When we look over this video

it's noted that some fuel is coming off from the left wing during the rotate. This is the position of the fuel vent within the wing, of course, 'leaking' due to new vertical and horizontal forces combined which, in turn, force fluids backwards and outwards in a swept wing, so not exactly a fuel dump procedure as published. My question is: is normal or correct for an airliner to have such an excess of fuel causing it to reach the fuel vent until it spills during take off? Shouldn't this wing carry less fuel?

EDIT: To help explain exactly what I am talking about, we are looking for a very thin trail of fuel coming from the fuel vent of the left wing. This vent is located under the wing and a little ahead the (left) aileron. We are not talking about a vortex from the engine vortex generators or from the flap edges. ;)

This: frame from the video with a red arrow pointing to the feature in question

This other video shows the same with much more detail:

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    $\begingroup$ No no, we are talking about the trail coming under and ahead the left aileron. It is fuel coming from the fuel vent, apparently on a overfueled wing. $\endgroup$ Jun 28, 2017 at 1:41
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    $\begingroup$ Fuel dumping is different to fuel venting. All big jets have fuel vents because in hot temperatures fuel expands and needs to go somewhere. Given that fuel is measured in weight rather than volume it is possible to 'overfill' it. The second video looks like it could definitely be fuel venting. How much venting is acceptable? That's a good question. $\endgroup$
    – Ben
    Jun 28, 2017 at 7:28
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    $\begingroup$ At first it looked like a vortex from the wing but looking the videos the second is clearly a leak and the first is looks likely a leak. Happens I know the first video airport and that town is hot tropical (almost equatorial) so @ben comment makes a lot of sense. It's common the tanks are filled at night and overfilled with the expansion at a warm morning $\endgroup$
    – jean
    Jun 28, 2017 at 10:44
  • $\begingroup$ "Excess" fuel leaking, by definition, is not normal. ;) $\endgroup$
    – user428517
    Jun 28, 2017 at 14:38
  • $\begingroup$ @sgroves Not true. For example, during a fast taxi, fuel may overflow into the surge tank then be vented. It is not "by definition" abnormal. It's also common for a full tanks operation to result in some spillage, again, perfectly normal. $\endgroup$
    – Simon
    Jun 28, 2017 at 17:01

3 Answers 3


No it is not normal. It can happen if the wing tanks are fully filled, typically for long trips or when the airline is tankering fuel to airports where fuel is more expensive. When cold fuel warms up it expands. With full tanks, the fuel has to go somewhere and it flows through the vent channels and will end up in the surge / vent tanks in the wing tips. From here the fuel can flow back into wing tanks (when there is space again) through a check valve.

737 fuel vent system Boeing 737 fuel vent system. Source: Aeronautics guide

There is a little NACA duct at the underside of each wingtip (about a meter from the tip) in the surge tank. That ensures a positive pressure in the surge tank and through the vent channels in the fuel tanks. This help the fuel pumps, reduces evaporation of the fuel and prevents a vacuum when the fuel is being used.

picture of the fuel tank vent NACA duct source: www.b737.org.uk

If the wing tanks are overfilled, this is where the fuel will come out.

picture of aircraft leaking fuel from vent while being over fuelled by fuel truck source: the aviation forum: user Keithnewsome

During the take-off the fuel tanks are changing their shape a little bit because the wing is flexing up. This may slightly reduce the volume of the tanks so if they are fully filled you will see a fuel trail spilling from the fuel vents at take-off.

Another situation where this may occur is at zero-gravity. I have seen this many times during parabolic flights with a Cessna Citation. At zero gravity the fuel will start to fly around in the tank and it will find its way to the exit (the vents at the wing tip) resulting in an impressive cloud of fuel from the wingtips. Whilst it looks like a big cloud, it is actually very little fuel that is lost.

In the video, you see the fuel trail starting below the wing (at 1:28), a couple of seconds into the 0-G part of the flight.

  • $\begingroup$ The fuel trail shows at 2:50 :) $\endgroup$
    – orique
    Jun 28, 2017 at 10:57
  • $\begingroup$ @orique Yes, at 2:50 again. The first time it is visible is at 1:28. $\endgroup$
    – DeltaLima
    Jun 28, 2017 at 11:36
  • $\begingroup$ Ah! I was seeking for the moment and just found the one at 2:50. Thanks! $\endgroup$
    – orique
    Jun 28, 2017 at 11:40
  • $\begingroup$ Fascinating stuff. As I understand it, the fuel tanks on some vehicles are not actually "empty", but are filled with some kind of open-cell foam or mesh (I believe the two reasons are (a) prevent sloshing (b) prevent fume buildup? ) In fact are the tanks on large airlines just "empty" or do they have this type of foam/mesh guts? $\endgroup$
    – Fattie
    Jun 28, 2017 at 11:45
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    $\begingroup$ @Fattie the answer can be found elsewhere on the site. Though airliners use subdiving of tanks and baffles, I believe the sponge would be too heavy and reduce usable fuel too much $\endgroup$ Jun 28, 2017 at 11:51

Lots of interesting information coming from this observation and it makes me smile. Ok, quick logic check; if fuel can come out the vent during flight, what will happen to fuel ballance? If the aircraft, and in this case, banks to the left, won't the fuel, that is still in the tip of the wing run out the vent? Short explanation on how the vent works and this is for automatic pressure refueling. The vent only opens during refueling. This is done by a sense pressure line that is taken off the main refuel line. What this does is it opens the vent valve to let the air out of the tank otherwise the tank would blow up like a balloon. Once refueling is stopped at the required level the vent closes. Ok, let's say the refueling does not stop automatically because there's a problem. The fuel system has what is called High Level Sensors that stop the refuel process when the tank get to max level. If both of these systems fail then, yes, the fuel will come out of the vent but, this only because there is still pressure from the refuel line on the sense line keeping it open. This is also a safety precaution. Ones the refueling is stopped, the vent valve will close.

The NACA duct let's air into the tank to prevent a vacuum from forming due the fuel being sucked out by the fuel pumps. It has one way valve in it to prevent fuel from running out.

Next; wing fuel tanks. Ribs form the shape and structure of the wing, leading edge to trailing edge ( front to back). Inside the wing the ribs also act as baffles but, not all of them though. The aircraft manufacturer designe the tank inside so it looks like compartments. Each compartment is connected to the next with one way flapper valves. So basically the fuel can only run towards the fuel pumps and not back again. This ensures there's always a positive feed towards the pumps even is the aircraft banks to one side.

So looking at all the above info, the problem with the fuel coming out of the vent was a faulty vent valve that didn't close as it should have. This would have been rectified before the next flight. And by the way, this isn't the first time this happened.


What you see is not actually fuel. It is the vortex coming off an engine strake in which some water vapour in the air condenses. You can see more of that phenomenon in this answer.

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    $\begingroup$ Hey, mate: we are talking about the discrete trail coming off under and ahead the left aileron. $\endgroup$ Jun 28, 2017 at 1:36
  • $\begingroup$ @user18674 The trail is coming from over the wing... $\endgroup$ Jun 28, 2017 at 1:50
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    $\begingroup$ I'm talking about this: s18.postimg.org/yvvd5dha1/DUBT.png $\endgroup$ Jun 28, 2017 at 2:07
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    $\begingroup$ I have to agree with the OP, that does not look like a condensation vortex. $\endgroup$
    – Federico
    Jun 28, 2017 at 7:09

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