"I am surprised nobody else posted this info yet. On Tue, 27JUL04, a UPS 752 caught on fire while under going maintenance for a fuel leak. I think it was a/c 410. I have got pics but I left my camera in my tool box at work, I will load some pics when I bring it home. The fire damaged the pylon, flaps, aileron, flap track fairings. Heat damage seemed to be the fillet fairing, and the lower fuel tank structure surrounding the pylon. The airport fire department responded very quickly and saved the rest of the a/c. Currently the a/c is parked inside DL's hangar, yesterday reps from UPS and Boeing spent the day examining/evaluating the a/c. I will post more info and pics as I get them. "
FWIW, I was involved in the investigation of that incident - in short it was the mother of all tailpipe fires.
Due to a procedural error while troubleshooting a strut fuel leak, they had managed to pool an estimated 10 to 20 gallons of fuel in the burner. They then decided to start the engine.
It doesn't take much fuel to make a good sized fire...