On 17 April 2016, a Boeing 787-9, registered N36962, operated by United Airlines as flight UAL870, departed Sydney New South Wales (NW) for San Francisco, United States.
During the departure, cabin crew switched on the aft galley ovens in preparation for meal services. As soon as the ovens were switched on, there was a short burst of smoke, which set off a fire alarm in a nearby toilet for about one minute. One of the ovens displayed a FAILURE message. Several cabin crew detected a strong chemical odour and an electrical smell as well as a blue haze. The oven interactive screen displayed a “Critical Error-Broken Fuse” message.
The cabin crew switched off all electrical sources to the aft galley. The captain was advised. After consultation with the ground-based technical operations maintenance controller, flight crew and cabin crew, a decision was made to return the aircraft to Sydney.
The crew advised ATC via a PAN call (uncertainty phase) and the aircraft landed without incident in Sydney with emergency services in attendance.
The airline had a similar incident with another Boeing 787-9 on 2 June 2016.
After extensive testing by the oven manufacturer and Boeing, it was found that the ovens had not been updated with the latest version of software, which addressed a known problem. The oven manufacturer is now updating the software in all relevant aircraft in United Airlines fleet.
This incident highlights the correct management of an abnormal situation with effective crew coordination. Each crew member responded effectively and the situation was professionally managed by the captain.
Aviation Short Investigations Bulletin - Issue 50
Boeing 787, N36962Source: John Richard Thomson
What happened
On 17 April 2016, a Boeing 787-9, registered N36962, operated by United Airlines as flight UAL870, departed Sydney, New South Wales (NSW), for San Francisco, United States. On board were 4 flight crew, 11 cabin crew and 238 passengers. During the departure, cabin crew switched on the aft galley ovens (Figure 1) in preparation for meal services.
After the two ovens were switched on, there was a short burst of smoke, which set off a fire alarm in a nearby toilet for about one minute. One of the ovens displayed a “FAILURE” message. Several cabin crew detected a strong chemical odour and an electrical smell, as well as a blue haze. Other crew described it as an ozone smell. The oven interactive screen displayed a ‘Critical Error- Broken Fuse’ message.
The crew immediately pulled all relevant circuit breakers, and switched off all electrical sources to the aft galley. The inflight service manager (ISM) advised the captain. The ISM and a relief pilot from the cockpit arrived at the aft galley with fire extinguishers. By this stage, the smoke had dissipated, but the odour persisted. As it could not be confidently ascertained that the ovens were the sole source of the problem, the captain contacted the ground-based technical operations maintenance controller (TOMC) by satellite phone.
Figure 1: Rear section of a B787-9 depicting aft galley
Source: SeatGuru modified by the ATSB
The discussion with the TOMC involved all flight crew and the ISM. It was agreed that the safest option was to return the aircraft to Sydney. The captain advised ATC by a PAN[1] call. ATC initiated an INCERFA[2] phase. About 110 km east of Port Macquarie, NSW, the crew commenced a return to Sydney. As the aircraft was well in excess of its allowed landing weight, fuel was dumped during the descent.
The aircraft landed without incident in Sydney at 1258 Eastern Standard Time (EST) with emergency services in attendance.
Post-incident engineering report
A post-engineering inspection quarantined the suspect oven, and after an inspection, a fuse was replaced. After appropriate testing, the aircraft was released back to service.
Boeing and the oven manufacturer investigated the cause of the ‘Critical Error’ fault displayed on the oven screen (Figure 2).
The manufacturer individually tested all oven components. They reported that all individual components worked correctly, however, an additional measurement of the oven motor current detected that the motor did not run smoothly. The motor temperature was also above normal, most likely from insufficient airflow. This known fault had been rectified with a new oven software release.
Boeing reported that the oven manufacturer is working with United Airlines to update the software in all relevant ovens in their fleet.
The exact cause of the odour could not be determined.
A second similar occurrence
United Airlines have advised the ATSB of a second similar occurrence involving another B787 aircraft. On 2 June 2016, a United Airlines B787 aircraft, N35953 experienced an electrical/heat odour in the mid B galley. The flight crew dumped excess fuel and returned safely to Melbourne. On this occasion, no emergency was declared.
Maintenance were able to isolate one oven, and confirmed the error was a broken fuse. The oven was removed and replaced, and the aircraft returned to service.
Figure 2: Error message from oven on N35953
ATSB comment
As part of the investigation, the ATSB obtained reports from the flight crew and cabin crew on board during the incident.
It was evident that all emergency procedures were carried out efficiently and effectively. The captain involved all relevant crew members and the TOMC prior to making a decision to return the aircraft to Sydney.
Safety message
This incident highlights the correct management of an abnormal situation with effective crew coordination. Each crew member responded effectively and the situation was professionally managed by the captain.
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