Staying on Top of the Freezing Level

Winter and airframe icing are like peanut butter and jelly; it’s hard to imagine one without the other.

Monday, April 10, 2023 at 09:46 AM ET

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Winter and airframe icing are like peanut butter and jelly; it’s hard to imagine one without the other. [Credit: iStock]

Key Takeaways:

Pilots must avoid using the standard lapse rate (2°C/1000 ft) to estimate the freezing level for airframe icing assessment, as this method is often inaccurate and can lead to dangerous misjudgments.
Actual atmospheric conditions frequently deviate from the standard lapse rate, exhibiting either a steeper lapse rate near the surface (Planetary Boundary Layer) or temperature inversions, which cause significant discrepancies in freezing level calculations.
For reliable airframe icing risk assessment, pilots should consult official and specialized weather resources, such as the Aviation Weather Center's freezing level forecasts (RAP model or G-AIRMETs), Skew-T diagrams, or vertical route profiles in aviation applications.

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Short of an erroneous forecast or calibration issue with your immersion thermometer, if you can remain below the lowest freezing level during your entire flight, there’s typically no chance for an encounter with airframe ice. Induction ice is certainly possible, but not airframe ice.

If you plan an altitude where the temperature aloft is zero degrees Celsius or less, airframe icing becomes exceedingly more likely while flying in visible moisture. Therefore, the freezing level is one key variable that you need to determine during your preflight analysis to better quantify your risk of airframe ice.