From Air Safety Week:
A Perverse Case of Battery Backup
The in-flight entertainment network (IFEN) installed in the Swissair MD-11 that crashed in 1998 was not the only one in which pilots could not completely deactivate the system with a single main power switch. Recall the Transportation Safety Board (TSB) of Canada remarks in the final report of this accident that the pilots were using a circuit breaker as a switch to depower the system, on top of which they did not know that cutting power to the cabin bus would not turn the system off, because it was connected to a flight essential bus.
An Air Canada A330 twinjet experienced a fire in its IFEN Jan. 17, 2002, while parked at the gate before boarding passengers at Vancouver International Airport. The fire occurred as a technician was trouble-shooting the malfunctioning video system management unit (SMU), located in the forward galley area. Even after he used the main switch to remove power from the entertainment system, smoke continued to pour out of the SMU. A portable Halon extinguisher was employed to put out the fire.
Why did the fire persist after power was cut? Investigators found that the main power switch used to turn off the system in an emergency did not completely turn it off. The SMU continued to be powered for up to two minutes by batteries, an intentional design feature for the SMU to complete a systematic software shutdown. That battery power is supplied "regardless of the position of the cabin entertainment system main power switch or the availability of aircraft power," according to the TSB report.
"The duration and intensity of the fire was not characteristic of merely a component failure," the TSB report said. "When main power was removed from the SMU, the processor board continued to receive electrical energy from the backup batteries, likely contributing to the duration of the fire. There was no effective mechanism to disconnect the battery from the SMU, a deficiency not addressed in existing Airbus service bulletins or Federal Aviation Administration (FAA) airworthiness directives."
The TSB noted that Airbus planned to issue three service bulletins between January and April of 2003 to rectify the hazard on A330 and A340 aircraft outfitted with the P@assport Entertainment system. Two were issued in December 2002. The third, SB A330-23-3122, which effects U.S.-registered A330s, will be issued in the middle of this year, according to Airbus.
That's Not All, Folks
Independent aviation data specialist John King has found more than 50 cases of significant system failures, smoke or fires involving cabin entertainment from 1988-2001, for an average incident rate of about four per year. A few are cited to illustrate the typical contents of these reports.
"Of course, all aircraft systems were designed and certified by the FAA and should not emit smoke or cause fires because proper circuit design expressly is supposed to stop this. The purpose of circuit breakers is not to protect the equipment but rather to prevent electrical overloads and wire overheating," King said.
He believes the frequency of these incidents is undercounted. "The FAA office of system safety databases provided only about 10 percent of the reports, as compared to direct accounts from pilots to the Aviation Safety Reporting System (ASRS), or from maintenance in the service difficulty reports (SDRs)," King added.
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