Aviation Investigation Report A98H0003
2.14.1 Operational Impact of the IFEN Integration
The design philosophy of the MD-11 is such that all "non-essential" passenger cabin equipment be powered by one of eight cabin buses. Activation of the CABIN BUS switch, located in the cockpit overhead switch panel, is designed to isolate all "non-essential" power to the cabin. This action is the first item in the Swissair MD-11 Smoke/Fumes of Unknown Origin emergency checklist and enables the crew to assess whether the smoke is originating from a component associated with the cabin bus system. During the initial review of the IFEN documentation it was determined that the IFEN power supplies were connected to aircraft power in a way that was incompatible with the emergency electrical load-shedding design of the MD-11.
Documentation shows that initially, the intention was to power the IFEN system from the cabin buses. However, the cabin buses could not provide sufficient power for the full 257-seat configuration of the IFEN system that was originally planned. Therefore, 115 volts (V) alternating current (AC) Bus 2 was used to satisfy the majority of the IFEN power requirements.
Powering the IFEN from the 115 V AC Bus 2 would not have constituted a latent unsafe condition if the design had included a means of deactivating the IFEN system (e.g., by use of a switching relay) when the CABIN BUS switch was selected to the OFF position. An alternate method of complying with the MD-11 type certificate would have been to seek FAA approval of an FAA-approved Airplane Flight Manual supplement to provide the pilots with relevant instructions on how to deactivate the IFEN system during emergency procedures. However, neither of these alternatives was pursued, and the design flaw was not discovered until after the SR 111 occurrence. Therefore, pilots were not likely aware that the IFEN system would remain powered after the cabin bus was deactivated.
The design of the IFEN system-to-aircraft power integration constituted a latent unsafe condition. However, as the fire was underway at the time the CABIN BUS switch was used (13 minutes, 7 seconds, after the initial smell was noted), no link was established between this latent unsafe condition and the initiation or propagation of the fire.
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