What happened
On 30 September 2010, a Cessna Citation 680 Sovereign, registration G-CJCC, was performing a commercial passenger flight from London Luton Airport to Milas-Bodrum Airport, Turkey. While climbing through FL300, the crew received a DC EMER BUS L amber alert on the Crew Alerting System.
Following the Emergency/Abnormal Procedures checklist, the crew identified a fault on the left main electrical bus and elected to switch the left generator to the OFF position. This action resulted in the loss of several systems, including the flaps, the commander's Primary Flight Display, and the left fuel quantity indication. The crew decided to return to Luton, which was only 20 minutes away.
During the descent and approach, the co-pilot noticed that increasing right aileron input was necessary to maintain a wings-level attitude. This was due to a significant lateral fuel imbalance that had developed during the flight. The aircraft eventually completed a flapless landing at Luton without further incident. No injuries were reported among the three crew members or five passengers, and no damage was sustained by the aircraft.
The investigation
The AAIB investigation, supported by the NTSB, examined the fuel system and the electrical bus failure. Investigators found that while the crew had not manually selected the fuel cross-feed, the loss of power to the left main electrical bus caused the fuel cross-feed valve to open and the right fuel boost pump to operate.
Ground testing conducted under AAIB supervision confirmed that when power is removed from the left main bus, the cross-feed valve opens and the right boost pump activates while the motive-flow shutoff valves remain open. This configuration allows excess fuel from the right engine to be returned to the left tank, creating an uncommanded transfer. Tests on a similar aircraft replicated these findings.
Findings
- The loss of power to the left main electrical bus caused the fuel cross-feed valve to open and the right fuel boost pump to operate without crew input.
- Because the motive-flow shutoff valves remained in their original positions, fuel was able to transfer from the right tank to the left tank.
- This uncommanded transfer created a fuel imbalance of 2,200 lbs, which exceeded the maximum permissible lateral imbalance of 800 lbs in an emergency.
- The resulting weight imbalance necessitated increased aileron control input to maintain level flight.