Freezing Rain and Tail-Plane Icing in a Cold Pool
Your turboprop is holding at 3,000 ft over an aerodrome in a shallow cold-air pool. Surface temperature is −3 °C, dew point −5 °C, wind 070°/06 kt, QNH 1028 hPa. The TAF reads broken cloud at 800 ft, overcast at 1,500 ft, with TEMPO 1200/1600 visibility 3,000 m in light freezing rain. A SIGMET warns of moderate icing between 2,000 and 9,000 ft. A pilot report from an aircraft that just landed states moderate rime icing in cloud between 3,000 and 6,000 ft, and freezing rain below 4,000 ft. The 1200 UTC sounding shows a surface inversion to 1,000 ft, temperature rising to +2 °C at 5,000 ft, then falling to −8 °C at 10,000 ft. The aircraft has wing and propeller anti-icing/de-icing but no tail-plane anti-icing. The alternate has a 600 ft ceiling and 5 km visibility, 20 minutes away. You have 55 minutes of fuel and a 30-minute final reserve.
1. Explain the temperature profile and why freezing rain, rather than snow, is occurring.
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The sounding shows a classic warm-advection pattern over a cold pool. A surface inversion extends to 1,000 ft, with cold air at −3 °C trapped beneath it. Above that, temperature rises to +2 °C at 5,000 ft, then falls again to −8 °C at 10,000 ft. Snow falling from the cloud above 5,000 ft melts as it passes through the above-freezing layer, then falls into the subfreezing air below 4,000 ft as supercooled water. Because the cold layer is shallow and the drops do not have time to freeze into ice pellets, they remain liquid below 0 °C and freeze on contact with the aircraft or the ground. The inversion is stable and inhibits vertical mixing, so the cold pool persists. Freezing rain is therefore not a marginal indication: it means supercooled large drops are present, which is a severe icing environment even when the reported intensity is light.
2. Assess the icing hazard to this aircraft, including the tail-plane, and explain why holding at 3,000 ft is a poor choice.
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The aircraft has wing and propeller protection but no tail-plane anti-icing. Holding at 3,000 ft places it in the reported freezing-rain layer below 4,000 ft, where supercooled large drops can freeze on the unprotected tail-plane. Tail-plane icing can reduce elevator effectiveness and produce a sudden pitch-down or loss of control, particularly with flaps extended. The pilot report of moderate rime between 3,000 and 6,000 ft shows that cloud icing is also present. Wing anti-icing may cope with rime but may not keep up with freezing rain, and the SIGMET covers the whole block from 2,000 to 9,000 ft. Holding at 3,000 ft is therefore the worst available choice: it is in the freezing-rain layer, in cloud, and with no tail protection. If a level change is possible, climbing above the supercooled layer or descending below it would reduce exposure, but the fuel state limits options.
3. Decide whether to hold, land at the destination, or divert. Include fuel, alternate and aircraft limitations in your reasoning.
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Do not hold. Of the 55 minutes of fuel, 30 minutes is final reserve, leaving 25 minutes usable. The alternate is 20 minutes away, so diverting uses 20 minutes and leaves only 5 minutes of margin; holding would consume the diversion fuel and remove the alternate option. Landing at the destination in freezing rain with an unprotected tail-plane is also high risk, especially with a 800 ft ceiling and a contaminated runway possible. The alternate has a 600 ft ceiling and 5 km visibility, which is the better environment even though it is not benign. Divert immediately, request priority if necessary, use anti-icing continuously, avoid prolonged flight in the freezing-rain layer, and plan the approach with extra landing distance for ice. If the alternate deteriorates below minima, declare a fuel emergency early rather than delay the decision.