B777 300ER Fuel Burn
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B777 300ER Fuel Burn
Some interesting findings:
-on this aircraft, a recent study compared (real life, not software predictions) data colected from flights operated either on company suggested CIs, on the lower 30s, wich translates into an average M.83 crz speed with other operated on typically LRC, CI around 180, giving M.84.
Given comparable TOW, atmospheric conditions, routes and aircrafts the only difference in operating at the lower CI is EXTENDED FLIGHT TIME!
The practice contradicts Boeing software predictions. Flying at a CI lower than say 150-180 does not actually provide any significant fuel savings. On the other hand, keeping the cruise around M.84 saves time without burning more fuel, thus reducing the overal trip costs.
These are cold FACTS, not guessing, with enough data to provide a definite trend.
Any clues?
-on this aircraft, a recent study compared (real life, not software predictions) data colected from flights operated either on company suggested CIs, on the lower 30s, wich translates into an average M.83 crz speed with other operated on typically LRC, CI around 180, giving M.84.
Given comparable TOW, atmospheric conditions, routes and aircrafts the only difference in operating at the lower CI is EXTENDED FLIGHT TIME!
The practice contradicts Boeing software predictions. Flying at a CI lower than say 150-180 does not actually provide any significant fuel savings. On the other hand, keeping the cruise around M.84 saves time without burning more fuel, thus reducing the overal trip costs.
These are cold FACTS, not guessing, with enough data to provide a definite trend.
Any clues?
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Interesting. Not much difference in cruise Mach within the extremes of cost index. This suggests to me that the point of least drag is between .83 and .84. My experience on the B777 seems to bear this out as aircraft weight seems to have little to do with it.
Where the big difference occurs between a high and low cost index is in descent profile speeds which due to ATC is out our hands most of the time anyway. Any way of correlating your results to the extent that ATC restricts your speed on descents?
It seems to me that if we could coordinate with ATC to hit an initial arrival fix at a specific time (easy with today's FMC's), that we would be more likely to be able to descend unrestricted which is where some real fuel could be saved. I think that I read somewhere that UPS is trying something like this for their arrivals into their Louiseville hub.
Where the big difference occurs between a high and low cost index is in descent profile speeds which due to ATC is out our hands most of the time anyway. Any way of correlating your results to the extent that ATC restricts your speed on descents?
It seems to me that if we could coordinate with ATC to hit an initial arrival fix at a specific time (easy with today's FMC's), that we would be more likely to be able to descend unrestricted which is where some real fuel could be saved. I think that I read somewhere that UPS is trying something like this for their arrivals into their Louiseville hub.
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One of the problems for us pilots is that the bean counters analyse fuel burn on a block hour basis. In other words, the amount of fuel burnt per hour.
If three identical aircraft flew from London to Frankfurt, one flew fast and did it in 50 minutes, another flew at LRC and did it in an hour, and the third aircraft took 75 minutes at minimum speed. If all three aircraft used the same amount of fuel e.g two tonnes, then the bean counters would show the third aircraft was flown the most efficiently.
How? The first aircraft's block hour rate was two tonnes in fifty minutes, and that equates to ablock hour rate of 2400kg/hr. THe second aircraft's block hour rate is 2000kg/hr, whilst the third aircrafts block hour rate is an incrediblly efficient 1600kg/hr.
So even if the first aircraft only burnt 1800 kg in doing such a flight in 50 minutes, the beancounters would show this as being less efficient than doing the flight slower and burning more fuel.
If three identical aircraft flew from London to Frankfurt, one flew fast and did it in 50 minutes, another flew at LRC and did it in an hour, and the third aircraft took 75 minutes at minimum speed. If all three aircraft used the same amount of fuel e.g two tonnes, then the bean counters would show the third aircraft was flown the most efficiently.
How? The first aircraft's block hour rate was two tonnes in fifty minutes, and that equates to ablock hour rate of 2400kg/hr. THe second aircraft's block hour rate is 2000kg/hr, whilst the third aircrafts block hour rate is an incrediblly efficient 1600kg/hr.
