Fuel leak quandary

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haykinson
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Fuel leak quandary

Post by haykinson »

My new DA40NG has been dripping fuel from the left wing, and I'm not sure what I'm doing wrong. Let me explain.

Yesterday, I had an FBO fill up my tanks. I verified that they left the left wing fueled to about two inches from the top, and then flew for about 3 gallons worth of distance to my home airport, in Normal fuel mode, never using the transfer pump. This morning, a local shop tells me that my wing is leaking, and indeed there was a puddle of jet fuel under the wing and drips coming out of the drain hole. The left tank was full to the brim, and I could no longer see fuel in the extended part of the right tank. At least four hours later I got a call from the airport that the wing was STILL dripping.

My airplane is at a 1⁰ incline towards the left wing. So theoretically if the fuel somehow was gravity-feeding itself from the right to the left, and then heat making the left side expand and drip, this would happen.

Has anyone else encountered this? I called Diamond and they had few suggestions other than making sure bugs didn't block the drain holes (but I'm not sure how any of that would lead to fuel self-transferring).
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Re: Fuel leak quandary

Post by Boatguy »

If the fuel selector was on Normal, and you flew for 3gal worth (half an hour?), the critical piece of missing information is what the tanks looked like after you landed. We don't know if this happened during the flight, or on the ground.

The fuel transfer pump shouldn't go on, even when switched on, if there is more than 13gal in the left tank. This is easy to test. Turn on the master and turn on the transfer pump. Nothing should happen.

I've never seen fuel gravity feed from right to left, but I've seen fuel systems on very expensive new boats that were plumbed incorrectly. I also saw a fuel system where the fuel valve itself was incorrectly assembled (at the valve factory no less) and caused fuel to move in a direction other than indicated by the position of the handle. For example if the fuel selector is plumbed incorrectly, or the handle is not actually in the position indicated (e.g., valve is rotated below), or the valve is not assembled correctly, so it was actually in the Emergency position, the engine would act as a fuel transfer pump and move fuel from right to left. If this plane is brand new, straight from the factory, those are possibilities.

I'd start with verifying the plumbing and function of the fuel selector.

To be clear, excluding the gravity feed scenario, you should have arrived home with 16.5gal in the left tank, and 19.5 in the right tank and no fuel leaking. I've only seen the "heat expansion" leaks, or small leaks, when a tank was filled to overflowing with fuel running across the wing (never talk to the line guy while they're pumping).

There was also an owner who reported something like this about two years ago, a mysterious transfer; you might use the advanced search to see if you can find that thread.
Last edited by Boatguy on Tue Jun 28, 2022 11:58 pm, edited 1 time in total.
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Re: Fuel leak quandary

Post by haykinson »

Thanks for your thoughts here!
Boatguy wrote: Tue Jun 28, 2022 11:12 pm If the fuel selector was on Normal, and you flew for 3gal worth (half an hour?), the critical piece of missing information is what the tanks looked like after you landed. We don't know if this happened during the flight, or on the ground.

The fuel transfer pump shouldn't go on, even when switched on, if there is more than 13gal in the left tank. This is easy to test. Turn on the master and turn on the transfer pump. Nothing should happen.
The left tank is still full. It’s been dripping all day, so I think fuel is still self-transferring from right to left. The transfer pump does not turn on. Indicators show full left and 13gal right at the moment and the dripping might have stopped.
Boatguy wrote: Tue Jun 28, 2022 11:12 pm I've never seen fuel gravity feed from right to left, but I've seen fuel systems on very expensive new boats that were plumbed incorrectly. I also saw a fuel system where the fuel valve itself was incorrectly assembled (at the valve factory no less) and caused fuel to move in a direction other than indicated by the position of the handle. For example if the fuel selector is plumbed incorrectly, or the handle is not actually in the position indicated (e.g., valve is rotated below), or the valve is not assembled correctly, so it was actually in the Emergency position, the engine would act as a fuel transfer pump and move fuel from right to left. If this plane is brand new, straight from the factory, those are possibilities.

