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Marenostrum

About the cost index and the derate/CLB setting

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Posted (edited)

There is something that I miss and you can better explain me.

the CI setting impacts (indirectly derived by its definition) the speed during the CLIMB, CRUISE and DESCEND phases.

So when I set a specific cost index in the FMC I am telling it how I want the speed to be managed in those phases.

But when I have to choose the N1 limit variables choosing the DERATE and the CLIMB type, am I affecting the desiderd cost index too or are they two uncorrelated concepts. If yes...why?

In other words...may I choose a CI 0 or 500 (for the 737 NGX) whatever CLB and DERATE I choose in the N1 limit page respecting the CI set?

Edited by Marenostrum

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I think -and I underline "think"- derated take-off and climb modes are engine modes whereas CI is related to fuel management.

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16 minutes ago, costamesa said:

I think -and I underline "think"- derated take-off and climb modes are engine modes whereas CI is related to fuel management.

CI is 

CI =(Time cost in $/hr)/(Fuel cost in cents/lb)

So yes...it is about fuel management...in the sense that if I choose  a LOW CI I am telling the FMC that the Fuel costs have an high impact compared to the other costs (speed in CLIMB will be lower...and the climb will be more steep to take the plane at the cruise altitude asap, I will save fuel...but the overall flight time will be longer).

On the contrary if I tell the FMC that the cost index is HIGH I am telling the contrary: the fuel cost has low impact in the overall costs (the CLIMB speed will be higher and the CRUISE altitude will be reached later...but the overall flight will be shorter in time).

Chosing the CI is like to tell the FMC to decide the speeds during the three phases...that means (in my confused mind) that the FMC will have to choose the thrust setting to achive the correct speed.

But when I set different options in the N1 page I am affecting them too. This is the reason of my confusion. I see a sort of interference between a choice of the CI and simultaneous choices of the N1 settings page.

 

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5 hours ago, Marenostrum said:

Chosing the CI is like to tell the FMC to decide the speeds during the three phases...that means (in my confused mind) that the FMC will have to choose the thrust setting to achive the correct speed.

But when I set different options in the N1 page I am affecting them too. This is the reason of my confusion. I see a sort of interference between a choice of the CI and simultaneous choices of the N1 settings page.

The thrust setting computed by the FMC is a maximum value for takeoff, climb and cruise. It is not setting the thrust to maintain the speed VNAV calculates. VNAV calculations use the climb thrust setting and the climb speed computed from CI to calculate the vertical climb path. Note, VNAV cruise thrust will almost always be less than the CRZ limit calculated by the FMC. VNAV descent is at idle so thrust limits aren't a factor.

Thust limit and Cost Index are effectively independent variables. Flight path is the dependent variable.

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Stefano, also note that the derate/flex thrust and derated climb variables "wash out."  The derated thrust last until thrust reduction after takeoff, and the derated climb washes out at either 12000 or 30000 depending on airline option.  The derated climb does not affect climb speed, which is affected by CI.  The derated climb means your are lighter and don't need as much thrust to achieve target rate of climb.  As Kevin explained, apples and oranges.

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22 hours ago, downscc said:

Stefano, also note that the derate/flex thrust and derated climb variables "wash out."  The derated thrust last until thrust reduction after takeoff, and the derated climb washes out at either 12000 or 30000 depending on airline option.  The derated climb does not affect climb speed, which is affected by CI.  The derated climb means your are lighter and don't need as much thrust to achieve target rate of climb.  As Kevin explained, apples and oranges.

Thank you Dan,

My understanding from the FCOM2 manual was that both CLB 1 and CLB 2 are "maintained for the initial part of the climb. Thrust eventually increases to maximum climb thrust by 15,000 feet".

Nonetheless, I cannot remember where, I think I read also that the CLB 1 and CLB 2 derate wash out at different altitude...but I cannot find the source.

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22 minutes ago, Marenostrum said:

Thank you Dan,

My understanding from the FCOM2 manual was that both CLB 1 and CLB 2 are "maintained for the initial part of the climb. Thrust eventually increases to maximum climb thrust by 15,000 feet".

Nonetheless, I cannot remember where, I think I read also that the CLB 1 and CLB 2 derate wash out at different altitude...but I cannot find the source.

I haven't really spent time looking at the N1 page to see what setting we're at, but I do watch for the indication to go from R-CLB to CLB at 15,000. You can see the split on the needles, too on my A/C's configuration. The caret for CLB is above the reduced and they slowly match up as you climb to 15k.

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5 hours ago, Spin737 said:

You can see the split on the needles, too on my A/C's configuration. The caret for CLB is above the reduced and they slowly match up as you climb to 15k.

Thank you...tomorrow I will look at it

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