Fuel Vent/Leak Issue

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Bede
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Fuel Vent/Leak Issue

Post by Bede » Fri Aug 31, 2018 3:17 pm

I just purchased an Avid Magnum (similar to Kitfox) homebuilt. I am blowing fuel out of the left tank in flight. Around 4 USG in 15 min, and then it slows considerably. There is no obvious fuel leak and the fuel doesn't go down when the plane is just sitting. I replaced the fuel caps with new caps. The only thing left that I can think of is a possible venting issue.

The fuel system is simple. 2 gravity fed tanks go through a small fuel valve to a 1 USG header tank on each side. Each header tank goes to a T and then to a simple on/off valve. The vent is a simple 1/8" aluminum tube coming out from the top of the tank into the air stream.

Any troubleshooting ideas?
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Re: Fuel Vent/Leak Issue

Post by PilotDAR » Fri Aug 31, 2018 6:14 pm

Is the vent valve in the fuel cap working properly? The low pressure over the top of the wing can suck fuel out in flight, particularly if the vent can crossfeed from the other tank. Thus fuel can be lost in flight, but not on the ground. The lower the fuel gets in the tank, the less it will flow out by low pressure above. I had a 177RG and a C 206 do this to me, losing most of the fuel from that tank. On the other hand, I found the fuel cap for my 150 on the runway at home after a day's flying, and I did not seem to have lost any fuel. The cap must have come off during my takeoff, and I flew several hours without realizing I did not have it - 150 has a different airfoil to the 177 and 206.

So check the correct function of the fuel cap vent first - it should let air into the tank, but hold pressure to hold fuel in the tank. It's the tank vent which will prevent any pressure build up in the tank. Check that the cap vent will let air in to the tank, but not out and check for a fuel tank vent being plugged, those are the most easy things to check.
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edmanster
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Re: Fuel Vent/Leak Issue

Post by edmanster » Fri Aug 31, 2018 8:16 pm

I know that the vent on a Cessna 172 is located directly behind the wing strut - in the "dead air" space. Odd that your vent would be exposed into open flow air ..
I had a harrowing experience with a Navajo. The engine oil vent lines sticking down through lower engine cowl. On this newly acquired aircraft - after takeoff - we watched the "oil pressure drop like a rock". We had to shut the engine of in-flight; all the engine oil was gone!. After landing - we noted that the vent lines did not protrude sufficiently below the cowl. We re-adjusted so they protruded at least an inch below to solve the issue.
... Just an idea .. get an airline & blow a stream of air over the top of the vent line (with full fuel tanks) .. see what happens. Tilt the vent lines forward & back.
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Re: Fuel Vent/Leak Issue

Post by Bede » Sat Sep 01, 2018 5:15 am

These vents are right in the air flow on top of the wing. There are 2 on each wing, I believe that they are balanced.

Why are some fuel vents out of the air flow (behind a strut or shield, and some aren't? That may be part of my problem.
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Re: Fuel Vent/Leak Issue

Post by PilotDAR » Sat Sep 01, 2018 7:01 am

Why are some fuel vents out of the air flow (behind a strut or shield, and some aren't?
In the case of Cessnas, I understand that the vent is behind the strut to keep it out of icing. I don't expect that your aircraft is intended to tolerate any icing whatever, so this was probably not considered. Cessna has a wealth of experience in all realms of systems design, and have documented well what a compliant design is. Less common types may have a little less definition of configuration.
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ahramin
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Re: Fuel Vent/Leak Issue

Post by ahramin » Sat Sep 01, 2018 10:09 am

I'm assuming you are running Mogas. Without the benefit of dye you are really going to have to apply some differential diagnosis to this problem. List everywhere it could be leaking from, order it from most likely / easiest to test, to least likely / hardest to test, eliminate the impossible ones, and start testing. 16 gal / hour may seem like a huge amount but it is within the realm of possibility to be just a loose fitting or a bad seam in the tank. Don't assume that because it only happens in flight with full tanks that it must be leaking from the vents or the top of the tank. It could be that it's much lower in the system but it only leaks with the combination of head and vent pressure.