I'd start with verifying the plumbing and function of the fuel selector.
It’s definitely plumbed correctly. On a long flight it clearly draws from the left and I have to transfer every so often for a couple of minutes. I have never moved the fuel valve but just turned it from Normal to Emergency and back so as to ensure that it was in the correct position, but looking at the fuel system diagram I think it’s unlikely that this would cause a transfer without the engine running.
Boatguy wrote: Tue Jun 28, 2022 11:12 pm To be clear, excluding the gravity feed scenario, you should have arrived home with 16.5gal in the left tank, and 19.5 in the right tank and no fuel leaking. I've only seen the "heat expansion" leaks, or small leaks, when a tank was filled to overflowing with fuel running across the wing (never talk to the line guy while they're pumping).

There was also an owner who reported something like this about two years ago, a mysterious transfer; you might use the advanced search to see if you can find that thread.
I read through that thread… viewtopic.php?t=6544 … but it was inconclusive in the end.
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Re: Fuel leak quandary

Post by Boatguy »

So the plane is not fresh from the factory and the long trips with the proper operation rules out the plumbing or valve issues.

My fuel selector is safety wired in the Normal position and requires breaking the wire to move to Emergency (which is really just having the engine draw from the right tank). Is yours not safety wired?

If this is a gravity feed issue, then it is bidirectional? Some pumps will allow a flow when they are off which would open a pathway, though I would think that the fuel would find a "level" point after which it would stop when the right tank was not higher than the left.

If it has stopped, you could test the gravity feed theory by pulling the left or right main up onto a block or a jack (from your local mechanic?). With the right up on a block, it should continue to feed to the left and flow out onto the ground. Or with the left up on a block, sufficient to raise it 2˚ so that it overcomes the parking spot and establishes 1˚ to the right, does fuel flow back to the right?

As always better to figure this stuff out on the ground than in the air!
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Re: Fuel leak quandary

Post by chili4way »

This morning, a local shop tells me that my wing is leaking, and indeed there was a puddle of jet fuel under the wing and drips coming out of the drain hole.
To make sure I have a clear picture, is the "drain hole" the place where you sump the fuel or the vent that's further out on the wing, contained in the circular "drain cover"? I only mention this because one time, my sump drain didn't seat correctly and there was fuel leaking from it. I just had to twist it a little to get it to seat correctly.

I looked at the schematics in the AMM (Fig 2, Page 3, 28-00-00) and the illustrations in sections 28-20 in the IPC. I can only imagine three ways that fuel could transfer from the right/aux tank to the left/main tank without the fuel transfer pump running.
  • the fuel transfer pump has an impeller defect where fuel can pass through it when the pump is not running, and there is sufficient gravity feed. Perhaps it's the odd chance that the fuel pump stopped in a weird position that allows the "transfer". You'd have to fly the plane enough (or otherwise safely drain enough fuel from the left/main tank) to be able to run the fuel transfer pump and see if it persists after the flight with a full fuel load.
  • the Restrictor on the right/aux tank isn't working to relieve fuel tank pressure and the fuel and air in the right tank are expanding enough to create enough pressure to turn the fuel transfer pump, perhaps with some gravity assistance. That's easy to test by opening the cap on the right fuel tank.
  • as was previously suggested, there's a plumbing error or defect in the fuel cooling loop; this seems unlikely as you'd see the effect of this during in-flight operation with the level of the right tank increasing.
I admit I'm grasping at straws on these three. Hopefully, the Diamond Customer Support folks willl give you a "real answer".
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Re: Fuel leak quandary

Post by Rich »

Looking at the POH and IPC:

Here's the important part: The transfer pump is of a type that in no way inhibits forward flow produced by other factors. It is the same part number used in the DA40-180 for the boost/standby pump. In this application it is routine for fuel to flow through at rates > 17 GPH, pulled through by the engine-driven pump with the boost pump turned off.

