View Full Version : Long Range Cruise vs Cost Index

Marty-Party

17th Apr 2004, 21:53

Can anyone please explain the difference between Long Range Cruise (on a B744) and the FMC Cost Index?

It appears that the 4 engine LRC is usually between M.85 and M.86. However, the minimum fuel economy cost index of 0 will give a broad range of Mach numbers depending on a whole heap of factors, sometimes much lower than the LRC speed.

The Cost Index 0 speed is usually below the LRC speed.

Cost Index 200 seems to get near to LRC when set to around 200 and slightly exceeds it with cost index 400.

I am lacking the knowledge to explain this.

Cheers

OPEN DES

17th Apr 2004, 22:43

Hi

As far as i know. LRC will give you 99% of max range. Cost index 0 will give you minimum fuel burn (fuel price infinitely high w.r.t. operating costs). These are 2 different concepts, hence it will give you different speeds. Cost index 999 will give you max speed (operating costs infinitely high w.r.t. fuel price). Flying according to a cost index´should´ give the company the least costs. In most companies the cost index is a function of fuel price and time based costs (operating costs).

(FYI: KLM on their 744 fleet uses a fixed CI of 150 and 400 for time critical flights.)

Hope this helps

S

SuperRanger

18th Apr 2004, 05:30

marty,

CI of zero is slower as it will give you MORE fuel on arrival at the expense of flight time (eng time cost!). LRC is a compromise between fuel burn and speed.

LRC will only give you .85-.86 AT optimum altitude. as the optimum alt increases, LRC WILL reduce (if you remain at lower alt).

the most significant difference between LRC and ECON (CI=0) is that LRC is based strictly on weight while ECON will take weight as well as WIND into consideration. ECON SPD increases with HW while decreases (to a certain extend) with TW. CI=0 will almost always be lower than LRC unless in very strong headwind conditions.

our company uses CI of 155 which approx equates LRC AT optimum altitude. again, this depends on the wind. therefore, it's prudent if one is unable to obtain a higher altitude to try other cruise speed such as LRC or select a lower CI. the WIND-ALTITUDE TRADE tables in the QRH comes in handy in situation like these.

SR

ttfset

28th Apr 2004, 15:44

The 744 FMC manual (manufacture's) states the units of cost index is pounds per hour. The formula is

CI = (cost of a/c time [$/hr])/(cost of fuel [$/lb])

the $ bits cancel out above leaving

CI in lbs per hour (confuses non Americans)

You can estimate the fuel required for a speed change eg 2 hours at CI 200 instead of CI 0 will burn an extra 400lbs of fuel

LRC is defined as 1% more fuel burn than max range (CI = 0) and is about 180 as a CI.

As an aside the N1 seems to be pretty constant at opt altitude (88% for RB211). In the old days did they not set a constant N1 and allow the a/c to slowly climb as weight decreased.