Cost Index
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Cost Index
Yeah, I know how it's supposed to work and I have read as many threads as I can find on the subject but bear with me.
My firm operate pretty full B737-800's about the Pacific on sectors usually around the 4 to 6 hour mark.
About a year ago in an effort to offset rising fuel costs a decision was made to move to a variable cost index (typically 6 to 10) rather than the CI30 that had previously been used. Possibly coincidentially an increase in PDA's was noted across the fleet at that time.
My dilemma.... I have found that operating at CI30, as best I can tell, achieves the same fuel burn or sometimes less (ref. the flightplan) with time savings of up to about 10-15 mins (obviously depending on the wind and sector length).
We use standard passenger weights and I don't want to go there but wonder if we are slipping up the other side of the curve.
Can anyone with any experience in this area shed some light or help me with what others have already concluded on this subject.
cheers
My firm operate pretty full B737-800's about the Pacific on sectors usually around the 4 to 6 hour mark.
About a year ago in an effort to offset rising fuel costs a decision was made to move to a variable cost index (typically 6 to 10) rather than the CI30 that had previously been used. Possibly coincidentially an increase in PDA's was noted across the fleet at that time.
My dilemma.... I have found that operating at CI30, as best I can tell, achieves the same fuel burn or sometimes less (ref. the flightplan) with time savings of up to about 10-15 mins (obviously depending on the wind and sector length).
We use standard passenger weights and I don't want to go there but wonder if we are slipping up the other side of the curve.
Can anyone with any experience in this area shed some light or help me with what others have already concluded on this subject.
cheers
From the airbus "getting to grips with..." series:
Is 15 minutes of maintenance time (and crew wages, if your crew are paid on flying time) worth the cost of 200kg of fuel?
As it can be seen, it is not really advantageous to fly at very low cost indices as fuel savings are not significant compared to time loss. Although using slightly higher fuel, a slightly higher cost index gives significant time gains.
For instance, for the A319, increasing the cost index from 0 to 20 reduces
the block time by 15 minutes (5%) for a fuel burn increase of only 200kg (2%) on a 2000nm sector.
For instance, for the A319, increasing the cost index from 0 to 20 reduces
the block time by 15 minutes (5%) for a fuel burn increase of only 200kg (2%) on a 2000nm sector.
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Should more-or-less be 'adjusted' in the CI along with maintenance and crew costs..
So, what is a 'PDA'? (yes, I used to have one - an IPAQ, but I don't think....................)
So, what is a 'PDA'? (yes, I used to have one - an IPAQ, but I don't think....................)
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From our manual:
"Performance % Deterioration Allowance (PDA) This is a specific percentage (%), calculated from inflight performance monitoring, fuel correction/allowance to provide for deterioration of that aircraft performance from standard book figures."
Each aircraft is allocated a % which is periodically updated and reflected in each flightplan.
I only bring up the (possibly coincidental) fleetwide increase because it may support where I am going with this.
As aircrew, we do notice that the slower ECON speeds result in higher nose attitudes (say 3 degrees) which may well increase the PDA but I'm thinking only if its a bit higher than the CI calculation assumes. You can probably see where I'm going with this..... Has anyone reverted to higher CI's due to this issue?
As far as LRC goes, yeah normally .78 to .79 and according to the FCTM about CI30. By definition using a bit more fuel but thats not what I'm finding.
"Performance % Deterioration Allowance (PDA) This is a specific percentage (%), calculated from inflight performance monitoring, fuel correction/allowance to provide for deterioration of that aircraft performance from standard book figures."
Each aircraft is allocated a % which is periodically updated and reflected in each flightplan.
I only bring up the (possibly coincidental) fleetwide increase because it may support where I am going with this.
As aircrew, we do notice that the slower ECON speeds result in higher nose attitudes (say 3 degrees) which may well increase the PDA but I'm thinking only if its a bit higher than the CI calculation assumes. You can probably see where I'm going with this..... Has anyone reverted to higher CI's due to this issue?
As far as LRC goes, yeah normally .78 to .79 and according to the FCTM about CI30. By definition using a bit more fuel but thats not what I'm finding.
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PDA is not a function of AOA or pitch angle. PDA is the result of engine wear resulting in more fuel being required to produce the required thrust and increased drag that results from airframe damage/pitting in service. The majority of the PDA is from engine wear.
Trying to draw a relationship between CI and LRC is not a logical or realistic concept. LRC is a fixed value above MRC and depending on the individual operator costs on a route the CI that generates the same speed profile as LRC will be different from operator to operator and route to route.
As for the discussion about pitch angle and AOA in cruise and the urban legend of "wasting fuel" due to increased drag, it is important to remember that in cruise the largest portion of drag results from ramming the blunt object through the air and is a function of the square of the speed.
Sadly, CI is one of the most misunderstood concepts in aviation and we devote the least amount of training footprint to increasing pilot knowledge on what it is, how it works and how to fly efficiently.
The Airbus publications "Getting to Grips" with CI and Fuel Economy offer a wealth of information, but need to be tempered with the fact they were written at a time when the price of fuel was significantly less. This skews a number of the graphs and nullifies many of the statements concerning lower cost index values.
Trying to draw a relationship between CI and LRC is not a logical or realistic concept. LRC is a fixed value above MRC and depending on the individual operator costs on a route the CI that generates the same speed profile as LRC will be different from operator to operator and route to route.
