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which speed conserve fuel the most?

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which speed conserve fuel the most?

Old 17th May 2012, 14:34
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which speed conserve fuel the most?

some one says LRC speed conserve fuel most, but others say use ECON Speed while CI=0 save the fuel most . today I took a look on 737, these two speeds are different but i can't figure out which one is better . can anybody tell? THX.
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Old 17th May 2012, 14:50
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You could use your FMC to find out. Make sure all your enroute winds are on the RTE DATA page. Have one MCDU with the fuel remaining at destination showing. Use the other MCDU to change the Cost Index ...without executing (give it time, it takes a few seconds to calculate)... until you find the CI with the highest fuel remaining at destination. Then Execute that CI. Do NOT use LRC. It isn't wind compensated.
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Old 17th May 2012, 15:16
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CI 0 gives minimum fuel used.
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Old 17th May 2012, 16:23
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Classic or NG?

FCTM under "Cruise Speed Determination":
ECON cruise is a variable speed schedule that is a function of gross weight, cruise altitude, cost index, and headwind component...
..Headwinds increase the ECON CRZ speed. Tailwinds decrease ECON CRZ speed, but not below the zero wind maximum range cruise airspeed.
In the same section you also find the that for NG, wind correction is not applied for LRC:
LRC is a variable speed schedule providing fuel mileage 1% less than the maximum available. The FMC does not apply wind corrections to LRC.
(Strangely it does apply wind corrections to LRC in the classic, thereby not being a "true" LRC according to the definitions).

Hence, in the Classic LRC may be the better option.
Edit, disregard - see my next post.

CI 0 gives minimum fuel used.
No, that would be best holding speed from the holdings page, which is more or less equal to max endurance.

Last edited by cosmo kramer; 18th May 2012 at 02:16.
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Old 17th May 2012, 16:27
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0 kt will always use the least fuel. Best to not start the engines either.
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Old 17th May 2012, 16:45
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LRC isn't even min fuel in zero wind. It is a touch faster than max range cruise because for a small penalty in fuel burn per mile (1%) you get a significant increase in TAS (5%) so it is a reasonable compromise between absolute min fuel burn and actually getting to where you want to go on time.
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Old 17th May 2012, 18:18
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You will use the least fuel for a fixed MRC speed but as your weight reduces with fuel burn, the AoA has to be reduced progressively. LRC can be flown with a constant AoA but to maintain the best SFC you would step climb to avoid reducing the performance to sub-optimal speeds.
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Old 17th May 2012, 22:34
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...as you don't take my word...

from 330/340 FCOM

CI is the ratio flight time cost (CT) to fuel cost (CF).
CI = CT/CF KG/MIN or 100 LB/H
The cost index is used to compute the best economic speed and Mach to be flown
considering the ratio between the cost of the flight time and the cost of the fuel.
CI = 0 corresponds to minimum fuel consumption (max range)
CI = 999 corresponds to minimum time

It is recommended to modify the CI in flight:
. In case a fuel problem is encountered, CI = 0 may be selected; the ECON SPD profile
is then computed to ensure minimum fuel consumption.
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Old 18th May 2012, 02:01
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CI = 0 corresponds to minimum fuel consumption (max range)
What a mix up of terms.

Fuel consumption is measured in amount pr. time unit (kg pr. hour). Hence, "minimum fuel consumption" would occur at the speed where the least kg pr. time unit occurs. This is according to all international definitions Max Endurance, not Max Range. For most jets, that would be slightly above clean speed (Les Green dôt spéed?).

It seems either Airbus has no idea of basic concepts (hope not) or that something go screwed up the translation from French. Perhaps it originally read:
"CI = 0 correspond to minimum (trip) fuel consumed (max range)".

At least, then it would make sense. I am sure they mean it all well, though. Alors! Eh bien..

So to reiterate if the original poster wants:
  1. Longest time in the air:
    Holding speed from the FMC (Boeing)
  2. Lowest trip fuel:
    ECON cruise CI =0 with forecasted wind in the legs page (*)
(*) Strike the previous comment about LRC being a better choice for the Classic (although wind corrected):
Boeing: AERO - Fuel Conservation Strategies
If faced with a low fuel situation at destination, many pilots will opt to fly LRC speed thinking that it will give them the most miles from their remaining fuel. As shown in figure 2, the best strategy to conserve fuel is to select a very low cost index, with zero providing the maximum range.
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Old 18th May 2012, 02:31
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Cosmo,

As we mostly use are aircraft to go from A to B, we are worried about how much fuel we comsume while traveling a particular distance, not how much we use per hour.

Fuel consumption is measured in units of fluid volume or mass as in how many Litres or Kilogramswe use. Whether we then want to know at what Rate that consumption happens, bee it per unit of time or distance is a seperate calculation.

Therefore we are most interested in max range, which can be calculated by using CI=0.

