Crash investigators for Swissair Flight 111 are looking at overhead aisle and emergency lighting as possible ignition sources on board the doomed flight.

The MD-11 jumbo jet went down off the coast of Nova Scotia on Sept. 2, 1998, killing all 229 people on board.

The pilots reported smoke in the cockpit prior to the crash.

The investigation is nearing its third anniversary
The investigation is nearing its third anniversary

An official with the Canadian Transportation Safety Board says heat discolouration was found in the ceiling panels.

Larry Vance says the discolouration occurs around the edging of the light sockets of the ceiling of the cabin. The 28-volt incandescent bulbs are used to illuminate the aisle during overnight flights.

Investigators have found more instances of overheating. In three other MD-11s, the aisle light bulb temperature rose as high as 200 C.

Additional testing showed that aisle light bulbs on MD-11s can draw electric current levels 143 per cent higher than they were built to withstand.

While Vance says that these lights are a possibility, nothing can be ruled out.

He says wiring for the in-flight entertainment system or overhead cockpit map lights are other possible sources.

The investigation is approaching its third anniversary and Vance says they may never find out what caused the crash.

But the lengthy process has yielded some results. There have been improvements to wiring and electrical systems, and the removal of the insulation believed to have fuelled the flames.