Winter Departure With a Marginal Icing Layer
It is 0600 local time in January at an inland prairie airport (elevation 2,300 ft MSL). Surface temperature is -6°C with a dewpoint of -8°C. A weak warm front lies 80 nm south and is forecast to lift northward through the morning. The 0600 upper-air sounding shows a surface-based subfreezing layer to 6,000 ft, an isothermal layer from 6,000 to 9,000 ft at -2°C, and a moist layer from 5,000 to 11,000 ft. Above 11,000 ft the air is dry and the temperature falls to -14°C at 14,000 ft. Your aircraft is a turboprop with a freezing-level restriction and no known icing certification for sustained operations in supercooled large drops. The planned route climbs southeast through the frontal zone to a cruising altitude of 16,000 ft.
1. Assess the icing threat for the climb-out and identify the altitude band of greatest concern, with reasoning.
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The threat is real but layered. The surface is below freezing, so any cloud or precipitation encountered between the surface and 6,000 ft is already a potential icing environment, but the air below 5,000 ft is relatively dry (temperature-dewpoint spread of 2°C at the surface), so cloud is likely thin or broken there. The critical band is 6,000-9,000 ft, where the sounding is isothermal at about -2°C inside a moist layer. An isothermal layer is a stable layer, and stable layers in a moist environment concentrate supercooled water because vertical mixing is suppressed and droplets persist. Temperatures near -2°C are within the classic severe-icing range for stratiform cloud (roughly 0 to -10°C), and the moist depth of 5,000-11,000 ft means a deep, sustained cloud deck rather than a thin wisp. Above 11,000 ft the air dries and the temperature drops, so icing risk decreases sharply. Conclusion: expect moderate icing in stratiform cloud from about 6,000 to 10,000 ft, with the worst conditions near the top of the moist layer where the temperature is coldest while still near freezing. The climb should be planned to pass quickly through this band, and the crew should treat any rime accumulation as a signal to request a different altitude.
2. Explain why the approach of the warm front changes the risk during the morning, and what observation would most change your assessment.
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As the warm front lifts northward, the moist layer deepens and the freezing level rises. Two effects compete. First, the cloud deck thickens and precipitation may begin, increasing liquid water content and the depth of the icing layer, which raises risk. Second, the warming aloft pushes the isothermal layer toward and eventually above 0°C, which converts the band from a supercooled-water environment to a above-freezing environment and reduces icing risk at low levels while shifting the risk upward. The most informative observation would be a pilot report or a sounding update showing the actual top of the moist layer and the height of the 0°C isotherm. If a PIREP reports rime ice at 8,000 ft, the model is confirmed and the band is active. If instead the freezing level has already risen to 9,000 ft, the low-level risk collapses and the concern moves to the mid-levels. A METAR showing the temperature rising toward -2°C with saturation (dewpoint depression near zero) would also signal that the airport itself is entering the cloud and that fog or freezing fog may replace the icing aloft as the primary hazard.
3. State the operational decision you would make and justify it against the aircraft limitations.
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Because the aircraft lacks approval for sustained flight in supercooled large drops and has a freezing-level restriction, the conservative decision is to delay departure until either the frontal passage is complete or a route and altitude can be found that avoids the 6,000-10,000 ft moist band. Climbing through a known moderate-icing layer in an aircraft without known-icing certification is not justified when a delay of one to two hours may remove the hazard. If departure cannot be delayed, the plan should be to climb as rapidly as practicable through the band, request a block altitude above 11,000 ft where the air is dry, and have a diversion airport with above-freezing conditions available. The decision rests on three facts: the isothermal moist layer at -2°C, the absence of known-icing approval, and the presence of a lifting warm front that will change the profile within hours. The crew should also brief the possibility of freezing fog at the surface if the temperature stays below freezing while moisture increases.