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lhe
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A sample fuel plan computed by SkyDemon looks like this:
Taxi/Takeoff 4,2 ltr
Enroute 2h 17m 73,6 ltr
Holding Time 45m 18,5 ltr
Landing 2,5 ltr
Contingency (10 %) 7,4 ltr
Minimum 106,1 ltr
Planned 3h 8m 106,1 ltr
How is the endurance 3h 8m calculated? No time is given for the contingency fuel, but clearly it contributed to the endurance. At the fuel flow for the chosen power setting (31.7 l/h), the contingency fuel would contribute 14 minutes for a total of 2h 17m+45m+14m = 3h 16m – 8 minutes more than the total computed by SkyDemon. (The times for enroute and holding are correct.)
Am I missing something or is there an error in how SkyDemon computes the endurance? (If there is an issue with SD, now would seem a good time to address it as the fuel plan calculations are about to be reworked anyway to include Final Reserve Fuel.)
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grahamb
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That figure is not endurance, but the planned amount required and the sum of calculated times for each of the various legs of your planned route based on the respective consumption and performance data in your aircraft profile. Tim can no doubt be much more specific on exactly how these legs are calculated.
Endurance is a calculation based on fuel loaded, not fuel required.
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lhe
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+xThat figure is not endurance, but the planned amount required and the sum of calculated times for each of the various legs of your planned route based on the respective consumption and performance data in your aircraft profile. Tim can no doubt be much more specific on exactly how these legs are calculated. Endurance is a calculation based on fuel loaded, not fuel required. No, the amount required is given as "Minimum". The amount intended to be loaded (and thus the basis for the endurance figure) is "Planned".
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grahamb
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+x+xThat figure is not endurance, but the planned amount required and the sum of calculated times for each of the various legs of your planned route based on the respective consumption and performance data in your aircraft profile. Tim can no doubt be much more specific on exactly how these legs are calculated. Endurance is a calculation based on fuel loaded, not fuel required. No, the amount required is given as "Minimum". The amount intended to be loaded (and thus the basis for the endurance figure) is "Planned". Apologies, you are of course correct. I misinterpreted what you'd written by not realising you'd set planned = required.
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PaulSS
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From what I can see, the 3:08 comes from Enroute + Contingency / fuel burn in the cruise. Take from that what you will but it doesn't seem to make much sense as far as endurance is concerned.
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lhe
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+xFrom what I can see, the 3:08 comes from Enroute + Contingency / fuel burn in the cruise. Take from that what you will but it doesn't seem to make much sense as far as endurance is concerned. That would indeed be what you expect. But, the contingency fuel / fuel burn in cruise is 7.4/31.7 * 60 = 14 minutes. Enroute + holding + contingency would be 2h17m+45m+14m = 3h16m. If the cruise fuel flow is used for the holding fuel, then it would be 2h17m+35m+14m = 3h6m However I try, I can't make out where the 3h8m comes from.
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Tim Dawson
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Right now, we take the amount of fuel planned and subtract the taxi/takeoff fuel and landing fuel specified for the aircraft, giving us the fuel available for the enroute part of the flight. That figure is divided by the fuel burn of the chosen power setting at the chosen overall level for the flight, giving the endurance figure.
If there's a better way to arrive at the figure, we are certainly open to it.
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lhe
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+xRight now, we take the amount of fuel planned and subtract the taxi/takeoff fuel and landing fuel specified for the aircraft, giving us the fuel available for the enroute part of the flight. That figure is divided by the fuel burn of the chosen power setting at the chosen overall level for the flight, giving the endurance figure.
If there's a better way to arrive at the figure, we are certainly open to it. I see. A problem with that is that the holding fuel (and the future Final Reserve Fuel) is determined with a (probably) lower fuel flow. Also, the trip fuel includes the climb which is at a higher power setting than cruise. When reading the fuel plan, I would expect that each item contributes to the endurance with its given time.
My suggestion is to compute the endurance as the sum of: 1) The trip time computed by SkyDemon 2) The holding time (and future time for FRF) 3) Time for the contingency fuel – this could be computed either using the cruise fuel flow or as a percentage of the trip time, although using the cruise fuel flow may make more sense. The difference would in any case be small. 4) Time for additional fuel above the minimum, using the cruise fuel flow.
In my example above, this would give an endurance of 3h8m.
Properly, a time should be included for the landing fuel as well, but the fuel flow is not known and the contribution would in any case be small
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grahamb
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The problem with computing any fuel endurance to the nth degree of accuracy is that a) real life rarely reflects the plan to that degree, even if the flight is straightforward, and b) if the plan goes out of the window, due to diversions or other unplanned events, all the initial assumptions (altitude, power setting, wind, climb and descent rates etc.) may be incorrect so the endurance will be what you can achieve, not necessarily what you thought.
I'm not for a minute suggesting that you should set off without a good grasp, and a record, of how much fuel the aircraft has on board, how much you calculate you will need, and what nominal reserves and endurance that represents, but agonising over whether it's 3:08, or 3:11, or 3:06 is frankly rather pointless.
On an IFR flight plan with full fuel I'll put 4:50 as my endurance, but I know I could easily stretch that to 5:15 or more without compromising my cruise speed too much if the chips were down.
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Tim Dawson
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Thank you, it's a suggestion that clearly has merit. Before making such a change though I would be interested in the other contributors to this thread weighing in also. One of the uses for the calculated endurance is on flightplans, where search and rescue want to know how far the aircraft could have got to if contact has been lost.
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