pcubine 15 Posted December 19, 2017 Share Posted December 19, 2017 I am using this PFPX feature for PMDG 744 and 777 Variants. The problem is the results are all over the place. On one flight the bias was from 96.0% to 104.3%. To get the raw data for the calculator I am using the aircraft's instruments and FMC positioned in such a way the I can take one screenshot and have all the data necessary. This only gives me the data for an instant in time. I do this for each flight level of flight. The flight I am doing now is RKSI-KORD and the screenshots will be taken at FL330, FL350, FL370, and FL390. I am using real time weather from AS16 and right now I have a tailwind of 259/164.THe component is a 139 knots. Maybe I should not use any weather. Any ideas as to what the problem is? Link to comment Share on other sites More sharing options...
VHOJT 57 Posted December 21, 2017 Share Posted December 21, 2017 Hi Michael, Okay! Are you ready? Basically, I have had a lot of issues trying to get PFPX bias good for PMDG aircraft. Through a lot of trial and error, I think I have found a solution. I suspect the PMDG aircraft don't quite burn at the correct rate, or some other simulator issue, rather than it being a problem with PFPX's inbuilt profiles - although please my note in the conclusion. First things first The 747 and 777 profiles that come with PFPX were created by a real-world dispatcher with access to very detailed data far in excess of any FPPM or FCOM data provided as manuals with the plane. I have found these profiles to work extremely well. I suggest using them when you can. I have found PMDG 747 and 777 both need negative bias, they seem to burn less fuel than PFPX predicts. I think it is fine to take measurements with weather, provided you have noted all the temperatures and winds - in fact, for me, this turned out to be quite important in finding a solution. It can be useful to take a few readings with zero weather in ISA conditions. Any measurements are only applicable to the speed/cost index at which you are using at the time. My experience is primarily with 747-400 with RR RB211-524GH2 engines. I have, however, found this method to be equally applicable to other aircraft, more recently the 777-300ER. My method for finding fuel burn This method takes a little practise, but I have found it to be the most accurate way to measure fuel burn over short periods. The engine instruments do not give a precise enough reading. Have the plane nicely established in cruise. Open one of the FMCs to the FS ACTIONS - FUEL page (the page where you would usually load fuel). Prepare yourself, and start a stopwatch on your phone. Note down the amount of fuel as you pressed start on the stopwatch. Go and note down data on the other FMC/displays, in this order: GW, SAT, ALT, TAS, GS. As your stopwatch approaches a minute, be ready looking back at the fuel page on the FMC. As a minute ticks by, note the amount of fuel on the fuel page. Subtract the second reading of fuel from the first, and then multiply the answer by 60 to get your fuel flow in lb or kg per hour. I suggest doing 2-3 of these in quick succession. Input the data into PFPX for each trial, and note the bias,drag figures and ISA deviation (ISA deviation is important!) Average the PFPX bias and drag figures by adding up the results of each of your trials, and divide by the number of trials. Use this as your bias/drag figure for this set of trials. Obviously, the longer the trial, the less error-prone your trials will be. Your results should only vary by 0-5 kilograms per minute. The less difference, the better. Remember, as you multiply the result by 60 to get per hour fuel burn, any error will multiply too. The results, and the problem After a few flights (all with weather) of measuring bias when heavy, medium and light, I plotted the results in a table in Excel. I noticed a number of fairly varying figures depending on gross weight and temperature. I then plotted a graph of PFPX's suggested fuel bias vs ISA temperature deviation and noticed a trend that PFPX would suggest less fuel (i.e. larger negative bias) at higher ISA deviation temperatures. Again, I am not sure why this is the case - as I said I suspect the PMDG aircraft or the simulator itself. I also averaged all the suggested fuel burn biases, drag biases and ISA deviation biases from my trials. This left me with one average fuel burn bias, drag bias and ISA deviation from all the trials. Back to my table and graph, I found that roughly: For each 10 degrees of ISA deviation, there was a ~3% change in suggested bias. In the case of the aircraft I have used, it is -3% fuel bias per each +10 ISA deviation. The solution From all your trials, calculate an average fuel burn bias, drag bias and ISA