What happened
On 30 September 2010, a Cessna 680 Citation Sovereign, registration G-CJCC, was performing a commercial passenger flight from London Luton Airport to Milas-Bodrum Airport, Turkey. During the climb, the crew observed a DC EMER BUS L amber Crew Alerting System message. Following established emergency procedures, the crew selected the left generator OFF.
This action resulted in the loss of several systems, including the flaps, the commander's Primary Flight Display, and the left fuel quantity indication. As the flight continued, the co-pilot noted that increasing aileron input was necessary to maintain wings-level flight. The aircraft had developed a significant lateral fuel imbalance, with fuel transferring from the right tank to the left. The crew elected to return to Luton Airport, where a flapless landing was completed without further incident. No fatalities or injuries were reported, and there was no damage to the aircraft.
The investigation
The AAIB investigation focused on why the fuel transfer occurred despite the cross-feed selector being in the OFF position. Analysis of the Flight Data Recorder (FDR) showed that the emergency left DC bus parameter became active 15 minutes after takeoff, followed by the loss of the left DC generator.
Testing by the aircraft manufacturer revealed that 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 activate. The investigation identified that a fault in the power distribution printed circuit board was the root cause of the initial electrical event. Furthermore, it was discovered that the fuel control system design allowed a loss of power on the left fuel control card to generate a false fuel cross-feed command.
Findings
- The initial electrical event was caused by a faulty power distribution printed circuit board.
- 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.
- The aircraft's fuel control system design meant that a power failure on the left fuel control card provided a low impedance input to the right card, triggering a false cross-feed command.
- The crew's adherence to the approved checklist procedures resulted in an unsafe aircraft configuration due to the underlying system vulnerability.