Mysterious Fuel Loss

[Shadrach]

1 hour ago, Bob - S50 said:

I just had another thought.  What if a tank had a slow leak that drained into a part of the wing that was sealed enough on the bottom to prevent dripping, yet had openings elsewhere that would allow evaporation?  Say for example out the front of the fuel tank into the inspection panel area.  If it evaporated nearly as fast as it leaked, the nose up attitude of the plane might keep the liquid from reaching the inspection panel to leak out, yet evaporation could allow it to escape through the weep holes.

The wing has holes in the skin for just this reason.  The scenario you imagine might be possible but is unlikely.

[carusoam]

Speaking of holes...

There are small sets of holes in the wing ribs internal to the tank...

holes in the ribs are put there to allow both fuel to flow from one area to the next...

when draining the tank / burning the fuel... fuel will be passing through these holes as the tank empties...

When filling the tank... air gets trapped for a while at the top, until it squeezes through the top holes to get out...

These holes are not very well known, and some sealing activities have a tendency to fill some of the holes or partially fill them...

So when you fill the usual Mooney tanks, Expect it to take a few minutes to press a few gallons of air through the tiny vent holes in the ribs...  

if the air is delayed in getting to where it wants to go... after some time, a full tank will appear to have lost a couple of gallons... as the fuel and air move around...

There are a few pictures of Mooney rib designs around here somewhere...

 

Some Mooney tanks are more complex, and have been broken into multiple compartments with tubes between them... making sure the tubes are working properly at both the top and the bottom will be important...

Some Mooney tanks have box-outs for speed brakes... I haven't seen any issues with filling related to brakes...

Some Mooneys got two fill holes in each tank, and a procedure to best fill them up all the way... and a flapper to really add confusion...

PP thoughts only, not a mechanic...

Best regards,

-a-

 

[wburger1]

On 5/20/2019 at 5:16 PM, takair said:

Is the F in the same hangar?

Not any more but it was in there for 3 months with no fuel loss.

[Prior owner]

On 5/20/2019 at 6:24 PM, carusoam said:

Speaking of holes...

There are small sets of holes in the wing ribs internal to the tank...

holes in the ribs are put there to allow both fuel to flow from one area to the next...

when draining the tank / burning the fuel... fuel will be passing through these holes as the tank empties...

When filling the tank... air gets trapped for a while at the top, until it squeezes through the top holes to get out...

These holes are not very well known, and some sealing activities have a tendency to fill some of the holes or partially fill them...

So when you fill the usual Mooney tanks, Expect it to take a few minutes to press a few gallons of air through the tiny vent holes in the ribs...  

if the air is delayed in getting to where it wants to go... after some time, a full tank will appear to have lost a couple of gallons... as the fuel and air move around...

There are a few pictures of Mooney rib designs around here somewhere...

 

Some Mooney tanks are more complex, and have been broken into multiple compartments with tubes between them... making sure the tubes are working properly at both the top and the bottom will be important...

Some Mooney tanks have box-outs for speed brakes... I haven't seen any issues with filling related to brakes...

Some Mooneys got two fill holes in each tank, and a procedure to best fill them up all the way... and a flapper to really add confusion...

PP thoughts only, not a mechanic...

Best regards,

-a-

 

AD 85-24-03 regarding plugged drain back holes in the fuel tanks... AD 85-24-03

Edited May 24, 2019 by PilotCoyote

[carusoam]

Nice link, PC!

thanks,

-a-

[Prior owner]

1 hour ago, carusoam said:

Nice link, PC!

thanks,

-a-

I should have included the SB: SBM20-230 Rev A

[carusoam]

The Service Bulletin is really cool because it has the drawing of the small holes in the ribs where air, fuel, and water gets to travel...

 

So.... if your fuel takes a really long time to settle.... expect that some air holes may be clogged....

So.... part II.... if your air holes are clogged, check the fuel flow holes.... because the same guy sealing over the air holes, sealed over the fuel/water holes at the same time... 

PP thoughts only, not a mechanic...

Best regards,

-a-

 

[Badmoonraising]

Sorry about the slow reply here. Hector has suggested something that I think has great merit. I will open the inspection panel and see if fuel is leaking along the main spar. A little fuel along a greater area would evaporate it without notice. Good suggestion! 

[Hector]

Sorry about the slow reply here. Hector has suggested something that I think has great merit. I will open the inspection panel and see if fuel is leaking along the main spar. A little fuel along a greater area would evaporate it without notice. Good suggestion! 

I suggested this not out of the blue, but because it is happening to me right now. I started to notice than if I filled up the tanks I would lose a 2-3 gallons very slowly over a week or two and then the fuel level would remain steady. No stains on the outside. This only happened if I topped it off and let the plane sit for a while. I thought it might be redistribution,change in temp, etc., BUT it was happening on the right tank only so none of that made sense. I removed the access panel just forward of the front spar and sure enough I spotted blue stains along the upper spar cap to skin interface near the wing root. Obviously I have a very slow leak that evaporates faster than it can find the drain holes. After I lose a few gallons and the fuel level lowers below the upper spar cap I stop losing fuel. My tanks have lasted over 30 years without a leak (as far as I can tell from the logbooks) so I guess I’m doing pretty well but it looks like it is that time again in the very near future.


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[Badmoonraising]

i really think you're onto something.