Aircraft and Flight Details
The incident involved a Cessna 525 Citation Jet, registration D-IPCS, powered by two Williams FJ44 turbofan engines. Manufactured in 1998 (serial no. 525-0264), the aircraft was conducting a commercial air transport passenger flight from Barcelona to Manchester on 31 October 2013 at 1820 hrs. On board were two flight crew and four passengers. No injuries or damage occurred. The commander, aged 45, held an Airline Transport Pilot’s Licence with 4,900 total flying hours, including 400 on type, and had flown 71 hours in the last 90 days and 10 in the last 28.
History of the Flight
The flight departed Barcelona at 1555 hrs. During cruise at FL 400, with a ram air temperature of -48°C, a fuel imbalance of about 120 lb developed. The crew transferred fuel from the right to left tank. Approximately 10 minutes after completing the transfer, the right engine FUEL FLTR BYPASS warning illuminated, indicating an impending filter bypass. The Quick Reference Handbook advised landing as soon as practical and to consider possible loss of thrust. The crew initially continued toward Manchester but, after another 10–12 minutes, the left engine warning also illuminated. They informed ATC and requested diversion, accepting London Gatwick. The aircraft landed at 1841 hrs after an expeditious but uneventful approach. Both engines operated normally, and no further abnormal indications appeared.
Fuel System Description
The fuel system comprises two integral wing tanks, each supplying its engine via a primary ejector pump. Electric boost pumps support starting, transfer, and backup. Fuel passes through an engine-driven pump and a filter before reaching the fuel control unit. Each filter has a bypass valve to maintain flow if obstructed, and a warning light alerts the crew. No heating is applied to the fuel before the filter; therefore, anti-icing additive must be mixed with the fuel to prevent icing.
Refuelling Prior to the Flight
The commander reported refuelling at Barcelona with 1,100 litres of Jet A-1 fuel, bringing total fuel to 2,700 lb (approx. 1,500 litres). Anti-icing additive, normally carried on board, was added. However, the commander inadvertently used a can containing only half the amount of additive compared to usual, resulting in only half the required concentration.
Aircraft Examination
AAIB investigators, accompanied by the operator’s maintenance engineer, inspected the aircraft. Fuel samples from both tanks showed no visible water or contamination and tested negative for micro-organisms. The engine fuel filters were removed and found clean, but water was present in the filter bowl fluid. The filters were replaced and bowls cleaned. Engine ground runs were normal. Subsequent laboratory testing confirmed the fuel samples contained significantly less anti-icing additive than required.
Conclusion
The investigation concluded that the obstruction of the fuel filters was most likely caused by ice forming on the filters due to insufficient anti-icing additive being added to the fuel during the previous refuelling.
Safety Actions
Following the incident, the operator implemented new procedures requiring crews to record the quantity of anti-icing additive used in the flight log. A safety message was issued to all crew highlighting the requirement for additive, advising of the new recording process, and reminding commanders to ensure sufficient additive is on board before departure from home base.