Incident Overview
On 15 August 2010, a Boeing 767-31K, registration G-DAJC, was operating a commercial passenger flight from Manchester Airport to Dalaman, Turkey. Shortly after reaching cruising altitude at FL370, the cabin manager reported smoke emanating from an oven in the forward galley.
Crew Response
The commander instructed the cabin manager to pull the circuit breaker for the forward galley ovens, and a crew member did so. However, acrid smoke continued to emanate, leading a second cabin crew member to discharge a bromochlorodifluoromethane (BCF) extinguisher into the right oven. Upon being informed, the commander isolated the utility busbar and issued a PAN call to Maastricht ATC, declaring an intention to divert to London Gatwick Airport.
Smoke persisted from the forward galley ovens, and cabin crew discharged five additional BCF extinguishers into both ovens, which did not stop the smoke as it appeared to originate from around the oven exteriors. No passengers were moved, as no complaints of smoke inhalation were received.
Diversion and Landing
Maastricht ATC transferred the aircraft to London ATC, which offered a direct approach to Runway 26L at Gatwick; the commander accepted. The aircraft landed normally at 0750 hrs, vacated via Taxiway FR, and parked on Taxiway J. Airport Fire and Rescue Service (AFRS) personnel entered the aircraft using a ladder and confirmed no fire was present. The aircraft was towed to a remote stand.
Aircraft and Oven Description
The aircraft had eight ovens: two in the forward galley and six in the aft galley. Each oven comprised an oven unit and a controller module mounted on top. The forward galley ovens were supplied with 115 V AC and protected by a 15-amp circuit breaker accessible to cabin crew. Each controller module had an additional 5-amp circuit breaker installed on its rear, not accessible without removing the oven.
Examination Findings
Both forward galley ovens were removed by maintenance engineers. The left oven was undamaged. Inspection of the right oven revealed an electrical short circuit within its controller module, and the module's 5-amp circuit breaker had tripped. The controller's printed circuit board (PCB) showed electrical overheat damage at the mounting location of the ON-OFF switch, with localised burning of copper tracks and soldered connections. This precluded examination of pre-existing defects in that area.
The oven unit was tested with a replacement controller module and found serviceable. The tripped circuit breaker was tested and performed within manufacturer specifications for current and cut-out time. Panel-mounted switches and timer switch were also tested and found serviceable.
Maintenance History
The oven had been installed eight days prior to the incident, after which the aircraft completed 30 flight cycles and 108 flying hours with no recorded defects related to the oven. In May 2010, the oven was serviced by an approved repair organisation, including cleaning, inspection, and testing in accordance with the manufacturer's component maintenance manual, and was recorded as serviceable.
Analysis
The damage indicated an electrical short circuit at the connection between the oven controller's ON-OFF switch and its PCB. The damage prevented further investigation into the cause of the short circuit. The 5-amp circuit breaker operated within its design specification, tripping to prevent further current flow and intensification of the overheating. Damage was limited to the controller's PCB and contained within its enclosure. Smoke from the overheating PCB escaped and circulated around the forward galley ovens, causing confusion about its source. A review of the oven controller's maintenance history did not reveal any defects that might have contributed to the short circuit, which was the most likely cause of the electrical overheating damage.