Fuel leak out of tank vent

[JerzyGeorge]

Has anyone experienced fuel siphon through tank vent tube?

 

It has happened to me twice , 1-st time long time ago while orbiting with full tanks shortly after take off.

 

The second time 10 days ago , noticed strong fuel spray ( like a spray gun) out of the vent shortly after take off in level flight , I think it started on the ground while taxing on uneven ground and I have not noticed.

 

Both times I tried to stop it banking to the left and orbiting for a while but did not work.

 

The only way I managed to stop it to land and take the cap of the tank to break siphon action.

 

Both times happened on the right tank, I think return line from the fuel pump may help to keep siphon event.

 

 

[facthunter]

The return of fuel may play a part but you are using fuel all the time from startup so there should be space.. I like to have both tanks selected for Take off and landing as a principle.. With them FULL it may be very critical to not have any SLIP. I don't know whether it is a siphon situation or not as I don't know the system It may be just filling up on the one side.. It should not be possible if it's vented correctly and you fly "balanced". Nev

 

 

[Happyflyer]

I don't see how a vent can siphon unless it the vent extends down into the fuel. We would have to know more about your system. I have heard of fuel leaks from over full tanks. Over filling with cold fuel on a hot day could see the fuel expand and over fill. Flying out of balance could see fuel transferring to the lower tank. Fuel return line going to the right tank while using the left tank could see the right tank overflowing if it was full to start with. Are your tanks linked, are you able to select "both" tanks or only left or right? Do your vents face forward to pressurize the tanks?

 

 

[JerzyGeorge]

In my Savannah there is no tank selector ( both tanks are linked together in the header tank - 6 liters behind the cabin)

 

Vents shaped in 180 degrees U ( up from fuel cap and down through the wing to under the wing ) so I can see the end of the clear tube.

 

Air flow bends them slightly backwards helping with the suction effect.

 

I have noticed leak on uneven ground and once when I had a flat tyre.

 

Happened only twice in flight , temperature difference may play some part in the problem as well , tanks mounted in the wing exposed to sun during taxing and perhaps warming up a bit.

 

Thanks guys for the feedback.

 

I am interested in stopping this without need to take the fuel cap off ( i.e. landing)

 

 

[bobcharl]

The standard Savannah fuel system allows this to happen with fullish tanks and parking/taxiing on unlevel ground. It can be rectified to a large degree by re plumbing the system with taps to select an individual tank as well as preventing any flow of fuel from tank to tank. However, be aware that this adds a little more complexity to the system and additional cockpit checks to include the extra tap/taps must be made (e.g. before takeoff etc.)Re the vent lines. check that the bottom ends are cut at a 45 deg angle facing forward, not rearward the way the manual shows. This provides some measure of tank pressurisation. Don't extend them too far below the lower wing skin to minimize the tendency for them to sweep back. In flight, I would have thought that flying in a slipping configuration ( kick the ball away from the overflowing tank) for a few moments would rectify the problem.

 

Good luck,

 

Bob

 

 

[Ultralights]

what tank does your fuel return line go to? i have found this happens when i have all tanks brimmed, i have to make sure i run off the tank the fuel return goes to first, i usually run it at least half empty, before switching to other full tanks, by the time i drain another main tank dry, the initial tank i ran on would be full again.

 

sometimes i have forgotten to do this and have lost some fuel overboard when the tank with the return line overflows.

 

also, with full tanks, and you fly unbalanced, you can have cross flow fuel cause an overflow. it can be fixed with the installation of a tap as Bobcharl mentioned.

 

 

[alf jessup]

My return line runs back to the left tank in the Tecnam Sierra, when I am brimmed I notice on occasions when I turn right or left while taxiing or parked on an uneven part of the hardstand it will vent out the left side wing tip vent or right depending on the slope, I do my run ups on left tank and select both for take off, after flaps up & fuel pump off and wait until I reach 1000ft I will then select the left tank only and run it for 1/2 hr before switching to the right, I never shut the other tank off until I have run on the new tank for at least 1 minute.

 

Once I have used 1hr of fuel I usually just run on the left side and cross feed when required to keep the fuel balanced, always have both tanks on when landing.

 

When full and flying out of balance you will start leaving a contrail of fuel

 

 

[JerzyGeorge]

Thanks guys , I will shorten a bit my vent lines and cut them to 45 deg.

 

 

[dan tonner]

Hi all;

 

I'm Canadian, waiting for some of the snow to melt and for spring to arrive and have too much "think-time" available.

 

Regarding the fuel leakage issues - I commented on this once before and was considering installing suitable one-way valves on each fuel supply line into the secondary tank behind the seat (permitting fuel flow INTO the secondary tank of course...) I didn't get much response.

 

Siphoning of either tank via the vent line will not occur once the fuel level drops below the opening in the fuel cap (the tube should not dip into the tank past the cap) BUT, if fuel in one tank is at a higher level than the upper arc of the vent tube in the lower tank, the fuel in the upper tank will pour through the secondary, into the lower tank and out the lower vent tube until the level in the upper tank is below the upper arc of the lower tank's vent tube. It doesn't take much of a tilt to have the upper tank's fuel level higher than the lower tank's upper arc - in fact, I think a 15 or 20 degree lean might completely empty the fuel tank on the higher side through the lower tank's vent.

 

Has anyone given the one-way valve a try.....I would prefer not to install shut-off valves at the wing roots.

 

Fly safely,

 

Dan,, spring to arrive

 

 

[JerzyGeorge]

Thanks guys for excellent ideas, Today I have shorten my vent lines under the wings and cut them 45 deg angles facing forward and gues what the problem disappeared . Thanks for excellent advice.