Is excess fuel leaking normal during take off of a B737?

Question

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:

This other video shows the same with much more detail:

Answer 1

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.

_{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.

^{source: www.b737.org.uk}

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

^{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.

Answer 2

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.

Answer 3

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.