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October 2004

Freezing drizzle: An aviation hazard that's no longer hard to see

Roy Rasmussen.

If you were out trick-or-treating in the Boulder-Denver area the last two years, you probably remember the weather: both Halloweens featured a nasty mixture of freezing drizzle and light snow. Those two storms cost United Airlines a total of $2.85 million in damage to 18 jet engines at Denver International Airport (DIA).

Aviation professionals have long known that heavy freezing drizzle can ruin jet engines; they even have procedures to prevent the damage. So what went wrong in the two Halloween cases? According to Roy Rasmussen (RAL, formerly RAP), the problem was not the weather itself but the fact that airport meteorologists either didn’t see the freezing drizzle at all or underestimated its intensity. Roy and his colleagues have created a method to give these and other aviation professionals an accurate reading of what’s falling—freezing drizzle or freezing rain—and how heavily it’s coming down.

In defense of the meteorologists who covered the two Halloween storms, Roy points out, “Freezing drizzle is hard to see. It kind of floats with the air in very small droplets [half a millimeter or less in diameter].” Aviation meteorologists currently don’t have an instrument that measures drizzle, so they must rely on their eyes to identify it and estimate its intensity by the effect it has on visibility. In the ’02 storm, the observer correctly reported mist and light snow but did not see the accompanying drizzle; in the ’03 storm, the observer recognized the freezing drizzle but reported it as light rather than heavy—and light drizzle is not a significant aviation threat. Roy notes that when freezing drizzle is estimated by visibility, it’s almost always rated light (96% of the time), but in reality it’s either moderate or heavy about half the time. “That’s causing a lot of trouble for the airlines,” he says.

In both of the Halloween storms, DIA ground personnel such as baggage handlers who had to be out in the weather recognized that it was heavy freezing drizzle, but it’s not their job to report it. “Pilots are trained to listen to the meteorologists,” says Roy. “If the official weather says light snow, it’s light snow to the pilot no matter what the ground personnel see. That’s not a bad thing, because most ground personnel aren’t experts on the weather; but this is a case where the estimation of drizzle intensity by visibility is inaccurate.”

Jet engine damage

The engine damage occurs while the jets are on the ground, waiting for takeoff. “Freezing rain falls right past a jet engine,” Roy explains, “but freezing drizzle falls at a much slower rate, so it gets sucked into the engine at idle speeds.” The droplets land on the engine’s hub, or spinner, and become ice. When the engine is revved up in preparation for takeoff, “that throws off the ice. It centrifuges off the spinner to the edge of the jet engines.”

The iced-up hub of a jet engine, the result of the aircraft's standing in heavy freezing drizzle. The white swirl is a design feature. (Photo courtesy United Airlines.)

In the Halloween storms, the major damage was to the delicate tips of the fan blades. “The blades generate the lift that makes the plane fly,” Roy says. “If they’re damaged, you lose thrust, because they’re not at the angle they were engineered to produce the maximum thrust. It’s probably not as dangerous as you might think, but they can’t use the [damaged] engines; they have to fix them. And that’s very costly.”

Better view of drizzle

Roy and his colleagues have created a long-term solution: a method using existing National Weather Service instruments to detect the presence and intensity of freezing drizzle in real time. The instrument is a rod that vibrates at very high frequency. When ice freezes onto the rod, the vibration frequency changes; drizzle changes the frequency less than rain does. The intensity of the rain or drizzle is measured by the amount of liquid falling, eliminating the faulty estimates of drizzle by visibility.

The method will become part of the Weather Support to Decision Making (WSDM) real-time winter nowcasting system in place at DIA. WSDM also supplies accurate estimates of snow and unfrozen rain, and the data are displayed in a color-coded, user-friendly format that can be easily read by pilots and other non-meteorologists.

But even if this Halloween brings more freezing drizzle and you have to fly in it, don’t worry. Last April, Roy trained the 30 pilots who fly United Airlines 737 jets. “The pilots needed to hear about this and realize that it could easily happen again,” he says. They’ve already changed their procedures as a result of his research. The old recommendation was to rev the engine almost to takeoff speed (called an engine run-up) every 30 minutes to shed any ice. “Now, if anybody says ‘freezing drizzle,’ they’re doing engine run-ups every 10 minutes. People are sensitized to the possibility that freezing drizzle causes engine damage.”

Roy emphasizes that there has not been an accident in the air as a result of freezing drizzle. “But that’s what we’re here for—to try to be proactive and figure out what’s going on before anything happens.”

• Carol Rasmussen


Also in this issue...

Hurricanes and climate change: Is there a connection?

Paul Swarztrauber looks back on 41 years at NCAR

Farewell to a well-loved tree

Digital Image Library now easily accessible

Delphi questions

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