Fuel siphoned out of tanks in flight?

[FloridaMan]

I was talking with another Mooney owner the other day who mentioned that in his M20J that he took an aggressive turn while taxiing and the fuel vented and siphoned the tank dry in flight. I frequently check my wing mounted gauges in flight. His was a non-event because he had plenty of fuel to reach his destination and was just surprised to find his fuel missing. 

Is this something that actually can happen? I know that I had a fuel cap fail after Signature at Key West improperly installed it and one of the little nubs broke off. The tell was fuel was getting sucked out over the top of the wing on my take off roll and a passenger panicked in the backseat. 

Edited January 16, 2018 by Antares

[201er]

I think this can only potentially happen to the first few gallons when topped off and not all the fuel.

[Mooneymite]

18 minutes ago, Antares said:

Is this something that actually can happen? I know that I had a fuel cap fail after Signature at Key West improperly installed it and one of the little nubs broke off. The tell was fuel was getting sucked out over the top of the wing on my take off roll and a passenger panicked in the backseat. 

Sounds like there's something wrong with his tank vent system.  I would definitely look into this issue before flying.  Is this just one tank, or both?

[RLCarter]

19 minutes ago, Antares said:

I know that I had a fuel cap fail after Signature at Key West improperly installed it and one of the little nubs broke off. The tell was fuel was getting sucked out over the top of the wing on my take off roll and a passenger panicked in the backseat. 

I had a CFI look in the right tank and not get the cap on correct, on climb out he noticed fuel coming out and got pretty excited. I agree with @201er once the fuel level goes down the siphon would be broken

[Oscar Avalle]

To review The fuel vent system would be key.


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[FloridaMan]

He doesn't have the plane any more and transitioned to a light sport.

One thing about having a Mooney is that people that have owned them in the past, even those who have moved on to top of the line acrobatic and corporate aircraft, have a place in their hearts for Mooneys. I can't tell you how many times I've encountered pilots who are far along in their careers who look at my old airplane with affection and love to talk about their experiences in them. I don't know if the same level of attachment applies to other aircraft, except for maybe Cessna 310s. People seemed to love those too. 

[Bartman]

I had some bad O-rings in the fuel tank caps and if I filled it to the top it would leak down the wing in flight. The outer rings were easy to replace at a moments notice at an oil change, but it made no difference and still leaked. Had the inner O-rings replaced and problem solved. So yes it can siphon and leak by bad O-rings if full and it wasn't much, but it stained the paint and took a while to get it cleaned up. 

[StinkBug]

I lost a cap entirely once (don't ask, I'm still mad at myself). The fuel level was about 2" down from the top. I flew on that tank until it was dry. The engine burned it all, none was coming out the top. I'm sure if it had been full a bit would have been sucked/splashed out by the air flow, but it was fine since it was down a little. I have a hard time believing that any level of maneuvering could cause a tank to drain overboard in flight. If there were some way of doing that I'm sure I would have found it with the way we fly the jump planes at work. 

[mooniac15u]

Siphoning is a specific process where fluid is pulled through an inverted u-shaped tube. The fluid travels up initially but ultimately flows down to a location that is lower than the starting point. I'm not sure if that's what the OP's friend really meant and whether he thinks that happened through the tank vent. For all of the gas to get siphoned out that way the tank vent would have to be connected to the bottom of the tank on the inside.

Sucking all the fuel out the top is also not realistic. 

[FloridaMan]

3 minutes ago, mooniac15u said:

Siphoning is a specific process where fluid is pulled through an inverted u-shaped tube. The fluid travels up initially but ultimately flows down to a location that is lower than the starting point. I'm not sure if that's what the OP's friend really meant and whether he thinks that happened through the tank vent. For all of the gas to get siphoned out that way the tank vent would have to be connected to the bottom of the tank on the inside.

Sucking all the fuel out the top is also not realistic. 

OP here. This is what makes sense to me as well, but I thought I'd pose the statement to see if there was anyone who had experienced this. I have had a stuck sump drain before that kept draining after I removed the fuel cup. I had to fuck it with the needle from the fuel cup for a bit to get it to stop. I religiously look in the tanks and verify that all drains have stopped draining after commanding them to. I am thankful to have the gauges on the wings as well to monitor fuel in flight. Also, I regard changes in tank selection to be one of the riskiest configuration changes to be made in flight. If I can spare it, I do it prior to descent or not at all. I used to run a tank completely dry before I had the engine monitor with fuel flow (if I got 3 hours out of the tank I climbed with, I knew I had 3 hours left), but I'd feel the imbalance in flight, so I switched to 16 gallons, then run the other tank dry or less than 10, and then back to the takeoff tank. That leaves you with only two tank changes in flight and keeps your reserves all together. I remember somebody here had a handle come off their fuel selector when between tanks. 

[carusoam]

Mooneys have two types of fuel tank vents...

1) old one is an internal tube within a tube, not able to siphon... a damaged tube could leak, but none have been mentioned around here...

2) newer one is external to the tank, not able to siphon...  

3) Caps have the ability to leak,  low pressure air in flight will vacuum a couple of inches out of the tank if given enough time...

4) Sump drains are pretty good at not getting stuck open. But, a continuous supply of rust particles from mild steel fuel necks can add to the challenge....

5) You know your caps need new o rings when... you are flying in rain and you see air bubbles forming around the fuel caps...  

PP thoughts,

-a-

[aaronk25]

Ya I’m in the not possible camp.....couple gallons when full though the cap seal, maybe...

I bet this guy had some good fish stories too.


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[jaylw314]

Only way to have a siphon would be if the fuel vent line went to the BOTTOM of the fuel tank.  Only a moron would design a fuel vent that way!

FWIW I did have a wing fuel sump stick last year.  It had been slowly dripping for a few months and I couldn't get it cleaned out.  Boy, that was a fun experience.  Frantically running around the hangar trying to find an empty bucket, then frantically trying to find a pencil to stick in it before the bucket filled up.  I stole someone else's pencil, broke the tip off, and jammed it in.  That gave me enough time to calm down.  After some attempts at jiggling the fuel tester, I finally got it unstuck.

Afterwards, the valve stopped dripping, so the 5 gallons of fuel must have cleaned out the debris.  I still carry around the pencil I stole in my flight bag...