LAX99FA106 · Cessna 172N

A factory installed map light and it's switch were mounted midway down the left doorpost. About 1/2-mile from the airport, a passenger grabbed the left forward doorpost with her hand for support and a 'pop' noise was heard. Immediately, flames shot out from both sides of the plastic doorpost cover at the map light's level and black smoke filled the cockpit. The pilot tried to smother the fire, but each time the flames came back and seemed to grow more intense. An emergency downwind landing was made on runway 1 as the flames completely involved the cockpits left side. The cabin was destroyed in the ground fire. It was unclear if AD 80-04-08 was complied with following its issuance on February 16, 1980. The AD was due to instances of fuel leaks and/or in-flight fires resulting from contact between the map light switch terminals and an adjacent fuel line. The AD required a one-time visual inspection of the left doorpost area to ensure that the switch and the fuel line were at least 0.50-inch apart. It also required the installation of an insulator cover over the map switch electrical terminals. Due to a pilot squawk, a mechanic had repaired the map light the day before the accident. He found a loose wire and reconnected it to the switch. In questioning, the mechanic first said that there was nothing between the switch and the fuel line, then stated during a second interview that after review of the AD and parts manual diagrams, he definitely had reinstalled the insulator cover over the switch terminals. Review of the 172's design history revealed that models prior to the 172N had only one fuel supply line from each tank that were routed down the aft doorpost. Map lights installed prior to the 172N were located at the top of the doorpost in an area with more internal clearance. With the 172N model change, a second fuel feed line was added from each tank and routed down the forward doorpost in the channel with the wire bundles and the map light switch, with nylon spacers added to physically separate the wires from the fuel line. Also, the map light and switch were moved to their present locations. The only flammable components in the doorpost are the wire insulation, the nylon spacers, the plastic doorpost covering, and, the fuel in the fuel line (0.75 psi head pressure at the map light level). No evidence of electrical shorts in the doorpost circuits or flammable fluid leaks external to the doorpost, which would account for the ignition and severity of the doorpost fire, were found during the investigation. Three accidents were found in NTSB accident records where fires originated in the left forward doorpost; all three aircraft were Cessna 172N's. The SDR data base had three instances (only 172N's) where electrical arcs from the map light switch had punctured the fuel line in the left forward doorpost and caused an in-flight fuel leak. Other SDR's documented over 12 instances where electrical wires had chaffed into the fuel lines in either the left or right forward doorpost areas. Examination of two exemplar Cessna 172N's revealed that the internal configuration of the doorpost components were not in conformance to original drawing specifications due to interior replacements and repairs over the years.

Verbatim NTSB analysis and probable cause from the NTSB dataset