You've installed new caps, but if the filler hole is the problem, a new cap still will not seal properly. Near the top of your list should be a pressure test of the tank. EAA has some videos on how to do that. If you think it's caused by vent pressure, make yourself something to shield the vent from wind pressure in flight.
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Re: Fuel Vent/Leak Issue

Post by photofly » Sat Sep 01, 2018 12:49 pm

If it will run on AvGas, put 100LL in it and look for the blue stains.
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Re: Fuel Vent/Leak Issue

Post by Bede » Sat Sep 01, 2018 7:25 pm

I put avgas in it. No stains. It's a fibreglass tank with flush mounted caps. I'm almost certain I would see a leak in the fittings as they're almost all exposed. I think that I'd be leaking gas if I left it sitting for a while if that were the case. There's only a leak when I fly.

One thing that tipped me off to a venting issue was that I closed the right fuel valve and drained the left tank. I filled the left tank to make a dip stick. When I opened the right valve, fuel started pouring out of the left vent, but it's not like the plane was sitting at an angle. The fuel seemed to be coming out under pressure.

Thanks for all of the suggestions.
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Re: Fuel Vent/Leak Issue

Post by edmanster » Sat Sep 01, 2018 9:32 pm

Looking at several pix of Avid Magnum aircraft on the internet; the Vent appears to be a L-shaped tube; approx 3 inch rise and 1.5 inch forward aimed into the air stream. This arrangement would definitely pressurize the fuel tank. But only 1 vent per wing is shown. Is this what your vent looks like ??
REF: https://alchetron.com/Avid-Flyer
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Re: Fuel Vent/Leak Issue

Post by Bede » Sun Sep 02, 2018 5:46 am

Almost identical but mine has 2 per wing. The lower one is about 1.5" below the upper one.

I'll look at the plans again when I get home but I seem to recall that the lower right one is connected to the upper left one and vice versa so they're balanced.
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Re: Fuel Vent/Leak Issue

Post by AirFrame » Sun Sep 02, 2018 7:49 am

Forgive a silly question, but sometimes the simplest answers are overlooked... Have you checked that the vent lines are clear? Take the fuel cap off and blow through the vent lines and see if you can hear bubbles blowing in the tank. If the vent is in the tank lid, see if you can blow through it. It's possible you've taken a bug into one of the lines in flight, and it's gone beyond sight but is still blocking the tube?
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edmanster
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Re: Fuel Vent/Leak Issue

Post by edmanster » Sun Sep 02, 2018 9:31 am

Since a vent is to be located at the high spot of the wing - where is the inlet for the second vent on same tank? What do you mean by "balanced" - is there an external tube between the vents. Now if 1 of the vent inlets was immersed in fuel & the second vent was producing positive pressure into the tank - that would explain an outflow of fuel ..
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Re: Fuel Vent/Leak Issue

Post by edmanster » Sun Sep 02, 2018 9:33 am

oops mis-spoke .... Since a vent is to be located at the high spot of the tank / not wing .. but wing as well
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Re: Fuel Vent/Leak Issue

Post by Bede » Thu Sep 06, 2018 6:29 am

So I did some more tinkering/investigating. The previous owner plugged the 2 lower vents because they were "leaking fuel". It looks like that caused the right vents (more prop wash) to pressurize the left like a juice box.

I unplugged the other vents and so far so good. Let's hope it works for good.
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Re: Fuel Vent/Leak Issue

Post by PilotDAR » Fri Sep 07, 2018 5:07 am

The previous owner plugged the 2 lower vents
Oh dear... an unapproved, undocumented modification - you gotta watch for those! I would not want to be the person who had been found to have done that if the result had been a fuel related forced landing!
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Re: Fuel Vent/Leak Issue

Post by anofly » Tue Oct 02, 2018 5:14 pm

well if its a homebuilt (amateurbuilt?) are you not an engineering test pilot for pretty well ever
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