So even if the first aircraft only burnt 1800 kg in doing such a flight in 50 minutes, the beancounters would show this as being less efficient than doing the flight slower and burning more fuel.
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A330
It would surprise me pretty much if the bean counters uses so limited and short sighted way of doing their math...
Make a lot more sense the overall trip cost, wich encompasses most of $$$ variables.
Make a lot more sense the overall trip cost, wich encompasses most of $$$ variables.
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We have had this discussion within our company. Problem which you run into as a pilot in this discussion is that you are only looking at one specific factor (in your case fuel consumption) and that is only one part of the equation within the cost index formula. Cost index is defined as:
CI= time cost/hr divided by fuel cost/hr.
The time costs are referenced as: 1) Fixed costs per trip (e.g. handling costs). 2) Time-related costs. These can be split in A)Commercial factors (e.g. early/late arrival, etc). B) Crew costs (longer or shorter flight time can influence the work and rest schedule of crews, etc). 3) Maintenance costs, these can vary significantly with small cruise speed variations, because most maintenance is done on the basis of condition and cycles rather than hours. A lower CI would lower the temperatures within the engine, increasing maintenance intervals. Also guarantees or contracts with the different OEM can influence your choices.
I agree that the fuel cost/hr has increased its presence significantly within the formula making your statement logical. But as a pilot I do not have enough information to say the economics are always better by increasing your normal CI to the 150-180 range (ours is 100 btw). I have posted a question to our company how often the data is updated within the CI formula (awaiting an answer).
CI= time cost/hr divided by fuel cost/hr.
The time costs are referenced as: 1) Fixed costs per trip (e.g. handling costs). 2) Time-related costs. These can be split in A)Commercial factors (e.g. early/late arrival, etc). B) Crew costs (longer or shorter flight time can influence the work and rest schedule of crews, etc). 3) Maintenance costs, these can vary significantly with small cruise speed variations, because most maintenance is done on the basis of condition and cycles rather than hours. A lower CI would lower the temperatures within the engine, increasing maintenance intervals. Also guarantees or contracts with the different OEM can influence your choices.
I agree that the fuel cost/hr has increased its presence significantly within the formula making your statement logical. But as a pilot I do not have enough information to say the economics are always better by increasing your normal CI to the 150-180 range (ours is 100 btw). I have posted a question to our company how often the data is updated within the CI formula (awaiting an answer).
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Some may remember Ernest K. Gann's comments in "Fate is the Hunter" about when his airline started flying Transatlantic flights in DC3s at the beginning of the American involvement in WW2.
The performance experts came up with plans to fly at really low power settings and (in the minds of the line pilots) ridiculously low cruising speeds to extend the DC3's range.
After two or three crossings, one of the older captains set out using the standard, higher power settings for the climb and cruise - and not only got to his destination quite a lot earlier, but burned no more fuel than they'd been burning using the low power/ultra coarse pitch settings.
Somethings never change?
I must add that from my observation on the 773ER, slowing down does save fuel, although I was taught when I first came on to the aircraft that it was a ".84 wing". Certainly anything over .84 is quite costly, and for minimal savings in time.
The performance experts came up with plans to fly at really low power settings and (in the minds of the line pilots) ridiculously low cruising speeds to extend the DC3's range.
After two or three crossings, one of the older captains set out using the standard, higher power settings for the climb and cruise - and not only got to his destination quite a lot earlier, but burned no more fuel than they'd been burning using the low power/ultra coarse pitch settings.
Somethings never change?
I must add that from my observation on the 773ER, slowing down does save fuel, although I was taught when I first came on to the aircraft that it was a ".84 wing". Certainly anything over .84 is quite costly, and for minimal savings in time.
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Too true
We are not talking about the benefits of CI, rather how do we achieve a given flight with the least fuel. I suspect CI is not always right due to variations in the aircrafts "actual" weight and trim.
We are not talking about the benefits of CI, rather how do we achieve a given flight with the least fuel. I suspect CI is not always right due to variations in the aircrafts "actual" weight and trim.
