Aviation Investigation Report A98H0003

1.14.13  Potential Increased Fire Risk from Non-fire-hardened Aircraft Systems

Under regulations in place at the time the MD-11 was certified, no requirement existed to determine whether a failure of any material used in an aircraft system would exacerbate a fire in progress. A premature breach of certain systems, such as oxygen, hydraulic, and conditioned air, could exacerbate an in-flight fire.

The crew oxygen supply lines in the MD-11 were originally manufactured from aluminum tubing. During aircraft manufacture, the aluminum lines were found to be susceptible to handling damage; therefore, the tubing material, along with the majority of fittings, were changed to corrosion-resistant (CRES) steel. Although McDonnell Douglas replaced the aluminum tubing and many of the fittings with CRES steel, McDonnell Douglas continued to use aluminum cap assemblies (e.g., AN 929-6) on unused but pressurized lines. Such an aluminum cap assembly was used on the 8-cm (3-inch) long stainless steel line that was branched off the main oxygen supply line located above the cockpit ceiling at STA 374. This cap assembly was installed in such a way that it protruded through the insulation blankets. During the fire, this area was exposed to flames and heat.

Furnace testing was conducted at the TSB Engineering Branch on a representative CRES steel line/aluminum cap assembly to observe the effects of elevated temperatures on the dissimilar metals. The normal operating range of the MD-11 crew oxygen system is 62 to 85 psi. During the tests, the line was pressurized to 70 psi, and uniform heating was applied. On some tests, leakage occurred at temperatures as low as 427°C (801°F); at temperatures above 427°C (801°F) the aluminum caps lost their installation torque.

Tests were conducted at temperatures between 566°C (1 051°F) and 593°C (1 099°F). After an exposure of approximately 10-and-a-half minutes, the aluminum cap assembly would typically fracture into two pieces, leaving the end of the line fully open. At this temperature range, leakage would occur prior to the fracture.

Tests were also conducted at temperatures of 649°C, 704°C, and 760°C (1 200°F, 1 300°F, and 1 400°F). The aluminum cap assemblies fractured at approximately 5 3/4 minutes, 4 minutes, and 3 1/4 minutes respectively. Metallurgical analysis of the cap fracture surfaces showed that these temperatures caused grain growth to take place, and that the failures occurred in the form of inter-granular fractures along the weakened grain boundaries. In these tests, no discernible leaking took place before the fracture, as the accelerated grain growth caused the deformation and fracture to occur before the time required to initiate a leak.

For comparison, similar testing was conducted using CRES steel caps instead of the aluminum caps. With the CRES steel caps, there was no loss of installation torque, and the caps did not leak or fracture, even when the assembly was exposed to a temperature of 760°C (1 400°F) for 20 minutes.

The testing was considered conservative in nature, in that uniform heating was used. In the occurrence aircraft, there would likely have been non-uniform heating effects. For example, since the cap assembly protrudes through the insulation blankets and the line does not, the cap assembly would be heated first by the fire and to a greater extent than the line. In addition, a thermal gradient would already exist between the supply line, which is adjacent to the cold airframe exterior, and the cap assembly, which is exposed to the warmer interior.

If this end cap were to leak or fracture during a fire, pure oxygen would enter the fire environment, significantly exacerbating the fire situation. Also, a cap failure would result in a loss of pressure in the line; this would stop the flow of oxygen to the pilots' oxygen masks.

Within the area of the fire damage, elastomeric end caps were used on the air conditioning ducts. Fire tests disclosed that the elastomeric end caps could be easily ignited by a small flame ignition source. (STI1-97 (video clip)) Once ignited, the integrity of the end caps was destroyed by a self-propagating flame front. Fibreglass hoses and connectors were also used on the air conditioning system throughout the aircraft. This material is heat tolerant; nevertheless, when tested in a cone calorimeter, the material ignited about two-and-a-half minutes after exposure to a heat flux of 25 kilowatts per square metre, which is equivalent to about 1 095°F (591°C). A breach of the air conditioning system in the area of the fire would introduce a significant flow of air that would exacerbate the fire. Heat damage to the structure in the reconstruction mock-up indicates that such temperatures were likely reached or exceeded in some areas where fibreglass hoses were installed.

FAR 25.1309 requires that a system safety analysis be conducted as part of a system's certification process. Although it is an established aviation industry practice, during the certification process, to consider the consequences of a system's failure, typically the system safety analysis does not include an assessment of the consequences of the system's failure as a result of a fire in progress.

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