That 787 that caught fire at Heathrow. The fire started when the emergency locator beacon caught fire. This gadget transmits distress calls on 121.5, 243, and 406 Mhz in the event of a crash. It was intended to guide searchers to the crash site, in the rare event that the crash of a big twin aisle jetliner isn't perfectly obvious from the air. Perhaps to guide rescuers to survivors floating around in liferafts after a mid ocean crash. The ones we had on the C-133 transport would detach from the aircraft and float, so they would not go down with the sinking aircraft. The C-133 model also had the annoying habit of going off accidentally while parked on the ramp. When this happened we had a real Chinese fire drill, we had to go out to each and every C-133 to see which one jamming the Guard frequency with beep-beep noises.
The emergency beacons on the 787 are Honeywell Rescu 406AFN . The cute spelling of the name is a Honeywell marketing idea. The aircraft carries two of them, one above the forward lavatory, and one above the galley. The one above the galley started the fire. The device was FAA certified back in 2005 and is in service on a variety of aircraft types. As might be expected, they are battery powered, and being right up to date, they use lithium batteries. It was not clear whether a short circuit inside the beacon started the fire or the batteries spontaneously combusted.
Which leaves Boeing with a huge burned spot in the carbon fiber fuselage in need of repair. There was a good deal of discussion in Aviation Week about just how such a repair might be made. Presumably they lay a big piece of carbon fiber mat over the hole and paint it up with resin. Like repairing a Corvette's crash damage.