deviation. Input the former two into your PFPX aircraft profile. When flight planning, check what the average ISA deviation is (temporarily display the plan in an OFP format that shows ISA deviation it if your OFP doesn't). Note the flight's ISA deviation. I will give an example below to explain what to do next: Your average ISA deviation: +6.2 Your flight's ISA deviation: +10 The difference (always do it in this order): Average - Current = 6.2 - 10 = -3.8 As a percentage of 10: -3.8 / 10 = -0.38 Multiplied by three: -0.38 * 3 = -1.14 These two steps are done because, for each 10 degrees ISA deviation, there is a 3% change in fuel bias. You want to find how much correction to apply. For example, if your average ISA deviation was +5, and this flight's ISA deviation was +15, you would subtract 3% from your average fuel bias to get the figure for this flight. However, you won't always have nice clean numbers, so this method calculates it for you. Clear as mud? Add it to your average bias figure: Average + Correction = -4.8 + -1.14 = -5.94 = -5.9 (rounded to one decimal place). Finally, input this figure into your PFPX aircraft profile, apply it, check your flight plan, and recalculate your flight. Notes I have found +3% per -10 ISA deviation from your average, and -3% per +10 ISA deviation from your average, to work very well for the 747-400. I consistently now get fuel burning accurately along the whole flight, and land with an error of only a few hundred kilograms usually. I have also found this rule equally applicable to other PMDG aircraft. I was lazy with the 777 and cheated a bit. I got an average bias in zero weather for a few different flight levels and weights, and applied the same -3% to fuel bias for each +10 ISA deviation on the flight. I have had very good results with that (but have only done a couple of flights) - with burn being within 300kg for the flight, and landing with a 300kg discrepancy. I haven't bothered for the 737 because I usually use it for short flights and fuel burn is pretty much bang on. The same for the FSL A320 profiles - they don't need any adjustment as far as I can tell, as they custom-made them for their aircraft, whereas the Boeings are PFPX real-world data vs PMDG simulated burn. My figures if you want to cheat (for these aircraft): 747-400 RB211-524GH2 Cost Index: 100 Average ISA dev: +6.2 degrees C Average fuel bias: -4.8% Average drag bias: +1.2% 777-300ER Cost Index: 100 Average ISA dev: 0 degrees C Average fuel bias: -2.0% Average drag bias: 0.6% 737-800 Cost Index: 20 Average ISA dev: 0 Avreage fuel bias: +1.0% Average drag bias: +0.2% Note that I have not done a lot of testing with the 737. Conclusion I hope this helps a bit, and hasn't confused you too much. I did note from forum posts a while back, that PFPX had issues with ISA deviation and fuel burn. I think this has been rectified since. However, those posts said to uninstall, and reinstall all the aircraft into PFPX, whatever that means. I haven't done this, and hence, I have a slight bit of confusion as to whether I am still suffering this problem. I am hoping another forum member might be able to chime in to clear this bit of confusion up. Also, if anybody else has suggestions, or a better way to work around it, please - let us know! Let me know if you have any questions. Cheers, Rudy Link to comment Share on other sites More sharing options...
pcubine 15 Posted December 21, 2017 Author Share Posted December 21, 2017 2 hours ago, VHOJT said: Prepare yourself, and start a stopwatch on your phone. Believe it or not, I don't have a cell phone. My wife has a cell phone I will use hers or my second computer for a stopwatch. That is interesting that the bias and drag change with different cost indexes. Thanks very much for the information on your procedure. I am certain that I will have some questions once I have started using your procedure. Thank you Michael Cubine Link to comment Share on other sites More sharing options...
phil747fan 52 Posted December 21, 2017 Share Posted December 21, 2017 Michael, we had a different issues with the queen you needed at the time of FOC adding +4 to 10% to get pmdg 747 having the flightplanning fuel right ... and at the time i have lovely discussions with Rudy .... i can find you some real flightplanning charts we were using to have a follow up on a cruise. it will give you some ideas between calculation, "real" fuel burn and actual one when landing. during your long cruise checks you should follow on your printed plan what you met for each point and what the expectation for fuel. and again the problem is not coming from profiles ... all the best. Phil Link to comment Share on other sites More sharing options...
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