to maintain assigned Mach cruise for spacing.