[Dreak]

I once observed ( while window seated next to a wing ) a flap open up and white smoke being ejected from our airplane. I could see clearly the small port from where it was coming from.

now..I asked a Friend that has been a pilot for a very long time ( flew 777's in his latter days )what it was.

he said it was a trail left for other pilots to determine the conditions of that particular airspace. Basically giving other pilots more of a heads up instead of getting there information from ground control.

Just relaying what was told to me.

I have seen some Interesting theories about these trails though.

Dreak