On 25 November 2003, at about 2010 Tokyo Local Time, during passenger boarding and while conducting pre-departure checks in preparation for a flight from Narita, Japan to Sydney, Australia, the flight crew of the Boeing 747-300 (747), registered VH-EBU, noticed that the 'No Smoking/Fasten Seatbelt' sign circuit breaker in the 'C' zone tripped when the sign was switched to ON. Shortly afterwards, the cabin crew reported that a flash was observed, and a burning smell was detected in the vicinity of seat 37K, on the right side of the main deck. The passengers and crew were not injured and disembarked, and the ground engineers were called to examine the affected area in the aircraft.
On removal of the side wall trim and stowage bins adjacent to seat 37K, the 'No Smoking/Fasten Seatbelt' sign circuit wires and other wires in the wiring loom were found to have been damaged. The engineers reported that the loom was pinched between the outboard corner of the stowage bin and the adjacent structural frame (refer to figure 1).
The wiring loom had chaffed against the structural frame and the wiring insulation had been progressively abraded until the conductors made contact with the metal frame. Electrical arcing resulted in localised damage to the wiring loom and the structural frame, extensive charring of the two adjacent insulation blankets and the tripping of the 'No Smoking/Fasten Seatbelt' sign circuit breaker. The damaged wires and the frame were repaired in Narita before the aircraft, without passengers, was ferried to the operator's base in Sydney for further examination and repair.
Damage to the aircraft
The damaged wiring loom contained wires from the 'Fasten Seatbelt' and 'No Smoking' signs, ceiling and emergency lights, rear bulkhead and the right and left cargo door position warning systems. All wires in the loom were damaged by mechanical chaffing and heat generated during the arcing event.
The outboard corner of one stowage bin was lightly charred and the arcing resulted in minor damage to the structural frame adjacent to the stowage bin.
While there was no fire, the heat generated during the arcing event resulted in extensive charring to two adjacent insulation blankets. The charring occurred when the blankets' outer reinforced plastic film melted due to the heat generated during the arcing event. The blankets sustained no fire damage. The blankets and the reinforced plastic material complied with the aircraft manufacturer's specifications and were approved for use on the aircraft.
Aircraft information
| Manufacturer | Boeing Company |
|---|---|
| Model | 747-300 |
| Serial number | 23223 |
| Registration | VH-EBU |
| Delivered new | January 1985 |
| Total time since new | Approximately 78218 hours |
| Last maintenance | 20 November 2003 at 78208 hours |
| Last 'D' check | 15 October 2002 |
Wiring loom compliance and examination
The wiring loom was properly constructed and supported. The individual wires complied with the aircraft manufacturer's specifications and were approved for use on the 747 aircraft. The aircraft manufacturer advised that the wiring loom location was in accordance with the original drawing specifications.
The operator advised that a review of the aircraft maintenance documents revealed that wiring in the 'C' zone was last inspected in 1997. At that time, no chaffing or damage to the wiring loom was reported. During the 2002 'D' check, the 'C' zone was subjected to a corrosion prevention inspection that, among other things, required wiring looms to be displaced as necessary to facilitate inspection of the underlying structure. No damage to wiring or looms was reported at that time. Since then, no major work that would have affected wiring in that area was reportedly carried out.
Stowage bins
The operator advised that the stowage bins were the units fitted since the aircraft was new. They had been removed and reinstalled during each of three 'D' checks that had been conducted on the aircraft since it entered service. The last time the stowage bins had been removed and refitted was during a 'D' check in October 2002.
The removal and installation of the stowage bin was detailed in the aircraft maintenance manual and carried out in accordance with the operator's task cards for the job. The task cards included information contained in the maintenance manual. In relation to the stowage bins, the maintenance manual stated: 'when bin is closed, ensure bin is aligned with adjacent bins and pull down lightly on the bin ….'. The aircraft maintenance manual and the operator's task cards did not contain a requirement to check for clearance between the stowage bin and the adjacent wiring looms and aircraft structure.
The aircraft manufacturer advised that the existing standard wiring practices manual, along with standard maintenance practices, are to ensure that stowage bins do not come into contact with wiring looms and the adjacent aircraft structure.
Fleet inspection
The operator inspected the wiring looms in the same area on all B747-300 aircraft in its fleet and reported that the subject loom on EBU appeared to have been routed slightly higher and the stowage bin was installed slightly lower and closer to the structure. The wiring loom on the opposite, left side of EBU, at row 37, was also found routed slightly lower. All inspected wiring looms were clear of the stowage bins and showed no evidence of chaffing or other damaged (refer to figure 2). The operator advised that there was no evidence to suggest that the wiring loom location may have been altered during the life of the aircraft.
Previous similar occurrence
The aircraft manufacturer reported that it was not aware of any similar event involving a wire being chaffed between the stowage bins and the structure, resulting in a short circuit.
The operator reported that in 2003, the 'D' zone 'Fasten seatbelt' and 'Lavatory flush' circuit breaker on EBU tripped due to the wiring loom having chaffed on a bracket. The loom was found to have been routed incorrectly.