What happened
On 31 October 2013, a Cessna 525 Citation Jet, registration D-IPCS, was operating a commercial passenger flight from Barcelona to Manchester. While cruising at FL 400 over the English Channel, the aircraft encountered a ram air temperature of -48°C. During this period, the crew managed a fuel imbalance by transferring fuel between the wing tanks.
Shortly after the transfer, a warning light indicated that the right engine fuel filter was bypassing. Following the Quick Reference Handbook, which advised an early landing due to the risk of engine thrust loss, the crew initially intended to continue to Manchester. However, approximately 10 to 12 minutes later, a second warning light illuminated, indicating the left engine filter was also bypassing. The crew subsequently requested a diversion to London Gatwick, where the aircraft landed safely without further incident or injury to the two crew and four passengers on board.
The investigation
The AAIB examined the aircraft's fuel system and sampled fuel from both wing tanks. While no visible contamination or micro-organisms were found, water was present in the fuel filter bowls. Laboratory analysis of the fuel samples revealed that the concentration of anti-icing additive was significantly lower than required for the operating temperature.
Investigation into the refuelling process in Barcelona revealed that the commander had used an additive container that held only half the standard volume of additive. Consequently, the amount of anti-icing agent added to the fuel was only half of what was necessary to prevent ice formation at high altitudes.
Findings
- The fuel filters became obstructed due to ice formation caused by an insufficient amount of anti-icing additive in the fuel.
- The commander inadvertently added only half the required amount of additive during refuelling in Barcelona.
- The aircraft's fuel system does not heat the fuel before it reaches the filters, making the use of additives critical in cold temperatures.