From the 10 or so long haul trips I've done HKG JFK and YYZ I would agree with the above posts. Going faster than .84 seems to burn a lot of gas but only save maybe 15 mins overall. Slowing down also doesn't seem to make any difference either.
On the 200's and 300's ( non ER ) we are told CI 20 is supposed to give the lowest fuel burn?? but the reality is that on short sectors of 3 hrs or less it makes no difference at all.
On the 200's and 300's ( non ER ) we are told CI 20 is supposed to give the lowest fuel burn?? but the reality is that on short sectors of 3 hrs or less it makes no difference at all.
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Another point to remember is that by staying airborne longer/going slower you are taking the aircraft that much closer to it's next major overhaul without achieving anymore by way of sectors. Even if only staying aloft 10 minutes a day by the end of a week that is over an hour, how does the cost of that hour compare to the cost of the fuel saved?
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As I already mentioned in my first post in this thread. Maintenance on modern aircraft is condition and cycle based. In the cost index formula, hours airborne only apply to time related issues such as commercial, and crew costs, and these could impact any CI decision.
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Otterman,
having been involved with one operators fuel conservation programme a few years back, I can rest assure you that block burn/hr was and probably still is the yardstick used. The bean counters argue that averaged over multiple round trips and routings, that block burn/hr is the yardstick to use.
Cannot say I agree with them. Merely by taxiing slowly I can push out my block time and appear to be all the more fuel efficient.
having been involved with one operators fuel conservation programme a few years back, I can rest assure you that block burn/hr was and probably still is the yardstick used. The bean counters argue that averaged over multiple round trips and routings, that block burn/hr is the yardstick to use.
Cannot say I agree with them. Merely by taxiing slowly I can push out my block time and appear to be all the more fuel efficient.
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Age old question
Did a fuel analysis for a year on same routes to cover all seasons in long haul. Higher CI compared to the bean counter's lower CI seemed to be most efficient use of fuel for time flown. Consistantly arrived with at least the same fuel remaining with much time saved. Knowing that, the proper CI should include additional factors such as fuel cost for that sector versus historical revenue stream for same sector. Obviously a freighter with higher revenue should fly closer to the minimum time/fuel available CI than a passenger flight. We are only talking long haul here; > 4 hours flight time.
I believe the sophistcated software does exist that could be employed every day for each sector to included pax/frt load, environmental factors and fixed costs. Could save a lot in the bean counter's salaries.
I believe the sophistcated software does exist that could be employed every day for each sector to included pax/frt load, environmental factors and fixed costs. Could save a lot in the bean counter's salaries.
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This is my point.
You wrote: "I believe the sophistcated software does exist that could be employed every day for each sector to included pax/frt load, environmental factors and fixed costs. Could save a lot in the bean counter's salaries".
The starter post indicates that such a sophisticated software still doesn't exist. At least for B777 300ER on medium/long haul sectors (6 to 9 hours for the particular study), facts (or actual numbers) do not show any significant fuel savings on employing CIs on the lower 30s, over M.84 cruise, or CIs on the 150 to 180. As for the engine wear, the difference is absolutely negligible.
This means your overall trip cost will be lower going on the later cost index. This suggests that CI theory is not confirmed on real life, when cold numbers came out, at least on this particular case.
B777 having a M.84 wing is a very interesting theory and confirmed by facts.
You wrote: "I believe the sophistcated software does exist that could be employed every day for each sector to included pax/frt load, environmental factors and fixed costs. Could save a lot in the bean counter's salaries".
The starter post indicates that such a sophisticated software still doesn't exist. At least for B777 300ER on medium/long haul sectors (6 to 9 hours for the particular study), facts (or actual numbers) do not show any significant fuel savings on employing CIs on the lower 30s, over M.84 cruise, or CIs on the 150 to 180. As for the engine wear, the difference is absolutely negligible.
This means your overall trip cost will be lower going on the later cost index. This suggests that CI theory is not confirmed on real life, when cold numbers came out, at least on this particular case.
B777 having a M.84 wing is a very interesting theory and confirmed by facts.
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What fuel burn does your performance manual give for different speeds?
