I wasn’t quit 100% right about surge tanks – usually they drain back into the main tanks through one-way valves – any overflow is also through 1-way valves but arranged in such a way that it only happens in the sorts of situations I mentioned. They also serve to allow expansion & contraction of the volume of fuel as it is used or with changing temperature.

Venting large amounts of fuel to atmosphere would be quite unusual

Thank you for the explanation. I was unaware about fuel surge tanks and surprised that the trail come from only one wingtip. Now I know that it is not uncommon and have found more similar photos:

http://jetphotos.net/viewphoto.php?id=6379939

http://www.b737.org.uk/fuel_tank_vent.htm
http://www.airliners.net/aviation-forums/tech_ops/read.main/183762/

It looks like fuel dumping out of just 1 tank/side

This thread – http://www.airliners.net/photo/KLM—Royal/Boeing-747-406/1666904/L/ – shows a 747 surge tank overflowing in flight and “dumping” which is not coming from the visible fuel dump nozzle.

This thread talks about an a/c dumoping from 1 tank only and compares it with surge tank dumping – http://www.pprune.org/archive/index.php/t-135875.html

Here’s another thread on the same topic with a photo – http://www.airliners.net/aviation-forums/general_aviation/print.main?id=3439237

The surge tank is a usually empty tank bay in the outer wing into which excess fuel can flow – or surge – if there is reason to do so – eg if the tanks are overfilled and the a/c banks or similar. Surge tanks vent straight to atmosphere.

http://www.flickr.com/photos/78526085@N05/6950905160/in/photostream/

Any idea what it is?