This is fuel coming out of the pipe located near the wingtip. This action is the so-called "fuel jettison procedure," executed by the pilots when an emergency landing has to take place, whereas the heavy fuel load would cause severe damage upon touchdown. That's why the pilots have to dump some fuel to decrease the landing weight and make the touch down as smooth as possible. I know what you're thinking now: this is a waste in fuel and a pollution of the environment. But here's the good news: the fuel dumped by aircraft doesn't reach the ground. It vaporizes in the air. ATC assigns the pilots a location and altitude high enough above the ground to allow the fuel to evaporate.
In this case, a Swiss Airbus A340 had to return to Zurich Airport on its route to Shanghai, after the oil temperature in the number 3 engine had gone up too high. Watch the action of the talented pilots at #pilotseyetv (link in bio, with English subtitles), and see how this maneuver is done. Enjoy! 😉✈ #pilot#pilotseyetv#pilotlife#pilotview#future#avgeek#aviation#aviationdaily#airbus#a340#swissairlines#flying#whyifly