WASHINGTON, D.C. — The National Transportation Safety Board (NTSB) has determined that the probable cause of a ground fire that caused extensive damage to a cargo airplane last year was due to the design of oxygen system hoses and the lack of positive separation between electrical wiring and electrically conductive oxygen system components. The Federal Aviation Administration's (FAA) failure to require the installation of new oxygen system hoses to remedy a safety issue previously identified by Boeing was cited as a contributing factor.
At 10:15 p.m. (PST) on June 28, 2008, at San Francisco International Airport, an ABX Air Boeing 767 cargo airplane experienced a ground fire just aft of the cockpit area before engine startup. The cargo airplane was operating as flight 1611 and was destined for Wilmington, OH. Airport rescue and firefighting (ARFF) personnel extinguished the fire, which had burned holes through the crown of the aircraft in the forward galley area, in a timely manner. The captain and first officer, the only two aboard the aircraft at the time of the fire, evacuated the airplane through the cockpit windows and were not injured. The fire started in the supernumerary compartment, which is located between the cockpit and the main deck cargo compartment.
Crew descriptions about what was heard when the fire started, combined with Safety Board testing, revealed that the ignition source had to be within the oxygen hose. The Safety Board's investigation determined that a short circuit to the supplemental oxygen system reached the oxygen hose.
The design of the hose included an internal spring, which could be heated by the inadvertent application of electrical current, causing the plastic hose to ignite. Safety Board testing found that the hose design brought together the three elements for a fire: the coil acting as an ignition source, the hose material acting as a fuel, and the oxygen to promote burning.
Boeing had previously identified safety issues involving conductive hoses and had issued a service bulletin instructing operators of aircraft with these hoses in the cockpit to replace them with nonconductive ones. The FAA approved the bulletin but did not issue an airworthiness directive to make compliance with the bulletin mandatory.
The Safety Board also found that other ABX 767 aircraft's supplemental oxygen system did not include positive separation between electrical wiring and oxygen system tubing. Electrical wiring that is near or in contact with oxygen system tubing creates the potential for electrical short circuits to reach the oxygen system hoses.
The involvement of oxygen in a fire can significantly expedite its growth and severity. Prior to the accident, ABX maintenance personnel performed numerous instances of oxygen system servicing on the accident aircraft, indicating a chronic problem on the airplane. However, ABX did not develop a specific action plan to resolve the identified discrepancies. The lack of further action was not stipulated by ABX's continuing analysis and surveillance program (CASP). The Safety Board determined that ABX's CASP did not properly address and correct the oxygen leaks. However, these previous oxygen leaks did not directly cause the fire.
"The hose design issue, which was one factor that gave rise to this accident, should have provided the FAA with plenty of warning that, if left unaddressed, could result in a serious accident, as we have seen here," says NTSB Acting Chairman Mark V. Rosenker. "Had the fire started when the plane was in the air, the result would very likely have been catastrophic."
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