What Cost index are you using B737NG?
TightYorksherMan
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What Cost index are you using B737NG?
Just as a matter of interest, what cost index are you all using?
Our airline states 4 for all routes.
Our airline states 4 for all routes.
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Will be changed for every flight, but usually somewhere between 10 and 13, highspeed OFPs at 200 (usually used to arrive before night curfew), we're supposed to change it to 8 if we're early.
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A Boeing survey showed that for the B737NG values of CI in the range of 10-30 are often used. Note that a CI=0 equals MRC. LRC is roughly around a CI of 35 on the B737NG. The actual, optimum CI can differ between airlines and routes. If you have a lot of connecting passengers, you might be better of flying at a higher CI than when you don’t have any connecting passengers on board for the same route/aircraft.
I noticed that you (Jinkster) are using a low CI at your airline. Flying at low cost indices can result in speed instabilities. Perhaps you have experienced a lot of movement of the AT when flying at this low CI? Note that not every airline reports these problems when flying at low CIs however. Flying fixed cost indices on all routes is not always the best thing to do either.
Boeing has written a number of interesting articles on this topic. See for instance:
http://boeing.com/commercial/aeromag...7_article5.pdf
I noticed that you (Jinkster) are using a low CI at your airline. Flying at low cost indices can result in speed instabilities. Perhaps you have experienced a lot of movement of the AT when flying at this low CI? Note that not every airline reports these problems when flying at low CIs however. Flying fixed cost indices on all routes is not always the best thing to do either.
Boeing has written a number of interesting articles on this topic. See for instance:
http://boeing.com/commercial/aeromag...7_article5.pdf
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In this time of economic problems, it seems even more important to fly at the correct cost index. The use of constant cost indices throughout the fleet or ignoring indices given in the flight plan doesn’t help getting the costs down. Note that a given cost index (assumed that it has been calculated correctly) ensures that the total cost (sum of fuel cost + cost of time) are minimum.
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As I get paid by the minute... I wish we used a CI of 1.
However our range is between 11 to 15 depending on sector length.
Average ECON speed for the NG varies from .77 to .78 at the higher altitudes, or about 5 knots... At the end of the day, not very significant for a 4 hour flight.
There are times during ETOPs operations, we are requested to fly a given and RTA request which throws the CI out the window all together... even though not resulting in much a total fuel burn deviation from the plan.
Bigger aircraft... larger difference however.
However our range is between 11 to 15 depending on sector length.
Average ECON speed for the NG varies from .77 to .78 at the higher altitudes, or about 5 knots... At the end of the day, not very significant for a 4 hour flight.
There are times during ETOPs operations, we are requested to fly a given and RTA request which throws the CI out the window all together... even though not resulting in much a total fuel burn deviation from the plan.
Bigger aircraft... larger difference however.
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Cost index surely should change dependent on network, fuel price fluctuations, operational costings and hedging agreements. Personally I believe that our standard cost index is due to our operational department being so understaffed and it's planning equipment being so inept. Besides, ECON speeds are rarely adhered to anyway, so fuel burn figures based on a perceived standard cost index will be largely inaccurate anyway. Maybe I'll punch in CI 500 next time I fly 
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Depends how you are payed by the minute. We are payed by the minute, however we are payed by the minute of standard block times which are a bit longer than flown ones. Usually i get payed 20 to 30% more time than i fly. However if i decide to hold for 2 hours and then land at the destination i certainly not going to make a profit, if i divert i get payed actual block time, sooo, hold for 1 hours and then divert 
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decurion:
Note that a CI=0 equals MRC.
Note that a CI=0 equals MRC.
With a strong headwind ECON (with cost index 0) will fly faster than actual MRC and LRC for that matter. Since we have no representation of actual MRC, the practical application would be LRC.
The reason being, that ECON will take actual wind into consideration whereas LRC doesn't. From practical experience even a Cost Index of more than 10 sometimes give a lower speed than LRC if you have a strong tailwind (FMC deems it more economical to hang around in the tailwind for a little longer and giving us a free ride).
If fuel is critical, I would suggest to compare and consider the options of LRC and ECON CI=0, if headwinds conditions, before deciding on the better option. Being stuck in a strong headwind at LRC may give you less fuel at destination than letting the FMC do it's magic.
Strangely you wont find that information in the FCOM, but in the 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.
..Headwinds increase the ECON CRZ speed. Tailwinds decrease ECON CRZ speed, but not below the zero wind maximum range cruise airspeed.
LRC is a variable speed schedule providing fuel mileage 1% less than the maximum available. The FMC does not apply wind corrections to LRC.
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Jhonny tightlips,
And why would you fly at 320/.80?please explain:suspect glad i aint your fo
Indeed ci 0 is the way to save the most fuel (MRC) and best L/D for descent however the optimum(maintenance cost/fuel cost) is ci12.
And why would you fly at 320/.80?please explain:suspect glad i aint your fo
Indeed ci 0 is the way to save the most fuel (MRC) and best L/D for descent however the optimum(maintenance cost/fuel cost) is ci12.
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We use a fixed CI of 20 for our -300/500/700 aircraft. That gives you a descent speed on 266 in the 300/500 and 261 in the -700. We modify the descent speed to meet published arrival or ATC requested descent speeds. Some folks (certainly not I) flt 280/300 knot descents routinely.
We are supposed to be looking at getting leg specific CIs from a computer program based on a variety of factors such as descent winds etc.
We are supposed to be looking at getting leg specific CIs from a computer program based on a variety of factors such as descent winds etc.
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I just dont see the point to fly at those speeds if not to burn more fuel,high descent rates from top of descent( unless you think idle isnt the way to go),increase in cockpit noise and be so much closer to overspeeds condition.
Last edited by de facto; 30th Sep 2011 at 02:04.