A Cessna 550 Citation II was operating a research flight on this day in aviation history. The aircraft was specially equipped for research on icing conditions in flight. The purpose of this mission was to locate icing conditions for a prototype helicopter's flight in icing tests.
The aircraft fuselage after part out. (Source baaa-acro.com © www.aeroprints.com)
On board the aircraft was a crew of four, two pilots and two research scientists. The flight had taken off from Fairbanks (Alaska, USA) at approximately 12.10 lt (local time) on an instrument flight plan.
While established in cruise flight while flying in icing conditions there was an ice built up on the wing leading edges with a thickness of approximately 7/8 to 1 inch. The commander activated the (pneumatic) de-icing boots which resulted in the ice being shed from the wings. A short while later (~4 minutes) a loud bang was heard from the rear of the aircraft and both engines spooled down. An emergency descent was initiated and attempts were made to restart the engines. As the aircraft descended through 6000 feet it broke out of the clouds the start attempts continued. At 3000 feet the crew was still unsuccessful in getting the engines started and further attempts were abandoned. At this point in time, the focus of the crew shifted to preparing the aircraft for an off-airport landing. A fairly clear area with some small trees was the best option and the aircraft landed with the landing gear retracted in the snow-covered field. During the landing, the aircraft struck several small trees causing substantial damage to the aircraft, resulting in the aircraft being written off as damaged beyond repair. The occupants sustained minor injuries in the landing.
The ice protection system of the aircraft consisted of two separate systems;
Anti-ice system, for the inboard section of the wings directly in front of the engines and the engine inlets. Using hot engine bleed air to avoid ice built-up and ice ingestion in the engines
De-ice system, for the main part of the wing using inflatable rubber boots
Both systems have to be manually operated by the crew, with the anti-ice system having to be activated by the crew prior to entering icing conditions. According to the aircraft manufacturer, it takes 2-4 minutes in flight (after ice has built up) for the wing leading edge to heat up sufficiently to melt the ice. The de-icing system is activated once ice has built up on the wing's leading edge. The de-ice boots will then inflate and deflate alternately breaking the ice away from the leading edges. The National Transportation Safety Board (NTSB) launched an investigation into the accident.
As part of the investigation, all occupants were interviewed and pictures taken inflight were analysed. Both engines were sent to the manufacturer where they were disassembled and inspected under the supervision of the NTSB. The inspection revealed that fan blades on both engines had failed as a result of FOD ingestion and had subsequently been ingested by the engines. The NTSB concluded that the probable causes of the accident were;
"The pilot's improper use of anti-icing equipment during cruise flight, which resulted in ice ingestion into both engines (foreign object damage), the complete loss of engine power in both engines and an emergency descent and landing on tree-covered terrain. Factors associated with the accident were the icing conditions, inadequate crew resource management, and failure to use a checklist."
The NTSB report with details of the investigation can be accessed by clicking on the .pdf file below;