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Check_Airman

Cost Index and OPOT ALT

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Edit: How embarassing. The topic should of course read "OPT ALTitude".PMDG continues to amaze me with the level of realism they have modeled.Can somebody in the know please explain to me what has got to be one of the most amazing features of the MD11 please? I

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As suggested by the variable name, it adjusts the trade off between performance and economy, which is in itself inadequate to optimize revenue per mile because of the multitude of factors that go into the selection of an optimum value. In order to select the best value, the primary determinate is probably fuel costs.A flight with a CI of 33 will cost less than a flight of 100 if fuel costs are relatively high, but if fuel is cheap compared to other costs then a CI of 100 could be more "economical," or at least provide a higher net cash flow for the owner.Given all the variables, it is good that in the sim world our costs are pretty much limited to our "utility costs" to use an economists term, or in lay terms the value of the time you spend playing verses the cost of buying the toys.When you used a CI of 400, I suspect that your economy speed was the max cruise speed available within the FMS. I am pretty sure I have edited speeds higher than econ, but I have never used a CI over 100. I like using 33 or 100 depending on how I feel when I get up in the morning. I've read a developer that always uses 77 for his own reasons, and a line pilot in the beta tests that always used 88. Go figure. I like 33 because I read once that that is what SWA used.Now, the central point you raised was optimum altitude as a function of cost. The lowest fuel costs are usually at the highest obtainable altitudes simply due to the thinner air, but the TAS will also decrease at higher altitudes; hence, block time increases. The FMS calculates the ECON altitude based on CI, trading off time enroute (one cost) for fuel cost (the other cost).This is just a layman's explanation by someone who only understands a part of all that is going on in the economics of flying an airplane.

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Thanks Dan,My normal CI is 100 in lieu of some RW data. I was using 400 to try and make up some time. I would have imagined that the FMS would be able to fly faster than .844 though. On other types, the FMS is limited to something like MMO-0.05. It seems I'll have to do some more experimenting so I can learn more about this behavio(u)r. Not that I mind, because it means I'll have to fly some more :DPaul

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Paul, keep track of your experimental data because I think it will be of value to others.I know, more flying but someone has to do it.

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Yes Paul, i'm very interested in your findings
Hello,Yesterday i took off from EHAM to CYYZ with 1 hour behind schedule, and I used a CI of 300 for a cruise flight level 320 and 340 then 360 reached by HO. It was also limitated to 0.844 MI also noticed that if you are in cruise, let say FL320 with a CI of 45, it will give you for a given a/c weight the max obtainable FL at 366 (my case yesterday). Modify the CI to 300 and tis FL will be reduced to FL337 (also the case yesterday).The other reason wy the FMS limits to 0.844 is when you reach your TOD, you will notice that you are rapidely and at hihg altitude arriving in the upper barber pole of the MMO/IAS, before the FMS switch to IAS (depend on your option).Cyrille

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