fuel vaporizing out of tank vent

[allflight57]

I was flying a Piper Arrow yesterday and we were climbing through about 3000 ft and it was quite choppy and i looked out on the wing and about a 1in. diameter trail of vapor was coming out from under the wing right about where the fuel tank vent is located, it only came out when we would hit bumps or increase angle of attack. It wasnt hot outside either so i doubt the fuel could have been expanding too much. Does anyone know why or how this happens? I also noticed it on the DA-42 twin star just when we rotated and fuel vapor was coming out form underneath the wing. Can anyone enlighten me on why or how this happens...thank you

Tyler

[Mark1234]

Assuming it is fuel, most likely because the tank is a bit on the full side - it's not really vapour that's coming out - you'd not see that - it's fuel coming out as a liquid, and being dispersed in the airstream.

Don't know the arrow, but at least one aircraft I am familiar with dictates use of the left tank first if full - the fuel that is pumped up front, but NOT used by the engine is returned to the left tank; therefore if you run on the right tank with the left full, it's got to go somewhere - out the vent.

[IO540]

Avgas expands about 0.1% per degree C increase in temp, and if you fill a tank completely at say +10C and then leave the plane on the ground at say +20C, you are likely to lose 1% of your tank through the vent. The stuff that will come out will be liquid, not vapour.

[llanfairpg]

Check you fuel tank vents are completely clear--check the tech log for evidence of operation into a grass strip, mud thrown up can block vents

[dublinpilot]

Any chance it's not fuel, but simply some form of condensation like you get in commerical jets, at high angle of attacks? I've NEVER seem fuel come out of our Arrow, even though on occasion it's been overfilled.

This bit is what makes me wonder about it
it only came out when we would hit bumps or increase angle of attack.

dp

[allflight57]

Hey thanks for all the replies guys, Dublinpilot you bring up a good point about that condensation...but is it possible to get any kind of condensation from small GA planes like that? The smallest plane I've seen produce a condensation effect is a EMB-120 brasilia.

Tyler

[llanfairpg]

it only came out when we would hit bumps or increase angle of attack.pressure differential over the tank vents?

[conflict alert]

I have observed fuel venting once from my Arrow IV. Moderate turbulence not long after departure with tanks fuelled to capacity. Used the right tank for start - runup - and departure. Fuel was venting from the left tank only. At the time I presumed it to be just the fuel sloshing around and going out the vent. Selected left tank and about 5 mins later it stopped. (Turbulence didn't!!) Never seen it again since I started the practise of stopping refuelling about an inch from the top.

[DFC]

You need to have a look at the tank vents.

Piper vents are prone to being snapped off or bent back.

What you are seeing is fuel being drawn out through the vent. Switch to that tank to use the fuel and bring the level down.

----------

Dublin Pilot,

DR right tank. Just because you did not see it does not mean it never happens. ;)

Regards,

DFC

[dublinpilot]

Just because you did not see it does not mean it never happens

Never said any different...not sure why you need to state the obvious? ;)

[llanfairpg]

Check you fuel tank vents are completely clear--check the tech log for evidence of operation into a grass strip, mud thrown up can block vents

[llanfairpg]

Duplication

[conflict alert]

This may be a dumb question but.....a number of you are alluding to blocked fuel vents as the problem, surely if the vent is blocked the fuel would not be venting????

Allflight57

Another cause could be poorly sealed fuel caps and fuel that is 'splashed up towards the cap within the tank ie turb or high AOA, could be getting sucked out due to a venturi effect around the cap and a poor seal. Look for tell tale signs of fuel discolouration behind the caps towards the trailing edge on top of the wing.

[TheOddOne]

Look for tell tale signs of fuel discolouration behind the caps towards the trailing edge on top of the wing.

Yup, saw that on a PA28 recently, looks like a streak of 'loo blue' across the upper surface of the wing. The onset was quite sudden, bearing in mind that it was caused by a worn seal on the filler cap...

TOO

[llanfairpg]

Yup, saw that on a PA28 recently, looks like a streak of 'loo blue' across the upper surface of the wing. The onset was quite sudden, bearing in mind that it was caused by a worn seal on the filler cap...Had an alarming amount of fuel come out of an Aztecs inner tank once in the cruise, it was the fuel tank vent blocked on that side