Guys say "I'll do .81 and save gas" etc, etc. 757/767 performance data shows them to be most efficient at .78 (typically). So they do .81 and come in ahead and think they 'saved' gas. The question that they can't answer is what would their arrival fuel have been in they'd flown .78/.79?
Guys say "I'll do .81 and save gas" etc, etc. 757/767 performance data shows them to be most efficient at .78 (typically). So they do .81 and come in ahead and think they 'saved' gas. The question that they can't answer is what would their arrival fuel have been in they'd flown .78/.79?
To make sure of the results dont you have to have 2 or more planes doing the same trip but using different CI's as the differences in winds at atlitude will have an influence?
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What most managers and bean counters tend to forget is, that the gain on CI might be substantial when you look at the cumulative effect of the whole fleet vs. one year of ops. However a badly executed descent and approach uses up much more fuel than any small gain enroute, if we consider just a single flight. Now looking at the rate of pilots trained in rush-syllabi and then the performance of such pilots enroute (mind you, i am not talking about safety, but skills that lead to more economy), there might be food for thought.
Another thing that impairs any CI gain, is the tendency of cheating-scheduling close and often even over the duty limits. Just to be able to roster single crew. What i have experienced there, is that almost all pilots looking at a cheating flightplan just increase the speed in anger.
I agree with the many posters who say that flying .84 is optimal. Faster burns definitely more, slower adds up to nothing. If companies are really serious about fuel conservation, they will have to integrate us pilots much more. Training needs to be adapted, just as sops. Pilots need to be motivated to save fuel and not simply forced to fly speeds and procedures that are set up by bean counters and any pilots knows they result in peanuts. You can motivate us in many different ways, gain participation is a magic word, but most certainly some fair and credible scheduling/rostering would be a start.
Another thing that impairs any CI gain, is the tendency of cheating-scheduling close and often even over the duty limits. Just to be able to roster single crew. What i have experienced there, is that almost all pilots looking at a cheating flightplan just increase the speed in anger.
I agree with the many posters who say that flying .84 is optimal. Faster burns definitely more, slower adds up to nothing. If companies are really serious about fuel conservation, they will have to integrate us pilots much more. Training needs to be adapted, just as sops. Pilots need to be motivated to save fuel and not simply forced to fly speeds and procedures that are set up by bean counters and any pilots knows they result in peanuts. You can motivate us in many different ways, gain participation is a magic word, but most certainly some fair and credible scheduling/rostering would be a start.
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B747-400
Boeing state that if fuel is the ONLY consideration, then fly at LRC which is M.859 at OPTIMUM. We fly ECON CI = 60 which is about M.845 at OPTIMUM.
On a LHR - SFO - LHR the difference is about 15 mins a day, 91.25 hours per year per aircraft. That 91.25 will give at least 4 more LHR - SFO - LHR trips with all that revenue in the kitty before the next heavy check (assuming the check is on flight hours). How many crews change the step size from ICAO to 2000' and fly at optimum or just above? How many crews adjust the Mach number to LRC if held down to a lower flight level? How many crews consider what effect a CI of 60 has on the climb and descent? How does the company calculate the CI?...getting the information from engineering is like getting blood out of a stone. The fact is that every aircraft is designed to have a sweet spot and the B747-400 does not like getting slow. The difference in fuel burn between M.86 and M.85 is minimal but remember...
TIME IS MONEY.
On a LHR - SFO - LHR the difference is about 15 mins a day, 91.25 hours per year per aircraft. That 91.25 will give at least 4 more LHR - SFO - LHR trips with all that revenue in the kitty before the next heavy check (assuming the check is on flight hours). How many crews change the step size from ICAO to 2000' and fly at optimum or just above? How many crews adjust the Mach number to LRC if held down to a lower flight level? How many crews consider what effect a CI of 60 has on the climb and descent? How does the company calculate the CI?...getting the information from engineering is like getting blood out of a stone. The fact is that every aircraft is designed to have a sweet spot and the B747-400 does not like getting slow. The difference in fuel burn between M.86 and M.85 is minimal but remember...
TIME IS MONEY.