whats it do?
It can be smoke that comes from the engines at takeoff and trails the aircraft, smoke that belches from the engines at startup, smoke that an aerobatic aircraft trails in colors behind it etc etc.
Bummer beat me to it well just have a look. Not FS9 but LOMAC instead but it is the same exact thing
Just click on Open Menu on the top left and click on Artwork then click on Virtual Reality then look down until you see them in formation with the smoke