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Celsius, wind 221° at 39 knots, light icing at 3,000 feet to 4,000 feet during climb. At 0905, a Mitsubishi MU2 at 7,000 feet MSL reported during climb out the sky was overcast at 900 feet with the tops of the overcast at 5,000 feet, light icing 2,300 feet to 3,300 feet during climb.
It was Christmas Eve. The airplane had a broken alternator switch, and the pilot’s final flight was an attempt a hop to nearby Jonesboro, Arkansas to have the switch replaced before flying to meet up with his family for the holiday. Weather near the depar- ture airport was 200 overcast, visibility 1.5 miles, and +1°C surface tempera- ture. Jonesboro (KJBR) was reporting 700 overcast, visibility 10 miles, with a 15-knot wind, +1°C surface temper- ature, and rapidly falling barometric pressure. The pilot may have thought he could quickly climb through the ice (after all, PIREPs called it “light”) and into an inversion above the clouds, then descend rapidly through the clouds in the approach to his destination. The holi- day may have increased his perceived stress to make the flight despite the adverse conditions.
Many Twin & Turbine readers’ aircraft are certified for flight in icing conditions – so-called “known ice” approval. But in what conditions exactly does “known ice” approval permit you to safely operate? Most pilots don’t know that ice certifica- tion provides a relatively small amount of ice protection. When is ice accumulation too much for even a known-ice airplane?
FAA certification for flight in icing conditions requires that the airplane’s ice protection systems be adequate to prevent or remove accumulations of ice in one of two conditions: continuous exposure and intermittent exposure. Known-ice airplanes are permitted to re- main in continuous icing conditions only in stratus clouds when water droplets are no more than 40 microns in diameter. That’s 0.0019 inches (0.05 mm). Even then known-ice certification assumes the pilot will exit icing conditions before traveling 17.4 nautical miles – any more exposure than that and the accumula- tion may exceed the system’s ability to remove accumulated ice.
In cumulus clouds, only very short and intermittent ice exposures are approved. The maximum droplet size un-
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Conditon
Cloud Type
Maximum Droplet Size
Maximum Exposure
Microns
Inches
Millimeters
Continuous maximum
Stratus
40
0.0019
0.05
17.4 nm
Intermittent maximum
Cumulus
50
0.0020
0.05
2.6 nm
Icing Certi cation Maximum Exposure Criteria (FAA).
Cross-section of the icing exposure. The pilot would have to climb through nearly 5,000 feet of potential icing conditions to get on top and into the warmer air above (inversion). A typical climb speeds is at least 2.5 minutes of continuous exposure, covering approximately six miles of horizontal distance – all in SLD conditions even “known ice” airplanes are not certi ed to handle.
Conventional wisdom is that if you encounter freezing rain, your escape path is to climb. In classic freezing rain scenarios, however, the moisture above a region of freezing rain often consists of supercooled liquid droplets – just as dangerous, and just as outside-the-approved- envelope as the freezing rain you’re trying to escape.
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