Incident
On September 30, 2010, at 0825 UTC, a Cessna 680, registration G-CJCC, operated by Bookajet, encountered a left main bus electrical system fault approximately 20 minutes after departing from Luton Airport (EGGW) en route to Milas-Bodrum Airport (LTFE) in Milas, Turkey. The flight crew performed the required checklists and returned to Luton. Upon landing, the left main fuel tank contained 5,500 pounds of fuel, while the right tank held 3,300 pounds, resulting in a fuel imbalance.
Investigation
The Air Accident Investigation Branch (AAIB) issued Special Bulletin S1/2010 on October 8, 2010. The bulletin detailed that ground testing following the incident revealed that removing power from the left main bus caused the fuel cross-feed valve to open and the right boost pump to operate without crew selection. Additionally, the motive-flow shutoff valves remained open. This configuration allowed motive-flow fuel to be returned to both tanks, resulting in uncommanded fuel transfer from the right to the left tank.
The bulletin stated that during the 20 minutes between selecting the left generator OFF and landing at Luton, sufficient fuel transferred from the right to the left tank via the left engine motive-flow system to create a 2,200-pound imbalance. The crew was unaware of the transfer because only the right fuel quantity indication was available.
Summary
The incident highlighted that a loss of power to the left main bus could lead to an uncommanded fuel imbalance. The AAIB continued to investigate the circumstances.