Description

On 28 June 2008 a Boeing 767-200 being operated as a Public Transport cargo flight by ABX Air (DHL) experienced a ground fire after loading had been completed and all doors closed and just before engine startup at night in normal visibility. The fire was located in the supernumerary compartment of the airplane. This compartment, which is present on some cargo airplanes, is located directly aft of the cockpit and forward of the main deck cargo compartment which is where the toilet, galley, and three non-flight crew seats are located (see diagram below).The flight crew evacuated the aircraft through the flight deck windows and were not injured, but the aircraft was substantially fire damaged and later classified as a hull loss.

Investigation

An Investigation was carried out by the National Transportation Safety Board (USA) (NTSB) which found that the initiation of the fire could be traced to internal ignition of a pvc oxygen supply hose in the crew compartment. The Investigation further concluded that the design of the oxygen hose assembly allowed its internal spring to become a source of ignition when it was electrically energised, the PVC hose material to act as a fuel and the oxygen within the hose to promote burning. The means by which the spring became energised was sought by examination of other Boeing 767s in the Operator’s fleet. These inspections found electrical wiring in contact with or routed near to stainless steel oxygen supply tubing. A short circuit from this close-contact wiring was considered to be the most likely source of electrical energy to the spring. No direct evidence of a short circuit on the accident aircraft, i.e. no beading on wires or arc marks on the oxygen supply tubing, could be found, but most of the wiring near the supply tubing and portions of the tubing was missing.

The investigation noted that the potential hazard from conductive oxygen hoses has previously been recognised by the aircraft manufacturer and new build Boeing 767 aircraft have had alternative oxygen supply hoses fitted since 1999.

Issues in respect of fire fighting methods were also disclosed during the investigation

Probable Cause

The NTSB determined that the probable cause of this accident “was the design of the supplemental oxygen system hoses and the lack of positive separation between electrical wiring and electrically conductive oxygen system components. The lack of positive separation allowed a short circuit to breach a combustible oxygen hose, release oxygen, and initiate a fire in the supernumerary compartment that rapidly spread to other areas. Contributing to this accident was the Federal Aviation Administration’s (Federal Aviation Administration (FAA)) failure to require the installation of nonconductive oxygen hoses after the safety issue concerning conductive hoses was initially identified by Boeing.”

The following 12 new Safety Recommendations were made as a result of the Investigation:

That the Federal Aviation Administration should:

• Require operators to replace electrically conductive combustible oxygen hoses with electrically nonconductive hoses so that the internal hose spring cannot be energized. (A-09-43)
• Prohibit the use of electrically conductive combustible oxygen hoses unless the conductivity of the hose is an intentional and approved parameter in the design. (A-09-44)
• Formalize the airworthiness directive process so that, when an aircraft manufacturer or other source identifies an airworthiness issue with an appliance, coordination with the appliance manufacturer occurs to ensure that the possible safety risks to all products using the appliance are evaluated and addressed. (A-09-45)
• Require airplane manufacturers, modifiers, and maintenance personnel to provide positive separation between electrical wiring and oxygen system tubing according to, at a minimum, the guidance in Advisory Circular (AC) 43.13-1A, “Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair,” and AC 65-15, “Airframe and Powerplant Mechanics Airframe Handbook.” (A-09-46)
• Require airplane manufacturers and operators to ensure that oxygen system tubing in proximity to electrical wiring is made of, sleeved with, or coated with nonconductive material or that the tubing is otherwise physically isolated from potential electrical sources. (A-09-47)
• Develop minimum electrical grounding requirements for oxygen system components and include these requirements as part of the certification process for new airplanes and approved supplemental type certificate modifications to existing airplanes. (A-09-48)
• Once electrical grounding requirements for oxygen system components are developed, as requested in Safety Recommendation A-09-48, require airplane operators and modifiers to inspect their airplanes for compliance with these criteria and modify those airplanes not in compliance accordingly. (A-09-49)
• Develop inspection criteria or service life limits for flexible oxygen hoses to ensure that they meet current certification and design standards. (A-09-50)
• Once inspection criteria or service life limits for flexible oxygen hoses have been developed, as requested in Safety Recommendation A-09-50, require airplane operators to replace those hoses that do not meet the inspection criteria or that exceed the service life limits. (A-09-51)
• Require transport-category airplane operators to (1) perform a one-time inspection of all passenger service unit reading lights installed on their airplanes to ensure that they include rubber boots or use other means to isolate the electrical parts of the assembly and (2) include, in maintenance manuals or other maintenance documentation, information about the importance of this electrical protection. (A-09-52)
• Require operators of transport-category cargo airplanes to install smoke detectors in the supernumerary or similar compartment of their airplanes. (A-09-53)

That ABX Air should:

• Modify (its) continuing analysis and surveillance program so that all identified chronic discrepancies, such as those affecting the oxygen system on the accident airplane, are effectively resolved. (A-09-54)

The full NTSB Accident Report, adopted on 30 June 2009, may be seen at SKYbrary bookshelf: NTSB/AAR-09/04/SUM

Further Reading

• Passenger Cabin Fire
• Ageing Aircraft - Electrical Wiring
• Airworthiness - The System