Icelandair B752 enroute on Sep 18th 2008, smoke in cabin due to audio jack overheat
Last Update: January 31, 2013 / 15:15:27 GMT/Zulu time
Iceland's Rannsoknarnefnd flugslysa (RNF, Aircraft Accident Investigation Board) released their final report concluding the probable cause was:
The Ethernet wire screen (ground) and pin 12 in the P2 plugs in cable assembly p/n 179169-117 were found disconnected from the plug and plug shield.
This fault can cause a ~32VDC load to feed through the SVDU and through the audio jack return lines, causing the resistors in the audio jack to overheat and burn.
The RNF reported that an investigation was opened, however no root cause could be determined at the time despite analysis by the manufacturer under supervision by the NTSB. It could only be determined that a voltage source of less than 32VDC had caused the audio jack to overheat. The occurrence was regarded as a single isolated event and the investigation closed.
On Jun 15th 2009 another Boeing 757-200 was pushed back from Keflavik's gate, when a passenger reported a hot armrest and a burning smell. The aircraft was towed back to the gate, an engineer deactivated the seat's inflight entertainment system, and the aircraft was pushed back again. This time another passenger in the neighboured seat reported a hot armrest and a burning smell. The aircraft again returned to the gate, a maintenance engineer disconnected power from both seats.
After getting notified about these occurrences the RNF decided to re-open the investigation of the Thales i4500 inflight entertainment system installed at least on Icelandair and Air Canada Boeing 757s (205 aircraft in total).
The investigation into the second occurrences determined that the shield of the Ethernet cables was not connected to the plug shield on both failed seats and pin 12 (ground) was also disconnected from the plug. Inspection of another seat showed the wire screen was not protected and could easily be disconnected from the plug shield. When monitored with a voltmeter and the plug being wiggled it was detected that after some time of wiggling a voltage of about 30VDC appeared on the audio return wire, that had been at less than 1VDC before, this condition was only possible with the cable shield and pin 12 disconnected.
An experiment confirmed that after disconnecting the wire shield and pin 12, effective removing ground from the device, 30VDC appeared at the audio return pin which caused the resistors to overheat.
The manufacturer of the inflight entertainment system took an immediate safety action to replace the connectors with new ones, that ensure grounding still occurs even in a loose plug situation.
This article is published under license from Avherald.com. © of text by Avherald.com.
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