Cost Index

[majuro]

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

[BOAC]

New TLA for me - PDA?

[John Citizen]

I have noticed the same in the Airbus A320.

Cost index 30 will be better (quicker/same fuel) than cost index 10.

[Checkboard]

From the airbus "getting to grips with..." series:

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.

Is 15 minutes of maintenance time (and crew wages, if your crew are paid on flying time) worth the cost of 200kg of fuel?

[John Citizen]

From the airbus "getting to grips with..." series:


Where can I find this document ?

[KBPsen]

Here. (http://tinyurl.com/2dcyuxx)

More filler.

[FE Hoppy]

You need to consider the cost of fuel now to when the GTG was written. What may have been insignificant in 1990 may well be significant now!

[BOAC]

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....................)

[B777Heavy]

I remember reading somewhere on B737NG FCTM which gives you the option of either LRC or CI30. Not sure if they meant that they could be pretty close the two.. :confused:

[majuro]

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.

[Canuckbirdstrike]

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.

[B777Heavy]

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

[Greenpilots]

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...

[MD83FO]

i've had controllers not give us our optimum level at speeds below CI30.
so no real saving.

[Canuckbirdstrike]

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.

[majuro]

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

[BOAC]

majuro - thanks for explaining your TLA - I am used to another TLA for that:)

(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). - 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)

[Canuckbirdstrike]

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.

[bArt2]

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.

[BOAC]

Canuck - how is the PDA prog going to know the excess burn is due to engine/airframe deterioration?

bArt2 - refer to company policy -as always!

[Old Smokey]

bArt2,

BOAC beat me to it by a few seconds, it's a matter of company policy.

As an example we use a standard CI=65 for our B777 operation, but company policy is to increase to 150 if more than 15 minutes late.

Regards,

Old Smokey

[bArt2]

@BOAC & Old Smokey: Thanks, As far as I know we don't have a company policy for that. I guess common sense can replace the lack of guidance on this matter :cool:

[BOAC]

bArt2 - in that case:ok: Up to you to decide on sector pay against get-home-quicker:)

[Canuckbirdstrike]

Good discussion and some thread creep. I'll try and address the issues in the last few posts.

First PDA and understanding weight effect vs. engine/airframe deterioration. If the correct GW is in the FMS then APM type software will derive good PDA values for the cruise phase of flight. If however, extra load is put on and not accounted for in the ZFW and GW then the PDA will be skewed by the weight difference. As discussed previously APM PDA values are not the whole picture and from a flight planning system perspective a PDA value that represents the whole flight is more beneficial. It is important to remember that there are two PDA values; one for the flight planning system and one for the FMS. These two values can be from 0-4% different. It is also important to understand that the flight planning system PDA will also contain a component that matches the flight planning system calculations to the manufacturer's calculations. This is a fixed value and varies based on flight planning system provider, aircraft and engine type.

Next there is the issue of comparing what other operators use for CI values. this is a mugs game. Many variables enter the equation for calculating CI and no two operators have the same cost structure. The next issue is are you operating in metric or imperial units - their CI values are different. lastly comparison would assume that all airlines used the same, robust process to calculate CI. That is the furthest from the truth.

Then the great issue of tactical speed management and use of CI. This is a great theoretical idea, but doing it successfully is very difficult. First, we must realize that just increasing CI while maintaining altitude is a very costly way to reduce flight time. CI was designed to be used in a 2D (altitude and speed) solution. 2D solutions work in flight planning systems that have lots of data on wind profiles and computing power. Current FMS hardware/software does not have this proper 2D capability and was really designed as a system to allow the autoflight system to strategically fly the aircraft efficiently based on a 2D optimized flight plan with a given CI. we must also understand the effect of wind on speedup/slowdown capability and realize that they yield completely different orders of magnitude results in a headwind vs. a tailwind. You also need to clearly understand the cost penalty for the late operation vs. speeding up. I have seen pilots speed up a flight and spend five times as much as what was saved.

