Econ V LRC
 

In our tech book it says the most fuel efficient way to fly is at LRC but everytime I have compared LRC to Econ speed, Econ has you arriving at destination with more fuel.
So how can LRC be more fuel efficient?

You aren’t comparing apples with apples..... :)

LRC is calculated to be 99% of the MRC for a specific weight/altitude, it doesn’t account for wind.

ECON is calculated using an airline specific cost index value and accounts for the wind, so it is quite conceivable that ECON will provide greater fuel savings than LRC.

Mutt

So what's the point of LRC if it does not account for the wind? And when would you use it?

LRC is a remnant from the Jurassic period when the pilots, flight engineer, and navigator had to figure out the best altitude and speed to fly. LRC gave a good reference speed that would allow faster flight than max range without significant increase in fuel use, and to which you could add/subtract wind corrections.

It's also good to know if the FMS craps out...

Greeings
And during the Jurassic era we had constant Mach, Max Endurance, Long Range Cruise.
The other negatif aspect of the LRC is that we are not far from the bottom of the drag curve, so any speed drop and you are in the unstable territory and it takes power to go back to LRC.

2ENGOP I also asked this question when I was doing a perf eng course (737ng) at Boeing. They said it was requested by most of the airlines? long time past - That LRC FMC should produce the same data as the aircraft perf manuals available to the pilots on the flightdeck? (no wind) as mentioned. What Mutt said.

But as to its usfulness as a function in everyday life without wind data, I dont know. But its why you find most Airlines (that I know anyway 73) use a standard CI which gives a good speed Vs fuel burn and ECON will ajust speed for wind which is what you want. Some will produce a CI for every flightplan. Irispective you will go further using LRC with wind data, Im not sure I agree its the most efficent way to fly as you tech book says - too many variables. Hence proper CIs and ECON.

Anyone care to add, correct?

Rgds

Way back in the BODs, bad old days, most aircraft cruised a fair bit faster than LRC and to decellerate to LRC gave a considerable gain in arrival fuel, although a longer sector time.

LRC is faster than MaxRC and was artificially set at 99%MRC, ECON is in the middle somewhere.

These days aircraft ECON speeds are a little slower that LRC and thus more fuel efficient.


Regards,
BH.

Actually, ECON is the same as MRC with 0 wind at CI=0.

ECON corrects MRC for wind, and will further adjust for crew costs if desired (CI).

On the 777, MRC is, as intruder said, CI=0. LRC is CI=100.

We regulally fly with CIs of less than 100 and, therefore, would indeed speed up and use more fuel at LRC.

LONG range cruise doesn't mean LONGEST rang cruise!!

LRC was originally defined as 99%MRC, that is it was slightly faster than MRC but only slightly less efficient. Part of the reason for cruising at LRC is that minor speed variations, faster or slower, will tend to average out whereas any variation from MRC will be worse. LRC is also a little more speed stable than MRC.

Regards,
BH.

correcting a value
 

Greetings
a decrease of 1% range with a gain of 5% to 7% in TAS, that was the idea :ok: