PPRuNe Forums Cost Index

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 8th Jun 2012, 14:19 #1 (permalink) Join Date: Jun 2010 Location: Wherever I hang my laptop Age: 53 Posts: 36 Cost Index Hi Can someone please explain cost index to a layman?
 8th Jun 2012, 15:53 #2 (permalink) ENTREPPRUNEUR   Join Date: Jun 2001 Location: The 60s Posts: 536 http://boeing.com/commercial/aeromag...7_article5.pdf It is a factor you enter in the FMC to indicate whether you want to save fuel or save time (ie time-based costs). It then chooses speeds based on your preference.
 8th Jun 2012, 15:56 #3 (permalink) Join Date: Jan 2006 Location: US Posts: 1,210 Calculation of fuel cost vs. other costs(crew variable costs, time related costs of components, on time performance, etc). Flying faster than your most efficient speed for the flight(adjusted for head/tail winds) burns additional gas. The company calculates the other costs and benefit of on time performance/connections and issues a CI to use. However, that's a predicted CI for when the paperwork is generated and the anticipated 'off' time for the flight. If we have a different 'off' time then planned we do not get an updated CI, that is based on our actual off time, to use. Then it's back to the 'old school'/common sense method. If you're airborne ahead of the estimated 'off' time you don't need to be using a high CI. If you're airborne later then the flight plan estimate, and saving time will allow people to make connections or get the entire crew/plane/system back on schedule, flying faster(burning additional gas) makes sense.
 19th Jun 2012, 02:40 #4 (permalink) Join Date: Nov 1999 Location: UK Posts: 172 For Airbus:- Cost index 0 = achieve minimum fuel. Cost index 999 = achieve minimum time. So basically, by entering a cost index, you're telling the FGMC/FMGEC how to adjust the flight profile, (when to climb, when to descend, what speed and flight level to fly at), to achieve the cost savings, or time savings that you need. U Last edited by Uplinker; 19th Jun 2012 at 02:47.
 19th Jun 2012, 14:26 #5 (permalink) Join Date: Sep 1998 Location: wherever Age: 44 Posts: 1,142 For the layman. The cost index is a value which represents a ratio between least fuel and least time for a specific flight or part of flight. It is a method of reducing or rather, optimising the overall costs of a specific flight or series of flights. The least fuel will be burned by flying at the maximum range speed. This is often quite slow. It may be compromised by the available cruise altitudes. The least time will be used by flying at max speed. It may also be compromised by available altitudes. For the cheapest overall flight costs there will be an optimum ratio of time/fuel. When fuel is cheap the ratio will be biased towards higher speed. When fixed costs are cheap the ratio will be biased towards lower speed. The actual speed will vary with weight and altitude. It will also need correcting for head or tail wind. Head wind is like making fixed costs higher as it takes longer for the flight therefore it will move the optimum ratio towards a higher speed. The implementation of the index value is different on different aircraft. Some will allow the crew to enter in directly into the flight management computer system. This will then calculate the optimum speeds for the planned climb, cruise and descent phases of the flight. Other systems will only calculate the cruise speed and some systems will require the crew to calculate the appropriate speed via tables or graphs and either enter this value into the the flight management system or even select and fly the speed manually. The concept is sound in that if correctly implemented it should reduce overall operating costs but like all things, if you start with the wrong data, for example real fixed costs, you will end with the wrong speeds and end up increasing costs rather than reducing them.
 19th Jun 2012, 14:39 #6 (permalink) Join Date: Apr 2009 Location: Earth Posts: 265 Another variable when flying to Vmd (max range) is that you are at the very cusp of being behind the drag curve, throw in a little turbulence, a temp or wind change and you find the A/T has to constantly adjust power settings to maintain the speed at Vmd. Power changes burn more fuel than a slightly higher power setting that doesn't require as much adjustment. A max range CI is fine in theory, but very rarely ends up doing anything other than burning more fuel than it theoretically should.
 19th Jun 2012, 15:27 #7 (permalink) Join Date: Dec 2000 Location: Canada Posts: 137 I think you might want to review that thought process..... Max Range Cruise (MRC) is the top of the Specific Range curve. The bottom of the Drag Curve (Min Drag) is the Maximum Endurance Speed; essentially your best holding speed. On a typical generic jet aircraft set of curves this is well separated. So it is incorrect to state that flight at low or CI=0 puts you on the cusp of the drag curve.
 19th Jun 2012, 17:29 #8 (permalink) Join Date: Apr 2009 Location: Earth Posts: 265 Canuck, please excuse my typo, I meant to say Vmd was effectively max endurance which is ostensibly max range (I was waiting for someone to spot the deliberate mistake) In the FCTM for the Boeing I fly it says "entry of zero for cost index results in maximum range cruise". Given that the total drag curve in this region is effectively flat, the speed differential in terms of IAS at representative cruise alts and weights between holding speed and CI 0 is 1-2 knots on the jet I fly, which brings us back to the problem of speed stability. Yes in theory CI zero gives max range, but in practice the fuel predictions slowly reduce to the point where they would have been if you flew .05 Mach faster. The only reasonable explanation I can come up with is that it's thrust adjustment to maintain an unstable speed that causes the increased fuel burn. In the military, the wingman flying formation always burns more gas than the boss
 19th Jun 2012, 18:13 #9 (permalink) Join Date: Dec 2000 Location: Canada Posts: 137 Falcon, relying on any of the FMS predictions for assessment of fuel savings is a mugs game. The tool is very primitive to start with, uses different parameters and algorithms than a flight planning systems and the fuel systems also have measurement errors. The net result is poor data fidelity for inflight evaluation. Using total flight data tracking smooths out the results and does demonstrate the savings from low CI operations. We must also remember that typical holding speeds are also not shown as the pure max endurance speed to address the speed stability issue you raise, so comparisons of a FMS generated hold speed does not necessarily accurately reflect what the true Max Endurance speed. I have spent 10+ years working in the field of fuel efficiency and CI. I work it from two angles, as a current line pilot that still flies and has flown multiple types from Boeing and Airbus, and from the engineering & fuel efficiency side. The number do not lie. Low CI operations are just different to what we are used to, the world has changed and we have to adapt to it.
19th Jun 2012, 18:35   #10 (permalink)