In a perfect world the best way to make the speed up decision is to have an in-flight flight plan generated to achieve the desired arrival time. this will adjust the vertical profile as well as the CI value to obtain the most speed up for the least cost. The incremental cost increase should be compared to the delay cost. Modern flight planning systems can do this very easily. LIDO has a module called VSOPs (Variable Speed Operations) that allows entry of a desired arrival time.

The issue of CI and efficient flight planning and operating is a complex multi-layered problem. I have been working in this area and teaching about it for over 10 years and still have only scratched the surface.

[St. Ex]

Here's the simplest explanation I've seen:

COST INDEX= ratio of FUEL COST ($ per kilogram)/ TIME COST (fixed operating cost $ per minute)

Where TIME COST includes the amortization of the aircraft, crew, maintenance, management, depreciation, etc.

For example, a CI=30 means the cost of 30 kilograms of fuel is equivalent to the cost of 1 minute of time cost.

Therefore, it is management who eventually determines which base reference cost index to use. As flight deck crew, we should initially set the CI according to what management instructs us then revise strategically as required to adjust for time savings bearing in mind the consequential effect on the EFOB or fuel remaining predictions at destination and/or alternate.

[Canuckbirdstrike]

St Ex, I agree with flying the company derived CI and following the flight plan lateral and vertical profile, as long as a safe and comfortable flight is maintained.

However, tactical change of CI in flight need to be approached with far more care than just a consideration of the EFOB requirements. Our business runs on very tight margins and speeding up a flight can result in large cost increases for very little measurable commercial gain. The problem is that just staying at a fixed altitude and increasing CI results in smaller changes in time and due to the fact that CI increase vs. fuel/time is non-linear the costs can spiral exponentially.

Unless you know for a fact what the commercial gain is for the speed up and what the cost increment is at the higher CI, you are dancing with the devil. As stated in my previous post the best way to approach this is through the use of 2D profile optimization using an in-flight flight plan from a good flight planning system and an airline specific data set of commercial costs for late arrivals, passenger missed connections etc.

[BOAC]

All of which returns us neatly to posts #20 and #21. If they do not specify what to do, you have at least 3 choices.

1) Just do what you want anyway ( or there are often mobiles and HF)
2) Stick with PLOG CI
3) Ask your company for policy

Often 'late' arrivals due to delays are easily absorbed in the programme and 'wasting fuel' to make up time un-necessarily can often, as Cbs says, cost a lorra money.

Obviously if you have an arrival landing slot to make, as at PRN in the 'early days' of KOSFOR, it make sense to adjust rather than have to divert.

[Willit Run]

on the 744; according to the boeing info; LRC figures are based on zero wind, and CI, the winds are taken into account.
Also, operations can "bias" a flight plan for a particular airframe, or route, if they so choose.

[Canuckbirdstrike]

Willit Run, you have identified an important difference between LRC and CI - the inclusion of wind. There are some LRC data sets that have values for different winds.

As for "biasing" for a particular aircraft, this is what PDA does.

As for route specific adjustments, this can be done based on fuel costs and then on a day-of-flight basis if there are specific constraints such as arrival time requirements. Again the caution here is using the right tools to adjust CI and flight profile to achieve an arrival time; slow down to arrive after a morning curfew or arrive before a night curfew, needs to be tempered with what is really possible. This is a complex discussion that really needs a number of graphs and data sets to illustrate. Unfortunately, very difficult to accomplish on a forum.

One of the other issues associated with this is the "planned" departure times and block times in an airline schedule. If not done properly this can lead to far to many operations requiring tactical speed adjustments. The challenge is that block/departure times are hugely influenced by commercial constraints to gain a competitive advantage when attracting customers. Many travel booking engines rank by departure/arrival time and this influences customer choice. Having participated in these discussions it is fascinating.....