Page 34 - January 15 Volume 19 Number 1

Air Temperature -12°C to -40°C
(
Supercooled Liquid Droplets (SLD)
Air Temperature 0°C to -12°C
temperatures exist from the surface, with no warm band of above- freezing air above the lowest layer. Instead, this is how freezing rain usually forms: Above a boundary defined roughly by the height where the temperature is at -12°C, small supercooled water droplets (SLD) are suspended in the atmosphere. These droplets collide with one another and fall into the lower levels, where the temperature is still below freezing, but closer to the freezing point. In this environment, larger supercooled droplets can form without themselves freezing. These large supercooled raindrops, upon striking surfaces chilled to below freezing by the cold air near the surface, flash-freeze onto those surfaces. This creates a thick and irregular coat of clear ice— freezing rain.
Why is it so vital to know this? Because everything we have been taught about freezing rain avoidance and escape is wrong,
32 • TWIN & TURBINE
Ground
Scott tells us, in 92% of all instances when freezing rain occurs. Since there is no band of above-freezing air overlying the freezing rain, flying at a higher altitude still exposes the airplane to SLD conditions—for which no aircraft is certificated. And trying to escape by climbing out of freezing rain conditions, hoping to melt off the ice accumulation, would only result in adding additional ice to the airframe.
The only workable strategy when freezing rain is reported, Scott tells us, is to avoid flight in clouds or precipitation anywhere near or above the freezing rain unless the outside air temperature is colder than -40°C. This applies to Known Ice airplanes as well as those not certificated or equipped for flight in icing conditions.
Many Twin & Turbine readers may never have been exposed to information such as was presented at the NBAA’s Single-Pilot Safety
Freezing Rain
Stand-Down. Yet, it’s this level of pilot education that makes the difference between the superb safety record of airlines and professionally crewed corporate aircraft, and the much less desirable accident history of business and personal IFR flying. IFR flying is a profession whether you pursue it as your livelihood or not. It takes significant continuing educational investment of time and, yes, money, to enjoy the benefits of personal aviation at an acceptable level of risk.
The 2015 NBAA Single-Pilot Safety Stand-Down will be held November 16 in Las Vegas. I hope to see you there. T&T
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JANUARY 2015