Thorp Air Command - T18.net

My Thorp has developed a fuel leak! I'm seeing a small amount of fuel leaking from the panel seam on the belly behind the landing gear. It's leaking from somewhere near the fuel tank, running back inside the fuselage, and dripping out at the first seam.

It looks like there's a small amount of fuel near the firewall in front of the pilot rudder pedals, although the spiral tube coming directly off the tank looks clean.

I'm about to take some panels off and look at it more closely, but has anyone seen a leak from this location? What could be the source of the leak?

Thanks!

Hey pal,
I would remove the carpet quickly. Then place white paper towels under the tank area. Hopefully they will show the source.
Please don’t fly it until it is fixed.
Cubes

Spiral tubing from the tank ? Are you sure that's not electrical wire wrapped in spiral wrap ? My initial guess about the fuel leak and the location would indicate the fuel vent line leaking ? Fuel like hydraulic fluid will end up in the places you least expect , like the pilot's rudder pedals . Not sure what type of vent line was installed , but it could be a loose clamp ? Possibly an old vent line (rubber) that has developed hairline cracks ? If the tank was completely filled with fuel , heat expansion may be causing it to over flow out the tank vent on the bottom of the AC . This has happened to me . I would start simple with the Cubes plan of attack and then move on to more aggressive measures . At least with the paper towels you might narrow the location of the fuel . As Cubes said...fix before flying .

RB

Where the fuel line valve connects to the tank has been overtightened on some aircraft and can do damage/crack the bung....so I've heard.... or the valve itself can sometimes be old and/or just leaking around the threads. surprisingly, that can amount to more fuel leakage than I would have thought. I had to replace that valve on my plane when I got it. It was leaking a small drip every 30 seconds or so. One other area to check is also the fuel gauge sender where it attaches to the tank. Mine busted where the sending arm attach tube attaches to the top plate. As I have a tuba like fuel vent that faces forward in the prop arc and pressurizes the tank in flight naturally (acting like a fuel pump) I had fuel flowing out of that fuel gauge attach plate area on climbout with a full tank and pouring on to my tender tootsies. Shut the master off right quick and landed and lived to flight another day. Scared the crapazoids out of me.

Fix before flight....

_________________
Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

Thanks all for your responses.

I took apart the center tunnel and found a seeping leak on the tank-side fitting of the electric fuel pump. The fuel was pooling around the threads, running down the fitting, and disappearing into the airframe.

I cleaned it up and tightened the nut carefully. The issue appears to be solved, as the fuel no longer leaks by the threads and I don't see any other source of fuel. That side of the pump is pressurized by the fuel, and I only had 10 gallons or so in there, less now that some had leaked.

I'll put some fuel in it and continue to monitor the area for leaks.

Thanks again for the help.

Well... I thought I had fixed the fuel leak by tightening the fitting, but I came back two days later to some fuel spots under the belly in the same spot.

Rich, this is what I meant by spiral tubing from the tank. It's solid tubing that loops twice after the fuel tank outlet before going into the center tunnel.

Here's where the fuel is leaking: There was fuel pooling at the bottom of the blue elbow. I tightened the nut, cleaned up the fuel, and watched it for a few hours the first time.


Here's another view of the center tunnel with the left side removed.


I haven't taken apart the center tunnel since it re-leaked, so I'm not certain that it's leaking from the same location.

If it IS from the same location, any suggestions on how to resolve the leak? I already tightened the nut quite a bit (is there a torque spec?). Do I need to replace the elbow fitting?

What about the solid tubing? Do you guys use flexible fuel lines, or solid?

Thanks!

My first guess would be a crack in the flare of the aluminum tubing . There may also be a small burr on the aluminum tubing or AN fitting . I used flexible fuel lines covered with a stainless steel sleeve . Guessing wont fix it , I would take out the fuel line and fittings where the leak is suspect for a closer look .

RB

That elbow looks like an o-ring adapter... there is probably an o-ring under that nut that may need to be replaced. At the very least the fitting should be removed and the o-ring replaced, as well as inspecting the surface that it mates against on the pump. When you say that fuel was puddling at the bottom of the elbow, I assume you mean where it connects to the pump. If you go here - https://www.anplumbing.com/adapters/o-ring-port-adapters.html, and scroll down a little you will see what I think you probably have. The connection at the top, or outlet of that fitting looks kind of funny also... the aluminum tubing where it exits the flare nut doesn't look good in the picture.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Are those fuel pumps flared internally or use a pipe thread? That elbow looks like a flared bulkhead fitting to me. https://m.aircraftspruce.com/catalog/hapages/an833.php
If the pump has pipe threads and I am correct about the fitting, a person would have a tough time getting it to hold fuel for very long. And the correct fitting would be https://m.aircraftspruce.com/catalog/hapages/an822.php

One of John’s would work too, maybe better, but I don’t think that is what you have.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

JIC fittings require an aircraft flaring tool on the solid lines. It is very common for people to think they can use a regular flare tool. It doesn’t work. I would replace those solid lines. You can either buy an aircraft flare tool, or use flexible lines that are professionally built.
At a minimum, remove and inspect all of those fittings. I had an airplane come through my shop that had 11 cockpit fuel leaks!!! Please don’t fly until those lines are fixed!
Cubes

This is off topic but if someone brought me that airplane to inspect, I would bust the turnbuckles. No more than three threads showing allowed at each end. There can’t be very many threads inside the barrel with the ugly safety wire. Especially at the front. They make different length barrels and forks so it is possible to mix and match. The last resorts would be to make a tab to come off horn to connect the cable to or to replace the cable. Regardless, the only way I would let that aircraft out of my shop like that is on a trailer.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

That looks like a good candidate for an upgrade to Teflon lined hose.You can buy the hose & fittings at Summitracing or Jegs and make the assembly yourself, then it will actually fit.

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Scott Emery
EAA Chapter 326
T-18 N18TE

I’m pretty sure it is what I suggested... I think that those Dukes fuel pumps may use that kind of an o-ring adapter. It is not a tapered pipe thread nor internally flared... it is a straight thread. It allows the fitting to be clocked at whatever angle you want, then locked down & sealed by tightening the nut and compressing the o-ring. It is not an uncommon type of fuel fitting. He should be able to tell if the leak is there or on the flared connection. Since he said that fuel was puddling at the base of the fitting I suspected it might be that.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Good advice. 45 deg. flaring tools are common in the plumbing & pipefitting trades. You want a 37 deg. tool... common also in the automotive field.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

None of the fittings in your link has the shoulder for riding in a bulkhead but the one on his pump does. Someone may have tried to convert a bulkhead fitting to work by cutting off the flare and/or installing an o-ring but that shoulder says bulkhead.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson