Since it is quiet in here on a Saturday evening, a non-expert reply!
This is unburnt fuel. In the same way that you may see a car with a slightly smoking exhaust. If you REALLY want to see smoke, watch a B52 departing and you will see why it is said that they are coal fired.
The reason, as far as I know, is that the engine is working at (or near) maximum and is unable to use all of the fuel BUT that the balance between pumping in enough 'go juice' and not quite enough is a fine balance. It is affected by the actual take off weight of the machine, temperature of air at the intake, and a number of other factors. The error is always on the side of enough fuel rather than the 'not quite enough fuel'!
Right, now let me sit back and be shot down in flames. Or would that be shot down in smoke?
