hi alli was flying today and on the fmc of the above mentioned ac, 738, there is an option on the FMC to select LRC, i selected LRC and after a while i selected 'Econ' however the mach speed on LRC was slower than that of Econ, i thought it would be viseversa to save fuel. Is this correct?I was getting a slower speed in LRC mode than Econ mode.

Expect LRC to be more economical in terms of fuel than Econ. Parasitic drag increases with the square of the velocity, plus several other factors.

ok i suppose LRC will give slower speed.

M 0.785 (TAS 450 kn) at FL410 is the LRC speed asccording to specs posted at www.b737.org.uk. Max range at LRC with max payload 2926 nm for the -800.KSAN to PHNL is about 2300 nm so you can see why the -800 is used for this Pacific hop, Just an FYI. Plenty of reserve for headwinds.

Depends on what you used for the cost index (and on whether the PMDG FMC models these function accurately). LRC stands for long range cruise. There is another speed called maximum range cruise (MRC), which is the speed at which the airplane will achieve the most nautical air miles flown per pound of fuel burned (nam/lb). LRC is a speed higher than MRC that results in a 1% reduction in nam/lb. Prior to the advent of FMC's and cost index-based econ speeds, LRC was commonly flown as it provides a fairly significant increase in speed with only a small fuel burn penalty, and it is an easier speed to fly from a speed stability standpoint.Don S.

thanksjust as a matter of interest i always use 99 as the CI.

Long Range Cruise is the most economical mode and Econ mode is the normal one. As is stated somewhere in the manual (also in the 747 manual and/or 747 type rating courses) LRC gives you the best economy for the actual altitude. The difference in fuel consumption is not big but after several hours it makes a great difference for air companies.For example PMDG 747, in my case, normaly flies at mach 0.86, if I select LRC from the CDU CRZ page it slows down to mach 0.80.

>Long Range Cruise is the most economical mode and Econ mode>is the normal one. This is not always a true statment. LRC and ECON don't take into account head/tailwinds (i.e. its a "static" environment). Sometimes ECON will give you the better fuel economy as well as the best time, especially if you've got a tailwind. LRC will keep you in the air longer because you are flying slower, therefore result in a higher fuel burn. However, if you've got a decent tailwind, one can take advantage and land with more fuel reserve in ECON than LRC.