Unbalanced fuel usage

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MMcfly
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Unbalanced fuel usage

Postby MMcfly » Sat Dec 02, 2017 7:42 pm

New to the forum but we have an issue with a DA42 Tdi and balanced fuel usage. The aircraft is a 2006 aircraft with low hours, approx 450, with no issues to date at all. However for some reason we have found that we are putting more fuel in one wing tank (incl aux) than the other by some margin ie 100% more in one wing tank than the other. Fuel is being used from both tanks but not in equal amounts. Fuel flow is even on both engines and aircraft is flown at normal cruise settings. After 3.5 hours flying one side used 26usg the other side 13usg in total (aux tanks fitted and fuel transferred). No fuel leaks and the total fuel used is correct but not even. The engineers are confused, has anybody experienced this before or can anyone offer any advice?
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Re: Unbalanced fuel usage

Postby Colin » Sat Dec 02, 2017 10:02 pm

How do you know that there are no fuel leaks?
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Re: Unbalanced fuel usage

Postby MMcfly » Sat Dec 02, 2017 10:30 pm

The total fuel used is as expected for a flight of this length. So no leakage otherwise the a/c is using less than the 11usg total/hour (verified by fuel flow meters and previous historic data). Also no evidence of fuel leak at all ie staining of aircraft. This is not one isolated flight but a result of a number of flights over a period of time.
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Re: Unbalanced fuel usage

Postby FlyingShrink » Sat Dec 02, 2017 11:46 pm

Hello All,
Well I am not an engineer but if there are no leaks and if the fuel burn is as expected for the flight , then the only logical answer is that one tank is cross feeding to the other , the only way the fuel can reach the other engine.

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Re: Unbalanced fuel usage

Postby SPPMNC » Sun Dec 03, 2017 8:54 am

Hi all,
I the fuel system the same on the VI and the TDI ? I am also going for FlyingShrink's guess. One tank may be cross feeding the other.
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Re: Unbalanced fuel usage

Postby MMcfly » Sun Dec 03, 2017 2:48 pm

That is what I think, but we are being told the design of the crossflow valve will not allow that to happen? Anyone heard of a similar issue?
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Re: Unbalanced fuel usage

Postby Colin » Sun Dec 03, 2017 7:12 pm

Well, I have a similar issue that I have been chasing over a year, but I am pretty sure it is some sort of leak.

Latest proof was that I topped the tanks, the plane sat for two weeks (very unusual for me), and when I returned nine gallons were missing from the left tank.

There wouldn't be staining with Jet-A, correct?
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Re: Unbalanced fuel usage

Postby MMcfly » Sun Dec 03, 2017 8:48 pm

The aircraft is infrequently used and kept in a hangar so we know it isn't leaking sat on the ground. 9 gallons is a lot to lose though.By staining I thought if it was leaking then dirt will stick to it wherever it is parked as opposed to fuel dye staining. Jet A1 (diesel) takes a while to evaporate.
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Re: Unbalanced fuel usage

Postby Rich » Mon Dec 04, 2017 1:26 am

Here's a thought based not on experience with a DA42 (never flown one) but with a M35 Bonanza which shared a similarity with the DA42, which is this: More fuel is typically delivered to the engine than it can use and the excess flows back to one of the fuel tanks. Now in the M35, this was always the left main regardless of the selected tank (this one also had aux tanks). It was built that way. So on a typical flight you used the left main first, then switched to another, watched the left main fill up, switched to the left main again for awhile - rinse and repeat.

Could it be that fuel from both engines is being returned to one tank, rather than to the respective tank from which it was drawn? This would be some sort of linkage/leakage problem in the valves, of course. As would possibly a failure of the crossover system itself.
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Re: Unbalanced fuel usage

Postby FlyingShrink » Mon Dec 04, 2017 1:38 am

In the Schematics, both fuels crossfeed pipes are completely independent and separate, including the fuel return lines and heat exchangers. So, IT is possible that the cross feed on the side you have more fuel is pulling from the other tank. If there was a leak, you would get a distinctive smell from Jet A. Alternatively you could use a fluorescent dye to detect this. http://spectroline.com/aviation-products/

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