Boeing late last year added a production-line inspection and issued recommendations to operators following an Etihad Airways Boeing 777-300 in-flight diversion caused by chafing and arcing of incorrectly installed wire bundles--the fifth incident linked to the faulty production process.
The incident occurred in October 2017 during a flight from Abu Dhabi to Sydney. As the aircraft neared Adelaide, the flight crew "noticed a burning smell coming from an air vent," the Australian Transport Safety Bureau (ATSB) wrote in its incident report. The issue soon triggered on-board warnings of a forward cargo fire. The crew performed its "non-normal" checklist, discharged forward-cargo fire bottles, and declared an emergency. The aircraft, carrying 349 passengers and 16 crew members, arrived "uneventfully" at Adelaide Airport about 50 minutes after the incident began.
A post-incident inspection found soot damage on the forward cargo compartment ceiling. A more detailed investigation traced the soot's source to heat damage and a chafed electrical wire in a bundle running between the cargo compartment ceiling and the cabin floor above. Boeing determined the entire wiring loom that contained the chafed wire, which that powered a re-circulation fan, was "incorrectly routed, likely during aircraft manufacture, and had not been installed as per the design drawings."
The aircraft involved, serial no. 41701, was delivered in November 2013 and had 21,493 hours and 2,284 cycles at the time of the incident.
Four years in service caused the mis-routed wire bundle to chafe on a nearby screw. This sent current "through the passenger floor carbon-fiber beam" at body station 508. The current generated enough heat to damage 14 ceiling brackets, and cause "several areas" of the beam to chafe and delaminate, ATSB said.
"The smoke generated from the arcing was of a magnitude that it migrated through the forward cargo ceiling liner into the forward cargo compartment and activated the forward cargo fire detection system," ATSB added.
Boeing determined that the Etihad incident was the fifth that included chafing and arcing in a 777 cargo area, but "the first event that triggered the cargo fire warning system and that had been detected in flight," ATSB said. All five were linked to incorrect wiring installation, "allowing screws to chafe wires and short circuit."
The OEM responded by issuing a service bulletin recommending forward cargo compartment wire-bundle inspections, looking for instances where wires have less than 0.13-inch clearance from nearby screws. Boeing also began inspecting 777 wire-bundle installations on the production line, starting with line no. 1529, which rolled out last fall. Finally, the OEM is "considering installation and design changes to new production aircraft to alter the position of the effected wiring loom to prevent recurrence," ATSB said.
While the incident was "serious," ATSB lauded regulatory requirements, fail-safe designs and the crew's actions for minimizing its severity. Materials in the space between the cargo compartment and the cabin floor are among those that must be made of self-extinguishing material, so they do not continue to burn when heat sources are removed. Arming the forward cargo fire suppression system cut power to the recirculation fans, which stopped the arcing, investigators determined.
"Discharging the fire bottles in the forward cargo space, even though procedurally correct, had nil effect on this occasion as the source of the electrical arcing was in the sealed zone between the cargo ceiling panel and the passenger floor compartment, not in in the cargo compartment," ATSB said.