The National Transportation Safety Board 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 PM PT, 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, Ohio. 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," said 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."
As a result of the investigation, the Safety Board voted to recommend that the FAA: require operators to replace electrically conductive hoses with electrically nonconductive hoses and prohibit further use of conductive hoses unless the conductivity is an approved design element; formalize the airworthiness directive (AD) process so it addresses all possible uses of an appliance affected by an AD; require positive separation between electrical wiring and oxygen system tubing; ensure that oxygen system tubing in proximity to electrical wiring is made of, sleeved with, or coated with nonconductive material or is isolated from potential electrical sources; develop and implement electrical grounding requirements for oxygen system components for all transport-category aircraft; develop inspection criteria or service life limits for flexible oxygen hoses to ensure that they meet current certification and design standards and require that airplane operators replace hoses that do not meet these criteria or life limits; and require operators of transport-category cargo airplanes to install smoke detectors in the supernumerary or similar airplane compartments.
During its investigation, the Safety Board determined that reading lights located in passenger service units (PSU) could become a potential source of ignition to nearby combustible materials. Because of this, the Board recommended that the FAA require transport category airplane operators to ensure that all reading lights in PSUs be installed with rubber boots or use other means to provide a greater level of electrical protection. The Safety Board also voted to recommend that ABX Air modify its CASP so that all chronic discrepancies are effectively resolved. And the Board reiterated a previous recommendation to the FAA about training on an emergency response firefighting device.
A synopsis of the accident investigation report, including the findings, probable cause, and safety recommendations, can be found on the Publications page of the Board's website. The complete report will be available on the website in several weeks.