Aero-Tips!

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Aero-Tips 11.09.06

Freezing rain. Sleet. Ice pellets. All may be encountered when surface air temperatures plunge below freezing. All are extremely hazardous to aircraft.

Freezing Rain

Freezing rain occurs when air near the surface is below freezing, while air slightly higher is above freezing. This temperature inversion, if moist enough, can generate rain that falls as liquid from clouds in the warm air atop the inversion. As rain falls into freezing air it rapidly cools, enough that when it hits a surface that has a skin temperature below freezing (such as the surface, or an airplane) the droplet spreads on the cold surface and flash-freezes. A huge accumulation of heavy, aerodynamics-altering clear ice coats the surface (or the airplane). The rate of accumulation can be phenomenal. Best practice is simply to avoid flight in areas of reported or forecast freezing rain -- with extremely rare exceptions even "known ice" airplanes are not certified for flight in freezing rain.

Going up

If freezing rain results from a temperature inversion, it's logical that an escape path from unexpected freezing rain is to climb, if the airplane retains climb capability. Don't count on it, but if the airplane will climb get into warmer air at the very first sign of ice accumulation.

Going down

If the band of below-freezing temperatures is only a few hundred feet high above the surface, precipitation will likely remain as freezing rain all the way to the ground. If the freezing band is more than a few hundred feet thick, however, falling precipitation will eventually freeze into tiny chunks of ice even if it doesn't contact a surface. The result is sleet or, in aviation-speak, ice pellets. The presence (or forecast) of sleet or ice pellets implies that freezing rain exists only a few hundred feet higher up. Consequently it's wise to avoid flight in areas of reported or forecast sleet or ice pellets to avoid flying in dangerous freezing rain. Encounter ice pellets or sleet in flight and you should not attempt to climb out of it, because you'll enter the band of hazardous freezing rain before you can get to above-freezing air.

The figure shows the basic relationship between rain, freezing rain and ice pellets, and how they can form in cold fronts and warm fronts if the temperature on the "cold side" is below freezing.

Aero-tip of the day: Avoid flight near areas of reported or forecast freezing rain, sleet or ice pellets.

FMI: Aero-Tips