On the NG there is a spring-biased check valve on the input side of the transfer pump. If this has a bit of debris or is worn it would allow fuel to flow from the right to the left in a non-level scenario.
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Re: Fuel leak quandary

Post by haykinson »

Boatguy wrote: Wed Jun 29, 2022 3:13 am My fuel selector is safety wired in the Normal position and requires breaking the wire to move to Emergency (which is really just having the engine draw from the right tank). Is yours not safety wired?
It was, I chose to break the safety wire (with engine off) to ensure that the selector was properly against its stops.
Boatguy wrote: Wed Jun 29, 2022 3:13 am If this is a gravity feed issue, then it is bidirectional? Some pumps will allow a flow when they are off which would open a pathway, though I would think that the fuel would find a "level" point after which it would stop when the right tank was not higher than the left.
...
Interesting suggestion. I'll have the shop try this if they get stuck.
chili4way wrote: Wed Jun 29, 2022 3:33 am To make sure I have a clear picture, is the "drain hole" the place where you sump the fuel or the vent that's further out on the wing, contained in the circular "drain cover"? I only mention this because one time, my sump drain didn't seat correctly and there was fuel leaking from it. I just had to twist it a little to get it to seat correctly.
I should have been more clear in my language. Yes, I meant the vent on the drain cover, mid-wing, and not the fuel sump.
chili4way wrote: Wed Jun 29, 2022 3:33 am Perhaps it's the odd chance that the fuel pump stopped in a weird position that allows the "transfer".
This is the second time this issue had happened. The first time I assumed that I'd somehow overfilled the left tank in the evening, and then expansion kicked in, but the cause was likely the same. It's hard to imagine the pump continually getting stuck in this off position — it would seem more likely that there's a defect.
chili4way wrote: Wed Jun 29, 2022 3:33 am the Restrictor on the right/aux tank isn't working to relieve fuel tank pressure and the fuel and air in the right tank are expanding enough to create enough pressure to turn the fuel transfer pump, perhaps with some gravity assistance. That's easy to test by opening the cap on the right fuel tank.
This was definitely not the case. There was no undue pressure when I opened the tank, and even though it was down by a few gallons already the transfer continued for hours after I opened the right tank. However... this in general makes me wonder. If the transfer pump impeller starts moving, might it just continue to do so?
chili4way wrote: Wed Jun 29, 2022 3:33 am Hopefully, the Diamond Customer Support folks will give you a "real answer".
Hm, so far they were significantly less helpful than I hoped. Their suggestion was to try to blow air via the vent and see if I get bubbles or if the vents are stuck.
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Re: Fuel leak quandary

Post by Rich »

Rich wrote: Wed Jun 29, 2022 2:19 pm Looking at the POH and IPC:

On the NG there is a spring-biased check valve on the input side of the transfer pump. If this has a bit of debris or is worn it would allow fuel to flow from the right to the left in a non-level scenario.
Test this: Disconnect the transfer pump input fuel line from the pump. If fuel flows from this line it is likely defective in some way. It should not allow this with the pump not running. Also try it with the right side fuel cap removed to ensure the tank is not somehow pressured.
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Re: Fuel leak quandary

Post by haykinson »

And now for the exciting conclusion — the shop had replaced the transfer pump, and leaking continued. They then discovered that there is a check valve upstream of the transfer pump — normally, the valve is supposed to prevent the flow of fuel from left tank to the right, but apparently there's also an undocumented feature where that same valve prevents the flow from right to left unless there's 2 PSI of suction. When the transfer pump runs, its suction opens this valve. In my case, for some reason the valve allowed transfer even without this suction. Once the valve was replaced, the uncommanded fuel transfer stopped.

I've yet to confirm this with a full tank, but the shop said they left it for a few days and observed no transfer, so they called it fixed. They also suggested to Diamond to document this feature of the check valve in the maintenance manual.
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Re: Fuel leak quandary

Post by Rich »

haykinson wrote: Sat Jul 23, 2022 3:12 am They then discovered that there is a check valve upstream of the transfer pump
OK, I have to comment. They discovered the check valve that I pointed out is clearly shown in the AFM?
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