Freezing Rain and Holding Fuel at a Mountain Airport
A twin turboprop is inbound at FL180 to an airport in a mountain valley. The 1250 UTC METAR reports wind 210°/18 kt gusting 32 kt, visibility 2,500 m, freezing rain, broken cloud at 600 ft, temperature -2°C, dewpoint -3°C, QNH 995 hPa. The TAF temporarily forecasts 1,500 m visibility, freezing rain and broken cloud at 400 ft. The freezing level is at the surface. The crew has 1,050 kg of fuel. Trip fuel to destination is 250 kg, alternate fuel 380 kg, final reserve 300 kg, and contingency 100 kg. Holding at 6,000 ft burns 600 kg/h. The destination has no ILS, only an NDB approach with MDA 720 ft and visibility 1,600 m. The alternate is 35 minutes away and forecast above minima, but has a PROB30 TEMPO of thunderstorms. The crew has already been holding for 5 minutes.
1. Calculate the fuel available for holding at destination while preserving alternate and final reserve, and the maximum holding time. The crew has already held for 5 minutes.
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Fuel on board 1,050 kg. Trip fuel to destination 250 kg, so fuel at destination = 800 kg. Preserve alternate 380 kg + final reserve 300 kg = 680 kg. Available for holding = 800 − 680 = 120 kg. At 600 kg/h, holding burns 10 kg/min, so maximum holding = 12 min. Five minutes have already been used, leaving 7 min. If approach fuel is also required before the diversion, the margin is smaller. Therefore the crew cannot wait 15 min for the forecast improvement, and should not plan another approach unless it can be completed and the diversion started with 680 kg remaining.
2. The destination reports freezing rain at -2°C, and the NDB approach has MDA 720 ft and visibility 1,600 m. The current visibility is 2,500 m but TEMPO 1,500 m. Should the crew attempt the approach, hold, or divert?
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The current visibility is above the 1,600 m minima, but the TEMPO 1,500 m is below minima and the weather is deteriorating. More importantly, freezing rain at a surface temperature of -2°C means supercooled water and a high risk of severe airframe icing on the approach and missed approach. The aircraft may not be able to hold or climb away cleanly if ice accretion is rapid. With only 7 min of holding fuel left and an alternate 35 min away, the safe decision is to divert now, not to attempt the approach. A diversion preserves the alternate and final reserve; an approach followed by a missed approach in icing could leave insufficient fuel.
3. Explain the icing threat from freezing rain and the immediate actions if the crew encounters severe ice accretion.
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Freezing rain contains supercooled droplets that freeze on impact. It can produce clear or mixed ice rapidly, increase drag and weight, reduce lift and stall margin, block pitot/static sources, and cause engine inlet or carburettor icing. The crew should turn on all available anti-ice/de-ice, including pitot heat, propeller heat and engine inlet heat, increase speed if the AFM permits, avoid abrupt manoeuvres, and leave the freezing-rain area. A climb may reach above-freezing air or drier air above the frontal inversion; a descent may be colder but still in freezing rain, so it is not automatically better. If the aircraft cannot maintain speed or altitude, declare an emergency, advise ATC and divert. Report the icing to ATC so other aircraft are warned.