BOULDER - Halloween weather has tricked, not treated, airport meteorologists the past two years in Denver. Heavy freezing drizzle--appearing to be harmless light drizzle--has cost airlines as much as $2 million in engine damage in a single storm as jets have waited for takeoff. Now Roy Rasmussen of the National Center for Atmospheric Research (NCAR) has developed a new system to identify the drizzle accurately. His research has enabled airlines to revise pilot training and on-ground procedures to avoid future damage. The new detection system will be installed this winter at Denver International Airport (DIA).
Rasmussen studied two cases of heavy freezing drizzle at DIA on October 31, 2002, and the same date in 2003. The two storms wreaked a total of $2.85 million in damage to 18 jet engines on United Airlines 737 aircraft. Trained meteorologists were on site throughout both events, but the freezing drizzle conditions were not accurately noted. In about half of all cases of freezing drizzle, the intensity is underreported, according to Rasmussen.
"Freezing drizzle is hard to see and its intensity is hard to estimate visually," says Rasmussen. "Often it goes undetected because the droplets are so small." The typical droplet diameter is about half of a millimeter, or half the thickness of a compact disk.
Rasmussen has worked with United Airlines to alert pilots of 737 aircraft on the hazard, and the airline has changed its procedures as a result of his research. Formerly, if an airport meteorologist observed heavy freezing drizzle, engines were revved close to flying speed (called an engine run-up) every 30 minutes to throw off ice.
"Now, if anyone says 'freezing drizzle,' they do engine run-ups every ten minutes," says Rasmussen. "Airline people are sensitized to the possibility that freezing drizzle can cause engine damage."
The real-time freezing-drizzle detection system developed by Rasmussen and colleagues will be part of Weather Support for Decision Making (WSDM), a system now at DIA that offers minute-by-minute weather reports tailored to aviation users. WSDM data are displayed in a color-coded, user-friendly format that can be easily read by pilots and other non-meteorologists. WSDM also provides data on snow and unfrozen rain.
Freezing rain falls right past an idling jet engine, Rasmussen explains, but freezing drizzle falls at a much slower rate, so it gets sucked into the engine. The droplets freeze on contact, and the resulting ice builds up on the engine's hub, or spinner. When the engine is revved up to takeoff speed, ice shards are thrown off the spinner into the rest of the engine.
In the two Denver storms, the major damage was to the delicate tips of the fan blades. These blades generate the lift that makes the plane fly, Rasmussen says. If they're damaged, the plane loses thrust, because the blades are not at the correct angle to produce the maximum thrust.
"It's not particularly dangerous," says Rasmussen, "but they have to repair the damage, and that's very costly."
The Federal Aviation Administration's Aviation Weather Research Program funded Rasmussen's research. NCAR's primary sponsor is the National Science Foundation.
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