View Full Version : Long Range vs Max Range Cruise

16th Jul 2001, 05:49
What is the difference between LRC and MRC?
Any idea what Assumed Temperature method is?

Thanks for the previous contributions.

Bally Heck
16th Jul 2001, 08:40
I believe LRC throws away the econ cruise index no which allows the FMC to work out the best speed for fuel cost v aircraft cost and gives best fuel economy.

The assumed temperature method is where the man gets.....oops

I'll start again

The assumed temperature method is where the engines are derated to an asssumed temperature where they produce they produce sufficient thrust to satisfy performance requirements for the prevailing WAT conditions. This saves engine life and builds in extra performance insofar as the air is denser than assumed and therefore the aircraft will perform in excess of that temperature. :rolleyes:

16th Jul 2001, 08:57
For jets, the max range speed is in a range where speed stability is very low. Picture yourself the diagram showing drag (vert) for speed (hor). At speeds higher than minimum drag, speed stability is positive: that is, you have to add power to increase speed. At lower speeds, the airplane is speed unstable: you have to add power to fly slower.

At the speed for max range, especially when the airplane is light, speed stability is too low for comfortable flight (lots of power changes). LRC-speed is about 1% higher, which increases speed stability sufficiently for comfortable flight, while having only a minor effect on range.


16th Jul 2001, 12:57
As far as I can remember, LRC speed gives a range of 99% of max cruise range. Meaning 1% of range is sacrificed for a higher speed. :rolleyes:

16th Jul 2001, 16:36

As previously stated, LRC is defined as 99% MRC. MRC is almost unflyable - speed unstable. As well, any variation, fast or slow, means you lose range. Hence flying LRC is achievable and, if the target speed is between MRC and LRC planned range is achieved.

Assumed temperature method is a way of reducing thrust at t/o when there is sufficient performance margin. Using rated thrust costs in terms of engine life, hence it's desirable to use less than max thrust where safe.
Roughly, assumed temp means determining the highest OAT at which the a/c with the current TOW could legally t/o from a given runway. After deducting a few degrees for a margin the thrust to be set is determined by entering the books with the "assumed temp" and setting the resulting N1/EPR.

16th Jul 2001, 19:19
I'm afraid I have to disagree on the speeds, gents.

LRC is about 5-6% faster (TAS) than the best specific (optimum) range. For this increase you pay about a 1% penalty in range. Max (optimum) range is a very stable speed, only a few knots indicated less than LRC.

Min drag (L/D max, or max endurance) is MUCH slower than max range. It is speed unstable if you get on the back side of it. Generally your holding speeds are 5-10% faster then L/D max to increase speed stability.

Wtih the fuel on board, you can do only two things. Either maximize your hours and minutes until flameout - max endurance. Or, maximize the miles you can fly - max range. Any other speed is a compromize.

The industry has adopted LRC as an acceptable compromize. A 1% penalty for a 5% increase in speed (for schedule) makes sense.

17th Jul 2001, 16:08
On what jet a/c would you say that MRC speed is "very stable"??? :confused:
As for speeds, I think we all agree that LRC speed will be a few knots faster than MRC speed. My (and others) omitting the word "speed" makes no real change to the intent of the posts. To recap, you give up 1% range to gain speed stability and a small margin for error (machmeter and airframe drag factor for a start).

17th Jul 2001, 23:20
You'd better check the FAA advisory circular for "assum temp".
please check AC 25-13

This will be a little help for you.
Good day!!

19th Jul 2001, 06:05

My "very stable" term is relative, not quantifiable. It was used mainly in response to erikv's statement about speeds above min drag to be "stable". Reading his post (first paragraph) might leave the impression that MRC is in the neighborhood of min drag. It's not.

Any speed above min drag may be "stable" in the technical sense, but in real life, at speeds around min drag, you will be paying a lot of attention to throttle position.

For example on my DC-8, at 300,000 lbs, the LRC is M.81. MRC is M.79, hardly enough difference to call it an "unstable speed". At Min Drag of M.69, it will be unstable, meaning it will require close attention.

19th Jul 2001, 16:34
What I mean by unstable is that there's a lot of thrust lever activity for the poor old FEO. From personal experience in B707 and B747 classic it is much harder to stay on speed (manual thrust) at MRC rather than LRC or MCC.
As you say, MRC is not Vmd.
It needs to be clearly understood what we're trying to achieve - MRC is for most miles, Vmd is for most time.