What happened
On 16 April 2014, a Singapore Airlines Boeing 777-200ER, registration 9V-SVC, en route from Singapore to Johannesburg, experienced a significant fuel discrepancy. Approximately one hour into the flight, the crew received a "FUEL DISAGREE" alert from the Engine Indicating and Crew Alerting System (EICAS).
Upon investigation, the crew noted that the fuel quantity calculated by the Flight Management Computer (FMC)—based on fuel burn rates—was lower than the quantity reported by the aircraft's Fuel Quantity Indicating System (FQIS). Crucially, the gap between these two figures was widening during flight. After consulting with the airline's maintenance center, the crew elected to return to Singapore. The aircraft landed safely without any injuries or damage to the airframe.
The investigation
The investigation focused on the refueling process prior to departure. During refueling, the operator's receipt indicated that 121.5 tonnes of fuel had been uplifted, which was significantly higher than the 8/86 tonnes targeted by the crew. A maintenance technician noticed this discrepancy and performed a manual check using "magnasticks." However, this manual check failed to detect the excess fuel, and the aircraft departed under the impression that the load was correct.
Technical analysis of the aircraft's Fuel Quantity Processing Unit (FQPU) revealed that the system was erroneously operating in a mode configured for a standard Boeing 777-200 rather than the 777-200ER variant. This error meant the system was not accounting for eight additional sensors located in the center fuel tank. Consequently, the FQIS was under-reading the actual fuel volume.
Findings
- The aircraft was overfueled by approximately 41 tonnes due to a failure in the Program Switch Module (PSM).
- The malfunctioning PSM caused the FQIS to treat the 777-200ER as a standard 777-200, ignoring the sensors in the center tank's middle compartment.
- The maintenance crew's manual magnastick check was ineffective, likely due to improper execution of the procedure.
- The technicians involved had received theoretical training for magnastick checks but lacked practical, hands-on instruction.
- There was no requirement for a duplicate inspection of the manual fuel check at the time of the incident.