What happened
On 27 June 2016, a Singapore Airlines Boeing 777-300ER, registration 9V-SWB, departed Singapore for Milan. Approximately 30 minutes into the flight, the crew observed a significant drop in the right engine's oil quantity, which had fallen to just 1 unit compared to 17 units in the left engine. While oil pressure and temperature remained within normal ranges, the crew consulted with the airline's engineering center. Following discussions with technical services, the decision was made to return to Singapore.
During the return flight, the crew experienced unusual vibrations in the cockpit floor and control column when increasing power to the right engine. A faint smell of burnt material was also noted in the cabin. Despite receiving a "FUEL DISAGREGE" message, the crew did not perform the full fuel leak checklist, believing the discrepancy was due to changes in flight parameters.
Upon landing at Changi Airport, the deployment of thrust reversers was followed by two loud bangs and visible flashes near the right engine. A fire subsequently developed under the right wing. Airport rescue and firefighting services responded immediately to extinguish the flames. There were no injuries among the 241 persons on board, though the aircraft sustained extensive heat damage to the engine cowlings, thrust reverser components, and the right wing area.
The investigation
The investigation focused on the source of the fuel loss and the subsequent fire. Investigators examined the Main Fuel Oil Heat Exchanger (MFOHE) and identified that fuel had leaked into the engine oil system. The inquiry also reviewed the crew's decision-making process regarding the fuel discrepancy message and the management of the engine parameters during the return flight. Technical analysis of the MFOHE revealed a design issue involving potential cracks in the tube structure.
Findings
- The primary cause of the incident was a fuel leak from the MFOHE into the engine oil system.
- The leak led to the development of a fire in the engine and wing area following the application of thrust reversers upon landing.
- The crew did not perform the prescribed fuel leak checklist because they believed the automated fuel calculations were inaccurate due to the altered flight profile.
- The presence of a burnt smell in the cabin was an early indicator of the leak, which the crew addressed by turning off the right engine bleed system.
Safety action
Following the investigation, the engine manufacturer accelerated a service bulletin to modify affected MFOHE units and developed enhanced algorithms to detect fuel leaks in the oil system during both flight and ground processing. The aircraft manufacturer also introduced interim procedures to minimize fire risks following a suspected leak. Additionally, recommendations were made to improve pilot training regarding non-standard situations and to utilize combustible gas detectors during maintenance to detect fuel odors more reliably.