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der known ice protection is 0.002 inches (0.05 mm). Exposure to ice accumulation is limited to 2.6 nm – requiring an imme- diate exit from icing conditions to avoid overwhelming the protection system.
Any water droplet greater than 50 mi- crons in diameter is considered a “large droplet.” If the water is in a liquid state and the temperature is at or below freez- ing, it is a “supercooled” large droplet or SLD. By definition, no ice protection system is certified for flight:
• inanySLDconditions;
• in icing in stratus clouds for more than 17.4 nautical miles of continuous exposure;
• in cumulus clouds for more than 2.6 nautical miles of exposure.
To put this in perspective, the diameter of a human hair is 90 microns, or 0.070 mm – nearly 150 percent of the maxi- mum water droplet exposure limit. What this means is that at or below freezing temperatures if water droplets are large enough to be perceived as individual drops or “streams” on your windshield or
wings, they are too big for even known ice airplanes to be protected. No matter what you’re flying, you need to exit this type of visible moisture immediately.
CIP and FIP
One of the most significant advances in aviation weather in recent years is the introduction of Current Icing Potential (CIP) and Forecast Icing Potential (FIP) charts. CIPs and FIPs allow the pilot to see the probability ice may form at a given altitude along a planned route of flight, and if it does, the anticipated rate of ice accumulation. The FIP charts also plot areas of forecast SLD. Remember that the CIP and FIP are just a few years out of the experimental phase, and they are far from perfect in their predictions. However, the FIP, especially, is an out- standing addition to a pilot’s weather briefing when freezing temperatures may exist where that pilot wishes to fly.
To use the charts, look first at the Probability chart for the altitude(s) you might fly. Any probability of ice at all means you need to investigate further – although of course, a probability of 60 percent (for example) means ice is far more likely than if the probability is less than 25 percent, the reality that even less than 25 percent likelihood of ice means ice may indeed affect your flight. If the FIP indicates an icing possibility, next check the expected intensity chart. Any route and altitude that includes an ice probability greater than zero and more than possibly “light” ice accumulation means even the known-ice pilot must plan for an escape. If the forecast is for greater than moderate ice accumula- tion or anywhere the FIP predicts SLD, make other plans, whether to fly a dif- ferent route, fly at a different altitude, or to fly at a different time – because even known-ice certification is not valid there, and will not keep you and your passengers safe.
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January 2019
Thomas P. Turner is an ATP CFII/MEI, holds a master's Degree in Aviation Safety, and was the 2010 National FAA Safety Team Representative of the Year. Subscribe to Tom’s free FLYING LESSONS Weekly e-newsletter at www.mastery-flight-training. com.