I don't think I've ever been damaged by a disconnected piece of aircraft hitting me. I've had collision damage by hitting an enemy, or part there-of, but never anything that had clearly broken off. I think the fuselage is the exception though, if it's still there (ie, not blown up), then you can always collide with it. If something is still attached to the fuselage, then you can collide with it, if not, then it's just a piece of harmless junk.
When colliding I find that I'm either getting blind-sided by someone (which is un-avoidable) or I'm not properly controlling my closure rate and end up trying to force a shot when I should be breaking off to set up another pass. Sometimes the closure rate issue also has to do with controlling my plane at too high a speed, where the controls are difficult to begin with.
-Soda
Aces High Trainer Corps.
The Assassins.