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DoNotFeed
26th Apr 2006, 14:01
Question to the techs out there?

Flying speeds lower than LRC is at the present a standard in our ops.
To me it was always clear the most economical speed is LRC (assumtion - no wind of course) taking into account a certain margin for cruise stability over minimum fuel flow.
Nowadays we fly well below this established speeds assuming a cost index of close to zero is minimum cost.
Flying nose high and always behind the drag curve with higher thrust setting is biting into the flight plan fuel.

In the case of a B767 there was in the old days when beancounters had no say a certain cost index, i recall something like 40 - 60 as minimum for the speed not getting too slow for real live.

Is there an aswer somewhere?

thanks for any story on this matter

hawk37
26th Apr 2006, 16:40
DNF, Where wind is not a factor, there is no reason to fly below MRC (Max range cruise) which equals minimum fuel used. LRC (Long range cruise) is defined to be the speed at which you would get 99% of the range of MRC. I'm guessing, but typically perhaps .02 mach faster that MRC (I'm not an airline pilot). The most economical speed will be where the sum of the cost of time and the cost of fuel are a minimum. This is where the cost index calculation comes into play. I can't really elaborate as I haven't read up on it. There is an airbus document available on the net somewhere called "getting to grips with cost index" which if you do a google search I'm sure you'd find.
However, if you're flying behind the drag curve, as you say, then not only are you below MRC speed, but your also at a speed below minimum drag speed. While this speed is readily available to modern airliners via a "green dot" annunciation, I'm not sure the 767 would have one. And if you were to decelerate much below minimum drag speed, it is possible you may be unable to maintain this speed, especially at high altitudes where thrust is limited.
Hawk

Intruder
28th Apr 2006, 09:50
The 744 FMS at Cost Index = 0 gives you max range for the current altitude INCLUDING wind effects. Theoretically, you cannot find a more fuel-efficient cruise speed. If you are at optimum altitude, you theoretically cannot find a better fuel burn. Also, since tailwind corrections will generate cruise speeds less than no-wind cruise speeds, all the way down to max enduarance speed (minimum fuel flow), you may indeed be significantly "nose cocked Up" in these situations, but NEVER on the "back side of the power curve."

Note that the FMS does not take into account wind differences at different altitudes, so some other way to determine best altitude for the wind profile must be found.

If the company is PLANNING your fuel for CI=0, you will have NO extra fuel available, and will burn more than planned if you run into ANY adverse weather/winds.

Jambo Buana
28th Apr 2006, 10:10
CI 0 gives you approx 1% more range than LRC. The speed schedule varies with weight, so the lighter you are the slower you fly.

Intruder
28th Apr 2006, 12:15
That is true only for the no-wind case. With a significant wind, CI=0 may yield more savings.

Old Smokey
28th Apr 2006, 12:31
Just a comment or two amongst some very well considered responses.....

Long Range Cruise (LRC) is a 'convenience' speed from a bygone era with not much practical application in the current high fuel cost era. As stated, it provides 99% of the Optimum Range available at Maximum Range Cruise (MRC). For those aircraft with which I'm familiar, CI=0 at MRC, although between different types CI's above zero are a bit arbitary, varying from type to type.

As Intruder has indicated, there is never a case where MRC could possibly be below Vmd, Minimum Drag Speed, and will in all cases, including cruise at well off optimum altitude, be somewhat above it. Vmd is for all practical purposes the best holding speed. Intruder, you have me a bit puzzled when you say "Note that the FMS does not take into account wind differences at different altitudes, so some other way to determine best altitude for the wind profile must be found". Perhaps so with the generation of the FMC/FMS that you're using, but, if a good range of winds for several thousand feet above and below the theoretical optimum altitude are inserted in, at least modern generation FMC/FMS units, the recommended altitude WILL consider wind gradient. It's not uncommon when pushing into a strong Head-Wind to see the recommended level drop to a lower level if all of the winds at all practical levels have been inserted. If only one wind has been inserted, the FMC/FMS is pretty happy to stay where it is and suffer the penalty.

With respect to the last two posts (Jambo Buana and Intruder), again for whatever CI is chosen MRC all the way through to High speed, if a full range of winds are inserted, full compensation is made for adjustment to the zero wind speed at the current level, and recommendation to an alternative level if the gradient indicates so.

Regards,

Old Smokey