Aviation Investigation Report A98H0003
1.16.2 Map Light Testing and Research
In December 1999, Swiss AAIB investigators, on behalf of the TSB, monitored a Swissair program to remove MPET-covered thermal acoustic insulation from their MD-11 aircraft. Investigators discovered that the insulation material adjacent to some of the map lights showed signs of heat damage. In many cases, the back of the map lights installed in the flight crew positions were in direct contact with the insulation material. This combination provided a potential source of ignition adjacent to a flammable material.
Subsequent examination of the Hella map lights, on the Swissair MD-11 fleet and other MD-11s maintained by SR Technics, revealed that some had damage to the insulating protective caps ranging from cracks to missing pieces; instances of arcing damage were also found on the metal contact spring and the carrier frame.
Based on this information, the TSB issued an Aviation Safety Advisory to the NTSB and relevant stakeholders on 3 March 2000. On 20 April 2000, the FAA issued an AD requiring an inspection of the Hella map light installations on US-registered MD-11s. Although this AD did not mandate that inspection results be reported to the FAA, some voluntary responses were forwarded to Boeing. These results revealed that about 40 per cent of the map lights sampled had some discrepancy, such as heat damage to the wire and cracked insulating protective caps.
Subsequent testing of the Hella map light demonstrated that, with pieces missing from the insulating protective cap, arcing between the metal contact spring and the carrier frame occurred at two locations on the carrier frame. To simulate potential in-service conditions, the Hella light was exposed to vibration during an arcing event. It was observed that the vibration extended the duration of the arcing, and the CB did not trip.
To simulate an actual aircraft installation, a map light was tested in a confined space, surrounded by, and in contact with, MPET-covered insulation blankets. Temperatures in the confined space stabilized at 151°C to 159°C (304°F to 318°F). After two months of continuous testing, the inside of the insulation blanket exhibited heat damage that was similar to the damage noted on in-service aircraft installations.
To assess the actual operating conditions, temperature measurement strips were fastened to the insulation blankets behind the Hella map lights in three MD-11 aircraft. A maximum temperature of 77°C (171°F) was recorded and there was no heat damage to the insulation blankets. One of the test aircraft had more space between the map lights and the insulation blanket. The space provided better ventilation around the map lights, which resulted in a lower operating temperature.
Further examination of the Hella map light revealed that there were additional failure modes, other than damage to the insulating protective cap, in which the light could be involved in an arcing event. The additional possibilities are as follows:
- A short-circuit can occur between the U-shaped universal joint suspension bracket and the wire terminal connection;
- A short-circuit can occur between the U-shaped universal joint suspension bracket and the ON/OFF microswitch assembly; and
- A short-circuit can occur between the spare bulb holder and the ON/OFF microswitch assembly.
Prompted by an FAA Safety Significant Finding unrelated to this investigation, Hella carried out tests to measure the heat developed by the MD-11 map light installation. As a result of the testing, Hella drafted an SB, for the replacement of the 11.5 W halogen lamp with a 7 W incandescent bulb.
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