Icing Risk in a Frontal Stratiform Cloud Deck
A flight plans to cruise at 9,000 ft for two hours. A warm front lies ahead, with its surface position 150 km south of the route and the frontal surface sloping at about 1:150. A temperature profile from the area shows 0 °C at 8,000 ft and −12 °C at 12,000 ft, with a deep layer of stratiform cloud between 6,000 and 13,000 ft ahead of the surface front. The aircraft is not certified for flight into known icing, but has a heated pitot and propeller. The crew observes that the cloud is smooth and argues that smooth cloud means little icing risk.
1. Estimate the height of the frontal surface at the route and determine whether the cruise level lies in the cloud and in the icing temperature band.
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Slope 1:150 means the frontal surface rises 1 m for every 150 m horizontal. At 150 km (150,000 m) from the surface front, the frontal surface height is 150,000/150 = 1,000 m, about 3,300 ft. The route is therefore above the frontal surface, in the warm air, not in the frontal cloud slab itself.
The cloud layer 6,000–13,000 ft ahead of the front contains temperatures from about +3 °C at 6,000 ft (using 0 °C at 8,000 ft and a lapse rate of 2 °C/1,000 ft) down to about −14 °C at 13,000 ft. The 0 to −12 °C band lies roughly 8,000–12,000 ft. At 9,000 ft the temperature is about −2 °C, inside the most hazardous icing band and inside the cloud layer.
So although the route is above the frontal surface, the aircraft is cruising in stratiform cloud at −2 °C, where supercooled water is most likely.
2. Evaluate the crew's 'smooth cloud means little icing' argument and state a decision.
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The argument is invalid. Smooth stratiform cloud often contains high liquid water content with small droplets, and it is precisely this type of cloud that produces the most persistent, widespread airframe icing. Smoothness indicates little turbulence, not little supercooled water. The temperature of about −2 °C at 9,000 ft is in the classic severe-icing band, and the deep frontal cloud gives a long exposure time - two hours of cruise.
Decision: the aircraft is not certified for known icing, so the flight must not be planned through this cloud at 9,000 ft. Options: cruise below the cloud layer (for example 5,000 ft, where the temperature is above freezing), route around the frontal cloud, or delay until the front passes. If icing is encountered inadvertently, the immediate actions are to change level (usually climb or descend to find a temperature outside the 0 to −12 °C band), turn back, and advise ATC. Pitot heat and propeller heat should be on before entering cloud.