Pilot Journal
Thursday, March 1, 2007

Severe Weather Flying

Dennis Newton’s book reviewed

severe weather flyingSevere weather. Who would ever think about flying in it, or around it? Yet a book about severe weather flying has been highly popular and successful for over 20 years, and is now in its third edition.
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Icing clouds. “If you don’t want ice, the first and most important thing to do is stay out of clouds with high liquid-water content. The primary cause of high liquid-water content is lifting of moist air.” Newton ticks off a list of cloud types—cumulus, stratus, stratocumulus and wave clouds—and details the specific icing hazard associated with each.

Ice forecasts. Several ice-related commuter airliner crashes in the 1990s led the FAA to turn its focus to improving ice-related flight forecasts. Freezing drizzle and freezing rain, in particular, were labeled “severe icing conditions”, and nearly every type of deice-equipped airplane was subject to an Airworthiness Directive that required its Pilots Operating Handbook be amended to prohibit flight in these conditions. Research and development of new and more usable ice reporting and forecasting models resulted in products like the Current Icing Potential and Forecast Icing Potential on the Aviation Digital Data Service Website. Progress, says Newton, “has been much better” than before, while in his opinion it is “still short of the ultimate goal of improving the quality and dissemination of icing weather information….”

Recognizing and avoiding ice in flight. “Avoiding ice, or getting out of it if you have been unsuccessful at avoiding it, really boils down to just two things. Either get to an area where the temperature is warm enough to melt the ice, or get out of the liquid parts of the clouds.” In addition to some caveats about temperatures aloft reports and the effect aircraft type and speed has on ice accumulations (and any resulting PIREP), Newton lists several rules for detecting and avoiding the liquid-water portion of clouds.

Effects of airframe ice. “A lot of data has been taken in icing [wind] tunnels and in flight testing,” reports Newton. “It is possible to give some ballpark numbers to point out the relative magnitude” of ice-related degradation. For instance, “the weight of ice does not, in itself, present a serious problem.” However, “even small buildups of ice on airfoil leading edges can decrease maximum lift coefficient by about 30 percent. Most of the damage to lift is done by the first accumulation…even a small buildup will significantly reduce the angle of attack at which an airfoil will stall.” Settling an argument about whether it’s better to keep the propeller(s) clean (for thrust) or the wings, “the propeller advocates lose the contest handily, according to the data.”

The legality of flight in ice. “Since about 1972, the FAA has been altogether prohibiting newly manufactured airplanes from flight into known icing conditions unless and until they have been tested and shown capable of safely operating in the icing conditions specified for transport category airplanes….” Earlier airplanes “were in many cases neither certified nor restricted” from flight in “known” ice. This topic is hotly debated (forgive the pun) and the whole concept of what constitutes “known ice” is under review by the FAA.


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