fuel flow discrepancy

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Aart
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fuel flow discrepancy

Postby Aart » Mon May 15, 2017 8:08 am

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 settingmanual gphActual readingdifference
45%3.33.56 % overread
50%3.63.97 % overread
55%4.04.25 % overread
60%4.44.32 % underread
65%4.84.72 % underread
70%5.24.96 % underread
75%5.75.45 % underread
80%6.25.95 % underread
85%6.76.45 % underread
90%7.26.77 % underread
100%8.37.411% 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!
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Re: fuel flow discrepancy

Postby Aart » Mon May 15, 2017 8:10 am

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
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Re: fuel flow discrepancy

Postby CFIDave » Mon May 15, 2017 1:20 pm

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.
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Re: fuel flow discrepancy

Postby Aart » Mon May 15, 2017 2:48 pm

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..
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Re: fuel flow discrepancy

Postby Lance Murray » Mon May 15, 2017 3:13 pm

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.
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Re: fuel flow discrepancy

Postby carym » Tue May 16, 2017 5:16 pm

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.
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Re: fuel flow discrepancy

Postby Aart » Tue May 16, 2017 5:20 pm

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.
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Re: fuel flow discrepancy

Postby carym » Tue May 16, 2017 5:24 pm

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).
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Re: fuel flow discrepancy

Postby Aart » Tue May 16, 2017 5:32 pm

That's a lot closer to what the book says (5.7) than my reading (5.4)..
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Re: fuel flow discrepancy

Postby carym » Tue May 16, 2017 6:08 pm

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.
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