As for the discussion about pitch angle and AOA in cruise and the urban legend of "wasting fuel" due to increased drag, it is important to remember that in cruise the largest portion of drag results from ramming the blunt object through the air and is a function of the square of the speed.
Sadly, CI is one of the most misunderstood concepts in aviation and we devote the least amount of training footprint to increasing pilot knowledge on what it is, how it works and how to fly efficiently.
The Airbus publications "Getting to Grips" with CI and Fuel Economy offer a wealth of information, but need to be tempered with the fact they were written at a time when the price of fuel was significantly less. This skews a number of the graphs and nullifies many of the statements concerning lower cost index values.
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Maybe a Poll might be appropriate, ofcourse I do realize that different airline/areas of operations come into play here. What cost indeces do u use on average??
Ours is a constant 30 on the B737NGs
Ours is a constant 30 on the B737NGs
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You also have to take into account that reducing the cost index from 30 to a lower level, will dramatically reduce your carbon and other exhaust emmissions. Might be good for our planet in the long-term...
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Ah, but do you know which burns more fuel the higher CI or the below optimum altitude?
Different answers for different aircraft......
Gathering the right information about your aircraft/engine/fms configuration and the airline cost structure and CI will help you make better decisions.
The problem is that CI is so poorly understood and rarely taught that facts are few and far between and rumour, urban legend and out and out wild guesses prevail rather than a fact based decision process.
Different answers for different aircraft......
Gathering the right information about your aircraft/engine/fms configuration and the airline cost structure and CI will help you make better decisions.
The problem is that CI is so poorly understood and rarely taught that facts are few and far between and rumour, urban legend and out and out wild guesses prevail rather than a fact based decision process.
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Canuckbirdstrike,
I agree with you about what comprises PDA. I agree that pitch angle should not be a factor in its value UNLESS it is higher than that assumed in the calulation of the speed by the FMC in accordance with the entered CI. (Say an aircraft was accidently overloaded everyday by the same amount...... I see that this would, by way of aerodynamic and induced drag increase the PDA).
I agree that LRC is a fixed speed (approximately 0.78M within 2000ft of optimum according to the FCTM) and equating to approximately CI30 (plus or minus the other inputs used when referencing ECON speeds). My reference to it may have been confusing. By definition it provides 1% less milage than the maximum available (CI0). What I'm saying is that in practice I don't think we're saving fuel by going much below it.
If this stuff is missunderstood, then I think it is a good thing to be discussing. Your reference to urban legend... sorry it wasn't one I knew about... just trying to get my head around my own observations.
cheers
I agree with you about what comprises PDA. I agree that pitch angle should not be a factor in its value UNLESS it is higher than that assumed in the calulation of the speed by the FMC in accordance with the entered CI. (Say an aircraft was accidently overloaded everyday by the same amount...... I see that this would, by way of aerodynamic and induced drag increase the PDA).
I agree that LRC is a fixed speed (approximately 0.78M within 2000ft of optimum according to the FCTM) and equating to approximately CI30 (plus or minus the other inputs used when referencing ECON speeds). My reference to it may have been confusing. By definition it provides 1% less milage than the maximum available (CI0). What I'm saying is that in practice I don't think we're saving fuel by going much below it.
If this stuff is missunderstood, then I think it is a good thing to be discussing. Your reference to urban legend... sorry it wasn't one I knew about... just trying to get my head around my own observations.
cheers
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majuro - thanks for explaining your TLA - I am used to another TLA for that
- as Canuck says - irrelevant. You would then have a 'PDA' for every flight, regardless of CI, so your performance monitoring programme (PMP) would pick this up and show that particular a/c as a 'rogue' performer and the fuel burns should be adjusted (eventually)
(Say an aircraft was accidently overloaded everyday by the same amount...... I see that this would, by way of aerodynamic and induced drag increase the PDA).
) would pick this up and show that particular a/c as a 'rogue' performer and the fuel burns should be adjusted (eventually)
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The comment with respect to PDA and aircraft operating at a higher than planned weight is not valid. PDA is only to address fuel burn increase due to engine/airframe deterioration.
One of the issues with APM type software is that only the cruise phase of flight is considered and corrections for weight changes are sometimes problematic. The ideal way to derive PDA values is to use APM values and also do comparative BURN studies (plan vs. actual) for each specific aircraft where the data is normalized for weight and time changes. This produces a PDA value for all phases of flight. The weight/time data normalization exercise is the key to getting the PDA value correct.
Technically LRC and CI 0 are not mathematically connected. LRC is 1% less than MRC. CI 0 and MRC are close but not the same.
One of the issues with APM type software is that only the cruise phase of flight is considered and corrections for weight changes are sometimes problematic. The ideal way to derive PDA values is to use APM values and also do comparative BURN studies (plan vs. actual) for each specific aircraft where the data is normalized for weight and time changes. This produces a PDA value for all phases of flight. The weight/time data normalization exercise is the key to getting the PDA value correct.
Technically LRC and CI 0 are not mathematically connected. LRC is 1% less than MRC. CI 0 and MRC are close but not the same.
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So since the CI optimises the fuel related costs versus the time related costs, would you still use the CI as noted on the OFP if you depart with a 30 minutes delay?
What if the aircraft has a 60 minutes turnaround for the next flight/crew at the home base and you are 30 minutes late.
I would think flying a bit faster would reduce overall costs, but I can't find official documentation about it.
What if the aircraft has a 60 minutes turnaround for the next flight/crew at the home base and you are 30 minutes late.
I would think flying a bit faster would reduce overall costs, but I can't find official documentation about it.