Forecasters Guide Pilots Into And Out Of Ice

February 05, 1997

BOULDER--As investigators seek clues to the January 9 crash of Comair Flight 3272 near Detroit Metropolitan Airport, atmospheric researchers are launching a new experiment to study one possible culprit in the accident: large-droplet icing. In the experiment, which is part of ongoing research funded by the Federal Aviation Administration, scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, are testing innovative forecasting methods to help pilots avoid dangerous in-flight icing conditions. The NCAR team is collaborating with colleagues at the National Aeronautics and Space Administration (NASA) Lewis Research Center in Cleveland, Ohio, and the National Weather Service's Aviation Weather Center (AWC) in Kansas City, Missouri. NCAR's primary sponsor is the National Science Foundation.

From January 13 through March 20, NASA-Lewis is flying a Twin Otter turboprop airplane equipped with measurement probes straight into treacherous icing zones identified by the NCAR team. NCAR scientists Ben Bernstein, Frank McDonough, and Marcia Politovich want to know whether their methods for forecasting these danger zones match up with the actual conditions pilots encounter in flight. NASA wants flight information to compare real-world ice shapes with those occurring in their experimental wind tunnel in Cleveland. "What's new and unique about this experiment is that it's specifically designed for large-droplet icing," says Politovich.

How Does Large-droplet Icing Bring Down a Plane?

The crash in 1994 of American Eagle Flight 4184 near Roselawn, Indiana, drew attention to the problem of large-droplet in-flight icing, especially for smaller aircraft. Large aircraft usually climb above dangerous conditions quickly. Smaller planes, which typically fly at lower altitudes, are at greater risk. Researchers studying the atmospheric conditions at Roselawn determined that relatively warm temperatures at cloud top (-5 degrees Celsius) may have combined with certain wind conditions to produce large drizzle drops of supercooled water (water that is still liquid even though its temperature is below zero degrees C). Supercooled droplets usually freeze when they hit the front of an airplane wing, and deicing devices on the wing are designed to remove them. However, some large droplets can flow back over the wing before freezing, or hit the wing behind the deicing equipment, creating crusty ridges of ice that can disrupt airflow and destabilize some aircraft.

The Experiment

The NCAR icing forecast team uses weather observations and output from numerical weather forecast models, combined with radar and satellite data, to target large-droplet icing conditions. The Twin Otter then heads into that area, taking precise measurements of the atmospheric conditions and the plane's response, as well as photographs and video images of any ice that forms. Since even experienced research pilots need direction out of dangerous situations, the NCAR forecasts include escape routes.

Bernstein and McDonough are also launching weather balloons in the Cleveland area from NCAR's instrumented weather van. The balloons carry sensing packages designed and built by Atek, Inc., of Boulder. The Atek sensors convey cloud structure data by measuring supercooled liquid as the balloons ascend through clouds.

Last year, at the request of the FAA, Bernstein created an algorithm, or mathematical problem-solving procedure, to automate freezing-drizzle advisories for areas smaller than those covered in general icing advisories. Due in large part to cooperative research by the AWC and NCAR, the areas of the advisories have become more specific over the past few years.

The research flights provide a good test of the new algorithm, which Bernstein calls the "stovepipe" because it uses data from observations on the ground to characterize what's happening in a stovepipe-shaped column of air at higher elevations. The experiment is also testing a new algorithm, developed by J. Vivekanandan of NCAR, that detects supercooled water droplets in cloud tops by examining infrared and visible-light readings from the GOES-8 satellite.

NASA-Lewis scientists will use flight data to improve icing simulation tools such as their icing research tunnel and their computer code for ice accretion, LEWICE. Working with AWC forecasters, the NCAR team will use the experiment's results to streamline pilot weather advisories issued at Kansas City.

Another step will be instructional materials for operational forecasters and pilots. Training modules, which include interactive CD-ROMs and Internet access, are being developed by the Cooperative Program for Operational Meteorology, Education and Training (COMET), a program of the University Corporation for Atmospheric Research. The modules will reach forecasters and pilots by mid-1998.

NCAR's icing research is sponsored by the National Science Foundation through an interagency agreement in response to requirements and funding by the Federal Aviation Administration's Aviation Weather Research Program. NCAR is managed by the University Corporation for Atmospheric Research.

-end-


National Center for Atmospheric Research/University Corporation for Atmospheric Research

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