Join Date: Apr 2009
Location: Earth
Posts: 265
Quote:
 I have spent 10+ years working in the field of fuel efficiency and CI. I work it from two angles, as a current line pilot that still flies and has flown multiple types from Boeing and Airbus, and from the engineering & fuel efficiency side. The number do not lie. Low CI operations are just different to what we are used to, the world has changed and we have to adapt to it.
Fair enough Canuck, I guess I'll have to keep playing a mugs game in the absence of anything more accurate and representative.
It makes me wonder why Boeing or Airbus for that matter promulgate information and thinking such that we actively use the FMS as a "how goes it" tool to fly as efficiently and effectively as possible given the inherent errors in the system.
Playing devils advocate, are you suggesting that these rather crude and inaccurate tools are in fact a hinderence rather than a help?

 19th Jun 2012, 19:59 #11 (permalink) Join Date: Dec 2000 Location: Canada Posts: 137 In short yes. There are newer tools coming on to the market that offer much better tactical and strategic fuel decision making methodologies. The FMS is a good tool to fly the aircraft using a strategic method for optimum profiles generated by better tools like a flight planning systems. The problem is that the data sets that pilots input and the loaded data coupled with poor algorithms in todays FMS's results in bad information to use to make inflight decisions. Given the time and cost to build and implement new and better FMS systems, the best solution is to disconnect the tactical and strategic decision making from the FMS and use the high powere and fast changing technology in PC's or tablets for this kind of work.
 20th Jun 2012, 12:32 #12 (permalink) Join Date: Jun 1999 Location: Brisbane Posts: 252 All A320 / A321 airlines I have flown with used a generic C.I. of 35. That was 10+ years ago before fuel prices really took a hike. Sometimes there were multiple ways to achieve efficiency for my airlines, including high speed operation following late departure, when anticipating ground delays between sectors including slot time difficulties. Of course the preference would be to get back on schedule with greater efficiency during the turnarounds rather than burning extra gas doing it airborne. Some years ago I wrote a comprehensive post re uncommonly used methods of achieving efficient operation. Accurate calculation of C.I. for each long range flight is probably more significant than an 8 or 9 flight hour multi sector day where a bit more animal cunning can be more beneficial. Just to repeat one old favourite. Tanker fuel to reduce refuelling time. Direct tracking and high speed descent. Inform company and crew of short turnaround regardless of company rep insisting no need due long slot delay. Push back and hold so tug unavailability will not delay ops. All checklist completed up to just before engine start. Inform company and clearance delivery that you can be airborne within 10 minutes of getting an improved slot. Let us say that Air France ask for pushback just as their slot is about to expire and they are caught out with no tug, cargo doors still open with baggage to load etc. When their slot is cancelled, don't wait for the AF response, jump in there advising ready to taxy in 3 minutes. Worked for me close to 100% of time from Canary Islands to BRU, turning a 2+ hour turnaround into 45 minutes. Means I could leave BRU late and return early. Better than any C.I. if another aircraft is U/S or late and company sweating on getting your machine for another flight. Also very handy if duty hours need trimming to prevent extended rest period following the flight. This stuff is pure gold for a holiday charter company with tight operating margins on everything. Direct tracking, visual approach, high speed descent and reduced APU ground probably offset additional tankering burn. Not going for thread drift, but just an example indicating that C.I. is not the simple answer to cost saving. There are other ways to save costs. This is just one.

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