fullforward

How does your airline operates B777 in terms of cost index and mach number versus sector distance?

fullforward

Please share with us how your airline genius think is the best way to operate the triple.

mutt

Ideally we would use CI=500 for all international flights outbound and about CI=60 inbound......

Mutt

Cubbie

Surely thats the wrong way around?-I want to go home=ci500!!?

777guy

If you evaluate fixed mach numbers ranging from .87 down to .76, you will see that the optimal mach in terms of arrival fuel quantity will always be .825-.83.

If you evaluate cost index values from 250 down to 0, you will find the optimal cost index in terms of arrival fuel quantity will always be 25-35.

My airline based in the USA uses these optimal values on ALL of our 777 operations. Everything from 800 mile domestic routes, to 6000 mile polar
routes.

Hope this helps.

Nil defects

Three years ago we were using cost index's of 150.

As the price of fuel has gone up the cost index has come down.

Today we are using for the:

B777-300 CI 75
B777-200 CI 65

These cost index are the same whether long haul or short haul and our sector's range from 30 mins to 16 hours.

In my experience on the B777 any speed below .83 doesn`t save fuel and there has been another thread running with the BA guys in the main forum, discussing this.

fullforward

Flew about 150 similar sectors, half using that ops genius recomended CIs (20s to 30s and M.83) and half with CI150 -180 and M.84.
Gather all concerned data (ZFW, avg winds, traffic, etcetc), compared.
Findings:
- the ONLY difference is that using the first we spent more time in the air.
ABSOLUTELY NO DIFFERENCE in fuel burn flying on the little faster mode...this for 777-300ER with GE115;

Looks like FMC predictions are way too optimistic or just simply INCORRECT...
Try it yourself at home if you have a chance!

Remember that it's all about real numbers, no empty guess.

fullforward

...looks like there's no much interest on the subject...

MrBernoulli

Despite the soaring cost of fuel my airline only seems to use Cost Index 0 or 100. Never anything else.

fullforward

C'mon guys, how your airline geniuses require you to operate the 777, in terms of CI, M number?

ACMS

Cathay:- before this little fuel problem we used:-

Rolls Royce 200 and 300's CI 60
GE 300ER CI 80

Now they want us to reduce to CI 20 ( for all models ) where it can save fuel.

However the Eagle flight plan system is still produced using the old CI's above, but they have included a table to show the fuel saving and extra time for a CI of 20 and we are then supposed to adjust the burn accordingly.

Funny thing is that for a Roller 200 and 300 the difference in burn for up to a 9 hr flight is 0, only extra time to get there!! So we don't bother adjusting anything.

On the ER the saving on a 15 hr Flt is about 300 to 500 KG's but a time loss of 8 mins.

fullforward

Good one!

This is exactly what I find out: using CI around 30 doesn't save a single kg over using CI 180 or M.84, only increase trip time in about 7 min for a 9 hr trip.
B777-300ER GE115
Boeing FMC data on predicting fuel burn is INCORRECT, as well as programs based on it.

ACMS

yeah it makes no difference on a 9 hr trip, but it does using the ER on a 15 hr jolly.
I guess if everyone of our ER flts to the and from NA were to go CI 20 and save 200 to 300 kg's it would add up( currently 6 flts a day x 300 kg's = 1800kg's or 2,300 lts or about $2,500 USD ). So worth doing, and it can mean extra money for the drivers too ( through our Hourly Duty Pay system, even better if you are on overtime )

So it can be a win win for all


Having said all that above it does make you laugh when the company expect you to save every kg of fuel and then the Engineers run the APU for 6 hours on an overnight stop in TPE. ( uses around 1,200 kgs or more )

fullforward

Thanks for your reply.

However, do you really compared fuel burn on this long trips using CIs around 20 and CIs aroung 180 and M.84? Take into account that these fuel burns comparisons will only make sense provided all other variables comes very close (TOW, taxi times, vectoring/holding, average winds and temperature, etc).
I sincerely doubt any airline has done it and got the real numbers. Ops geniuses normaly rely on manufacturer FMC predictions.
They may have a surprise in case decide building a data base, with a significant number of flights going "fast" (M.84 or CI 180) and the others going on M.83 or CIs around 20.
As far as the data I built reveals, the only significant difference is flight time.
The bottom line: in effect, will be more expensive to go slow, putting into the equation time related costs.

ACMS

Extensive studies by CX have shown that a CI of 20 does save fuel on long flights ( 15 hrs )

It wouldn't be too hard to examine the results of CI 80 flts v CI 20 flts on the ER with 6 ULH flights per day for the last 8 months. ( 1,440 flts, of which every one is tracked ) And this is just our ER fleet.

So don't go getting too involved or worried over it, just do it !!

Old Smokey

A lot of comment in this forum that speeds below M0.83 make little or no difference to the total fuel burn. That's not surprising.

If you're at or close to Optimum Level in a B777, go to the INIT REF Page on the FMC, and insert CI=0 (Maximum Range Cruise). Inevitable, the scheduled Mach Number is in the range of M0.825 to M0.827, just short of the M0.83 mentioned. With Max Range Cruise JUST below M0.83, it's little surprise that there's little significant difference. You might note a slight gain in fuel on a very long sector.

Of course, if your cruise level is somewhat "Off-Optimum", you won't see these results.

The bottom line is that Climb and descent at CI speed will make a SIGNIFICANT difference to overall fuel burn, not advisable to fiddle with these. On the other hand, "fiddling" with speed during cruise, will, generally, have only small impact on overall fuel burn. Of particular value during climb is deleting the low level 250 KIAS speed limit as soon as possible. Deleting the speed limit at about 5000 feet will give you an immediate 400 to 500 Kg fuel "bonus".

Best Regards,

Old Smokey

fullforward

My point is simple like this: I don't think any airline simply put their fleet (same aircraft on same routes and similar conditions) flying on M.84 or CI around 150 for some time, collected all significant data, did the same on lower CIs or M.83 and compared everything.
They seems to rely mostly on FMCs software fuel burn forecasts, which aren't accurate enough.
If they do it I guess they will have a surprise.

mutt

Yes we did, spent a few months doing it.

Mutt

Old Smokey

Ditto to mutt and his airline. We spent a lot of time researching it, NOT Boeing predictions, but actual results.

Not just a past exercise, it's an on-going programme.

Best Regards,

Old Smokey

fullforward

Iīm VERY curious to know about the results. For time being I stick on my numbers. Unfortunately we donīt fly sectors longer than 9:30 hr.
I concede that, if thereīs any gain on longer sectors, it would be on the 200-300 kg, IF ANY.
But fly times will exceed 10 min on an 13-14 hr. Does it worth?
Itīs just an academic exercise...as good boys weīll have to follow our Fltops geniuses sops...