Beechcraft King Air 200 Severe Icing Incident

Flight Details and Icing Encounter On 1228 Pacific standard time, a Beechcraft King Air 200 (registration C-FCGL, serial number BB190) departed Prince George Airport, British Columbia, on an instrument flight rules medical evacuation flight to Cranbrook, British Columbia. The aircraft carried two pilots and two paramedics and was dispatched to transport two patients from Cranbrook to Kelowna. During cruise at 15,000 feet, the aircraft entered icing conditions. For approximately 45 minutes, the ice-protection equipment functioned effectively, but ice accumulation then exceeded the system's capabilities, causing airspeed to decrease and requiring a descent despite maximum engine power. The aircraft descended from 15,000 to 10,800 feet, below the minimum obstacle clearance altitude, with Vancouver air traffic control issuing emergency vectors to avoid high terrain in the Arrow Lakes area. The pilots later reported being clear of cloud and proceeding to Kelowna, where accumulated ice up to six inches thick was shed during approach and an uneventful landing was made. ## Weather Assessment and Aircraft Limitations Approximately 20 minutes before departure, the pilot-in-command reviewed TAFs, METARs, and SIGMETs via an Internet weather site, none of which indicated icing conditions along the intended route. Based on this information, the pilot determined the weather was suitable for an IFR flight at 15,000 feet from Prince George to Cranbrook to Kelowna. However, the graphical area forecast, which the pilot did not review, called for mixed moderate icing between the freezing level and 16,000 feet for about two-thirds of the planned route. The Transport Canada-approved flight manual for the Beechcraft King Air 200 states that while the aircraft is approved for flight in icing conditions as described in the U.S. Code of Federal Regulations, it is not approved for severe icing conditions or conditions exceeding the ice-protection equipment's capabilities. The manual notes that flight into icing conditions outside CFR-defined conditions is not prohibited, but pilots must be prepared to divert promptly if hazardous ice accumulations occur. ## Descent and Recovery In cruising flight, the aircraft's true airspeed was 230 knots, providing an estimated time en route of 1 hour 30 minutes. For about 40 minutes after departure, icing conditions were light to moderate, and the ice-protection equipment prevented excessive accumulation. However, ice accumulation then exceeded the system's capabilities, and airspeed began to decrease. The pilots began planning a descent and diversion but determined there were no suitable airports within range. As airspeed decreased to 150 knots, the crew initiated a descent. The pilots requested the lowest possible safe altitude, and the controller issued a descent to 13,900 feet, the 100 nautical mile safe altitude for Cranbrook Airport. Upon reaching this altitude, the descent could not be arrested despite the power levers being advanced to the engines' redline inter-turbine temperature (ITT). Airspeed decreased to 140 knots, descent rate reached 1,500-2,000 feet per minute, and significant airframe and control surface buffeting occurred. As the aircraft descended in a power-on stall condition through 12,000 feet, the pilots notified ATC that they were unable to maintain altitude. The pilot advanced the engine power levers to the forward stops, causing both engines to continuously exceed maximum ITT and torque limits. ATC instructed the King Air to turn left 90 degrees to bring the aircraft over Upper Arrow Lake and clear of high terrain. Although the aircraft stopped accumulating ice as it descended through 11,000 feet, the accumulated ice prevented levelling off. Radar data indicate the aircraft descended as low as 10,800 feet. Shortly after, the controller advised the pilots they were likely over Arrow Lake and 10,000 feet was a safe altitude for their location. An overflying de Havilland DHC-8 informed ATC that weather appeared better to the west of Arrow Lake, and the controller vectored the King Air southwest toward Kelowna. Shortly thereafter, the King Air entered visual meteorological conditions, and the pilot was able to retard the engine power levers to bring all engine operating parameters within approved limits. During the severe icing period, the aircraft nose attitude was maintained 5 to 10 degrees up, with significant airframe and flight control buffeting, including rudder pedals buffeted almost half-deflection. Although the visible portion of the horizontal stabilizer's leading edge appeared to be shedding ice, the aircraft was in a full power-on stall during much of the descent to 11,000 feet. When instructed to turn left to follow the lake, the pilots had great difficulty banking the aircraft more than 5 degrees. ## Post-Incident Actions and Safety Measures Subsequent to the incident, both engines were removed from service because they had been operated above temperature and torque limits for about seven minutes. The pilot-in-command did not review the available graphical area forecast weather information and was not sufficiently informed to avoid the forecast icing conditions. The severe in-flight icing conditions caused ice accumulation that the aircraft's ice-protection systems were unable to prevent or remove, resulting in a power-on stall condition and uncontrollable descent. The pilot-in-command did not detect the severe ice accumulation in sufficient time to alter the flight route to avoid the icing conditions. Following an internal investigation, the company distributed a memorandum as an interim safety action to advise flight crews to review all available weather data before flights. The company subsequently developed a syllabus, examination, and emergency checklist regarding severe icing and implemented them as part of its training program to provide flight crews with more in-depth knowledge of severe icing conditions and exit strategies. This report concludes the Transportation Safety Board's investigation into this occurrence, and the Board authorized the release of this report on 07 June 2005.