Boeing Co 747-338, VH-EBU

Aircraft operator

As a result of this occurrence, the aircraft operator reported
that the task cards for the job were amended and that they now
require that the wiring looms and other components in the area of
the stowage bins are to be inspected to ensure adequate clearance
from the bins and the structure.

Aircraft manufacturer

During the comment period for the draft report, the aircraft
manufacturer was requested to review the installation procedures
for 747 overhead bins, to assess the benefit of including a
requirement to ensure adequate clearance between the stowage bins
and any adjacent structure, wiring looms and other components.

The aircraft manufacturer responded to the draft report and
advised:

Based on this incident, our Service Engineering and Maintenance
Manual departments reviewed the need for an inspection note for
wire bundles when installing the overhead stowage bins. The results
of that review were as follows:

As stated in the draft report, the current Boeing task cards do
not provide instructions on how to manage wire bundles upon
re-installation of the stow bins. Instead, wire separation
information including minimum clearance requirements for wire
bundles is provided to operators in section 20-10-11 of Boeing
Standard Wiring Practices Manual (DC-54446). Thus, the wire
separation requirement is already documented to the maintainer,
just not on the specific task card.

We depend on the maintainer to possess the necessary minimum
skills to return the airplane to a serviceable condition. As such,
a maintainer installing stow bins should already be aware of
minimum wire bundle clearance requirements. Thus, we believe that
the existing maintenance documentation, along with standard
practices expected of the maintainer, already addresses this
issue.

We searched our service history records and could not find any
other occurrences of this issue. Thus, the 30+ years of service
history we have on the 747 fleet shows that this has not been a
problem with the in-service fleet.

As a result, we do not feel it is necessary to amend the stow
bin installation task cards to include a requirement for inspection
of wire bundle clearances. Of course, each operator could include
such a requirement at their discretion.

¹ The
'C' zone is the zone on the main deck between the main cabin doors
2 and 3.

² The 'D'
check is the most intensive maintenance check on an
aircraft.

The following factors were identified as being significant to
the development of the occurrence.

The aircraft maintenance manual and operator's task cards did
not contain a requirement to check for clearance between the
stowage bin and the adjacent wiring loom and the aircraft
structure.
The stowage bin was installed without ensuing adequate
clearance with the aircraft's structure and the adjacent wiring
loom.
The wiring loom to the 'No Smoking/Fasten Seatbelt' signs in
the aircraft's 'C' zone became pinched and subsequently chaffed
between the stowage bin and the fuselage frame.
The wiring loom insulation had progressively abraded over time
until the conductors made contact with the metal frame, resulting
in electrical arcing.
The electrical arcing resulted in localised damage to the
wiring loom and the structural frame, extensive charring of the two
adjacent insulation blankets and resulted in the tripping of the
'No Smoking/Fasten Seatbelt' sign circuit breaker.

The wiring loom to the 'No Smoking/Fasten Seatbelt' signs in the
aircraft's 'C' zone became pinched and chaffed between the stowage
bin and the fuselage frame. As a result of that contact, the wiring
insulation had been progressively abraded until the conductors made
contact with the metal frame, resulting in electrical arcing.

Pinching of the wiring loom most likely occurred when the
stowage bins had been installed during the last 'D' check in
October 2002.

Neither the aircraft maintenance manual, nor the operator's task
card detailing installation of the overhead bins, call for
inspection of the wiring looms and other components in the area of
the stowage bins to ensure their adequate clearance from the
bins.

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.