Investigators at the US National Transportation Safety Board (NTSB) faulted the FAA and cargo carrier ABX Air for deficiencies that led to a ground fire on board an ABX Boeing 767-200 at the San Francisco Airport late in the night of 28 June 2008.
The fire, which broke out in the supernumerary section directly behind the cockpit, began after cargo had been loaded but before pilots started the aircraft's engines for taxiing. The aircraft was substantially damaged though neither the pilot nor copilot was injured.
The probable cause of the fire, revealed by the NTSB during a final hearing on the incident this morning, was the design of the supplemental oxygen system in the supernumerary compartment installed by Israeli Aircraft Industries (IAI) during the conversion of the aircraft from a passenger-carrying to cargo-carrying configuration.
In particular, the supplemental oxygen hoses used to supply oxygen to masks in the supernumerary seating area were electrically conductive, and when put into contact with adjacent wiring that had contacted the oxygen lines and developed a short-circuit, caused a torch-like fire to break out from the hose and ignite nearby materials in the ceiling portion of the aircraft.
NTSB investigators simulated several ignition scenarios to determine the most likely chain of events.
The board faulted FAA for failing to require operators through an airworthiness directive (AD) to replace all oxygen hoses found to be electrically conductive, an issue first discovered by Boeing more than a decade ago. The airframer in 1999 had issued its own service bulletin (SB) to 76 operators advising them to change out certain hoses with a new version that included a plastic spacer at each end of the flexible hoses. FAA participated in the development of the SB, but considered the problem to be one of reliability, not safety, according to NTSB officials, and therefore did not release a companion AD.
ABX had been in the process of replacing its hoses, though the SB was focused only on the cockpit oxygen supplies and did not apply to the supernumerary area that IAI had installed.
To correct the problem, the NTSB has issued a recommendation to the FAA to modify its AD process to look more broadly at accessories of similar design to those identified as problematic by service bulletins.
For ABX, the NTSB faulted its in-house continuing airworthiness programme for not solving what had been recorded as a persistent problem with the incident aircraft's oxygen system for 18 months before the fire. The Board issued a recommendation that would require the cargo carrier to solve such problems earlier.
Also included were recommendations to better separate, isolate and ground oxygen and electrical lines, require smoke detectors in supernumerary areas and perform checks of passengers reading lights that during the investigation were found to be capable of generating sparks.