Aviation Investigation Report A98H0003

1.12.5  Examination of Cabin Overhead Aisle and Emergency Light Assemblies (STI1-66)

The passenger cabin was equipped with overhead aisle and emergency light assemblies manufactured by Luminator Aircraft Products (PN 0200486-001). These light assemblies were installed in the bridge/gap assemblies that support the ceiling panels used throughout the passenger cabin above the left and right aisles. Each light assembly includes both an aisle light and an emergency light; however, only the aisle light is illuminated during normal operations. A few recovered bridge/gap assemblies exhibited a single black circular mark and some brown discolouration adjacent to the area where the aisle and emergency light assemblies are mounted.

During subsequent examinations of other MD-11 aircraft, the same discolouration pattern was found on the bridge/gap assemblies. In addition, some aisle light lens covers were found to be deformed. Testing was conducted using temperature measurement strips to determine the temperatures reached within the lens cover adjacent to the lamp and also adjacent to the outside of the aisle light assemblies during normal aircraft operations. Peak internal temperatures of approximately 200°C (392°F) were noted, with average temperatures between 143°C and 160°C (289°F and 320°F). Temperatures between 110°C and 138°C (230°F and 280°F) were measured on the outside of the light assemblies. Discolouration on the bridge/gap assemblies was a result of the heating effects of the aisle light lamp. It was noted that the amount, and frequency, of heat damage increased the longer the light assemblies were in service.

Some of the cabin overhead aisle and emergency light assemblies examined were found contaminated with a heavy build-up of dust and lint. Some contamination was also noted on light ballasts, wire harnesses, and electrical terminal strips and connectors, as well as in some areas above the forward cabin drop-ceiling and elsewhere in the aircraft. In some forms, dust and lint can be highly combustible and may be ignited from a small ignition source.[58] Concentrations of dust and lint could provide a path for fire propagation. Microscopic examination and analysis of dust and lint collected from filters removed from Swissair aircraft indicated that the deposits consisted of a mixture of different materials, such as textile and paper fibre fragments. The deposits were determined to be flammable[59] and easily ignitable from a small ignition source.

[58]    An ignition source, such as a small diffusion flame produced by a burning paper match, or a short-duration electrical arc.

[59]    For the purpose of this report, a flammable material is defined as a combustible material that is easily ignited and readily sustains a self-propagating flame front.

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