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fuel flow discrepancy
Posted: Mon May 15, 2017 8:08 am
by Aart
A fellow DA42 owner and myself (Conti CD135 engines) have concluded that the the FF meters of our aircraft seriously over-read at cruise power settings and under-read at low power settings.
See the following table, comparing the actual FF (as per the manual, very accurate) to the FF reported by the FF meter
Power setting | manual gph | Actual reading | difference |
|
45% | 3.3 | 3.5 | 6 % overread |
|
50% | 3.6 | 3.9 | 7 % overread |
|
55% | 4.0 | 4.2 | 5 % overread |
|
60% | 4.4 | 4.3 | 2 % underread |
|
65% | 4.8 | 4.7 | 2 % underread |
|
70% | 5.2 | 4.9 | 6 % underread |
|
75% | 5.7 | 5.4 | 5 % underread |
|
80% | 6.2 | 5.9 | 5 % underread |
|
85% | 6.7 | 6.4 | 5 % underread |
|
90% | 7.2 | 6.7 | 7 % underread |
|
100% | 8.3 | 7.4 | 11% underread |
|
Would be great if any of you flying this engine (or the 1.7) could record what the FF meters are saying and report back. Thanks!
Re: fuel flow discrepancy
Posted: Mon May 15, 2017 8:10 am
by Aart
messed up table.. First column is power setting, second one FF by the book, third one FF by the FF meter and last one % difference
Re: fuel flow discrepancy
Posted: Mon May 15, 2017 1:20 pm
by CFIDave
I think some of the discrepancy is due to things like altitude and temperatures.
The "load" % set by the pilot with the power lever(s) may not reflect the actual % power produced, but is just one of the inputs to the ECUs in managing the engine. There is a strong correlation between load % and RPM based on a published engine map (electronic lookup table), but ECUs will vary fuel flow rate to accommodate the specific temperatures (ambient air, coolant, gearbox, etc.) and barometric pressure/altitude.
Re: fuel flow discrepancy
Posted: Mon May 15, 2017 2:48 pm
by Aart
Noted Dave, but the end result of all this is that I have -say- 5 gallons less in my tanks than what the totaliser says after a 5 hours flight. More or less regardless of altitude flown btw. This cannot be right. I was under the impression that the combination of the FF meter and the totaliser had to be pretty accurate..
Re: fuel flow discrepancy
Posted: Mon May 15, 2017 3:13 pm
by Lance Murray
Possibly the "K Factor" can be adjusted in the settings? Not the pilot settings page but the Garmin settings page that the avionics tech has access to.
Re: fuel flow discrepancy
Posted: Tue May 16, 2017 5:16 pm
by carym
Aart wrote:Noted Dave, but the end result of all this is that I have -say- 5 gallons less in my tanks than what the totaliser says after a 5 hours flight. More or less regardless of altitude flown btw. This cannot be right. I was under the impression that the combination of the FF meter and the totaliser had to be pretty accurate..
There must be something wrong with your settings. I never found more than a 1-2 gal discrepancy between the totalizer and what was actually put in the tanks. Usually there was no discrepancy at all. I would occasionally lose up to 1 gal out the vent from the right tank after filling the tank to the top. I avoided this by putting about 1 gal less than full in each tank, and when resetting the totalizer I would decrease the full totalizer by 2 gallons.
Re: fuel flow discrepancy
Posted: Tue May 16, 2017 5:20 pm
by Aart
Thanks Lance and Gary.
Gary, I know you don't fly the DA42 any longer but do you recall your fuel flow figures at various power settings?
You must have used a basis for flight planning.
Re: fuel flow discrepancy
Posted: Tue May 16, 2017 5:24 pm
by carym
Aart wrote:Thanks Lance and Gary.
Gary, I know you don't fly the DA42 any longer but do you recall your fuel flow figures at various power settings?
You must have used a basis for flight planning.
I almost always flew at 75% power and FF was between 5.5 and 5.6 gal/hr (more often at 5.6 gal/hr).
Re: fuel flow discrepancy
Posted: Tue May 16, 2017 5:32 pm
by Aart
That's a lot closer to what the book says (5.7) than my reading (5.4)..
Re: fuel flow discrepancy
Posted: Tue May 16, 2017 6:08 pm
by carym
I would occasionally see 5.7, but usually it was 5.6. There must be some setting that is off in your G1000. By the way, I had 1.7 engines but I don't know if that makes any difference.