Prevailing Cultural / Organizational Factors

The Supplemental Type Certificate which approved the design and installation of the IFEN system on an MD-11 airplane was issued by an FAA-approved Designated Alteration Station (DAS. The IFEN system was designed by an outside organization (not the DAS), and the DAS submitted a letter of intent (LOI) to the FAA describing the intended system design and plan for certification. The original design intent had been for the IFEN system to be powered from the cabin electrical buses. Inclusion on the cabin buses would have allowed electrical isolation of all "non-essential" electrical loads. In the event of an in-flight emergency, crew procedures for smoke/fumes of unknown origin required deactivation of the cabin electrical buses. Deactivation of the cabin electrical buses is via an overhead switch in the flight deck.

An early design review had discovered that the cabin buses did not supply adequate electrical power for the full 257 seat configuration, so the system was redesigned to draw power from AC bus 2 (an essential bus). This would have been an acceptable design if: an independent switch for deactivating the IFEN system had been included; or a switching relay connected through the cabin bus switch. However, no options were provided, and deactivation of the cabin bus switch resulted in the IFEN system remaining electrically powered.

The TSB could not determine if the design changes were shared with the DAS. The TSB postulated that under normal circumstances, a design change of this significance would have resulted in a revision to the LOI. No revision to the LOI was submitted. The TSB also stated that FAA oversight of this project would likely not have uncovered the design change in a timely manner, as the FAA was reliant on the DAS to inform them of changes in the scope of the project.

The TSB believed the flight crew was unaware that the IFEN system was powered by AC bus 2, and further believed that having accomplished the emergency procedure associated with smoke/fumes of unknown origin, which deactivated the cabin electrical buses, the IFEN system had been eliminated as a source of ignition. With the IFEN system continuing to be powered electrically, it remained as a potential ignition source, and the TSB believed electrical arcing in the IFEN system wires did, in fact, ignite the MPET in the area of the flight deck attic.

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