Last edited by Wizofoz; 18th May 2012 at 02:37.
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Old 18th May 2012, 03:09
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today I took a look on 737, these two speeds are different but i can't figure out which one is better . can anybody tell? THX.
Cost index :0 = MAX Range Cruise and Max L/D during descent(239kt on NG).

LRC is a good compromise if you need to go somewhere fast but still save fuel (airport closing/engine failure...).
LRC does not correct for wind(ie you fly into tailwind and your target speed will be the same),it uses 1% more than ECON CI:0.
On the NG,LRC is about CI 40-45.

Source:Boeing

Last edited by de facto; 18th May 2012 at 03:15.
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Old 18th May 2012, 03:31
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Wizofoz,
I agree with you .And thanks to all the reply above , it makes me learned a lot .
I think the answer is CI=0 saves most.
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Old 18th May 2012, 04:40
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This is a bit basic, isn't it?

Minimum Trip Fuel is achieved at Maximum Range Cruise. This is achieved at Cost Index = 0, both Airbus and Boeing. This is FULLY compensated for Wind Input (both manufacturers).

Long Range Cruise is now an out-dated concept, being a compromise speed achieving 99% of optimum range. Most currently used CIs are less than LRC.... forget LRC.

Maximum Endurance (or Minimum Fuel Flow) is achieved at Recommended Holding Speed (Boeing) or Green dot speed (Airbus). Wind has no effect.

Best Regards,

Old Smokey
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Old 18th May 2012, 11:00
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Mr Smokey, are you able to comment on the following?

1. Re Descending at CI 0, does this imply idle power? If not, where does one get the thrust setting?

2. de facto said "Cost index :0 = MAX Range Cruise and Max L/D during descent(239kt on NG)" Is the speed for Max L/D necessarily a CI 0 descent?

3. ReverseFlight says "LRC can be flown with a constant AoA" Does anyone see a requirement for a jet at LRC to have a constant AOA, as fuel burns off (or altitude changes)?
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Old 18th May 2012, 12:51
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There is a boeing powerpoint presentation floating around the net that explains it nicely. The short answer is CI 0.
This will have you arriving overhead destination with more fuel in your tanks than LRC will and I think that is what you are trying to achieve yeah?
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Old 18th May 2012, 14:08
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Wizofoz,

As we mostly use are aircraft to go from A to B, we are worried about how much fuel we comsume while traveling a particular distance, not how much we use per hour.
I am objecting to the wording in the Airbus FCOM, not the concept of Max Range Cruise.

Definition of CONSUMPTION

2 a : the act or process of consuming <consumption of food> <consumption of resources>

Comsumption therefore implies a rate. Fuel Consumption will therefore always be "amount per something".

The consumption that will get you the most miles is in an aircraft (Max Range Cruise) not the minimum fuel consumption (Max Endurance). When the engines consume the minimum possible (Max Endurance) you will not fly very fast and hence not cover that many miles.

But since you actually did write practically the same as myself (your emphasis "comsume" vs. Airbus "consumption") it seems you agree.


We are as pilots expected to do our job with great precision, I expect the same precision from those writing our manuals. And actually there is a great difference between Max Range Cruise and Max Endurance, it's not a small mistake.

And no, sometimes it's actually more interesting how long we can stay in the air, than flying from A to B. If you never fly a holding, consider yourself lucky!

Last edited by cosmo kramer; 18th May 2012 at 14:18.
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Old 18th May 2012, 14:41
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Cosmo,

I disagree.

The process of consuming can be fast or slow. A Cessna can consume a ton of fuel, as can a jumbo. Either way a ton has been consumed.

The Rate of consumption is different between the two.

We are just arguing semantics, but there is nothing in the verb "To comsume" that imlplies a rate.

Does the sentance "To travel two hundred miles" imply how fast it happens?
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Old 18th May 2012, 14:47
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Cosmo says

"2 a : the act or process of consuming <consumption of food> <consumption of resources>"

"process" is not a rate, neither is "act". The above has nothing to do with the RATE of consumption.

Wiz is correct
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Old 18th May 2012, 16:32
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I disagree.

The process of consuming can be fast or slow. A Cessna can consume a ton of fuel, as can a jumbo. Either way a ton has been consumed.
No you don't You still use other words (consuming, consumed) than "Consumption".

We are just arguing semantics, but there is nothing in the verb "To comsume" that imlplies a rate.
Again correct, but "TO consume" (verb) is not the same word as "THE consumption" (noun). E.g. "THE consumption is high on the left engine, look at that fuel flow it's up to 3 tons per hour".

The semantics of an official critical document should be correct.
Maybe Ole did mean Max Range, but want he wrote (based on that very document) could easily be misunderstood as Max Endurance. That is why such a document should be wrote in an unambiguous manner.

Last edited by cosmo kramer; 18th May 2012 at 16:32.
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Old 18th May 2012, 20:01
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Next time you are doing a 12 hour sector into 60 knot headwinds, just punch-in LRC or your C.I. into the box, the results are interesting!
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