Icing Encounter and the Decision to Divert
You are flying a single-engine turboprop at FL200 in IMC. The outside air temperature is -8 degrees C. You enter a layer of supercooled large droplets and notice ice building on the windshield and boots. The aircraft has a known-ice certification with boots and a heated propeller, but no bleed-air anti-ice for the windshield. The ice rate is moderate to severe, and the aircraft is beginning to lose 10 kt of airspeed. The nearest suitable airport is 25 minutes away with a 1,000 ft ceiling and 5 km visibility. A closer airport is 10 minutes away but has a 300 ft ceiling and 1 km visibility with a non-precision approach. Your alternate is 40 minutes away with good weather.
1. What are the immediate priorities, and which airport should you choose?
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Immediate priorities are to exit the icing conditions and maintain control. Reduce power and pitch to maintain airspeed, avoid abrupt manoeuvres, and turn toward the nearest area of warmer air or clear conditions. The loss of 10 kt indicates performance degradation; do not wait for further loss. The closer airport at 10 minutes has 300 ft ceiling and 1 km visibility with a non-precision approach, which is below or near minima and offers little margin. The 25-minute airport has 1,000 ft and 5 km, which is more suitable for a stabilised approach. The 40-minute alternate has good weather but is farther. The best choice is the 25-minute airport if the aircraft can maintain altitude and airspeed; if performance continues to deteriorate, the closer airport may be the only option despite poor weather. The decision must be made early, before the aircraft is unable to climb or maintain altitude.
2. How does the aircraft's certification affect your options, and what should you report to ATC?
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Known-ice certification means the aircraft is approved for flight in icing conditions within the certified envelope, but it does not guarantee protection in severe icing or supercooled large droplets. The absence of windshield anti-ice means visibility forward may degrade, increasing workload. The crew should report the icing intensity and type to ATC, request a change of altitude or route to exit the conditions, and declare an urgency if performance is affected. ATC can provide priority handling and vectors to the nearest suitable airport. The report also helps other aircraft. The key judgement is to treat the certification as a limit, not a licence to remain in severe icing.