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 observed a DC EMER BUS L amber Crew Alerting System message.
Following established procedures, 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 left fuel quantity indication, and the commander's Primary Flight Display. The commander transferred control to the co-pilot, and the flight was diverted back to Luton.
During the descent and approach, the co-pilot noted that increasing right aileron input was necessary to maintain a level wing attitude. The aircraft eventually completed a flapless landing at Luton without further incident. There were no injuries to the three crew members or five passengers on board.
The investigation
The AAIB investigation, supported by the NTSB, examined the aircraft's fuel and electrical systems. Ground testing conducted under AAIB supervision revealed that de-energizing the left main electrical bus triggered the fuel cross-feed valve to open and activated the right fuel boost pump, even though the cross-feed selector remained in the OFF position.
Testing on a similar aircraft confirmed that this configuration allowed fuel to transfer from the right tank to the left tank via the engine motive-flow system. During the flight, this uncommanded transfer created a fuel imbalance of approximately 2,200 lbs, which the crew could not detect due to the loss of the left tank's quantity indication.
Findings
- The loss of power to the left main electrical bus caused the fuel cross-feed valve to open and the right boost pump to operate without crew input.
- Uncommanded fuel transfer from the right to the left tank occurred because the motive-flow shutoff valves remained open during the electrical failure.
- The crew was unaware of the growing imbalance because the left fuel quantity indicator was non-functional following the generator shutdown.