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Science Briefing

Snowfall, Visibility, and Airline Crashes

Some long-held beliefs about visibility and aviation safety may have to be revised, thanks to a recent report from the Research Applications Program (RAP). Common sense holds that the lower the visibility, the more risk of an accident for a given flight. But this study, conducted by Roy Rasmussen, Jeff Cole, and Kevin Knight (RAP) and R. K. Moore and Murray Kuperman (United Airlines) with the support of the Federal Aviation Administration, contradicts that view. Looking at five major icing-related crashes over the past 16 years (including the Stapleton International Airport disaster of 15 November 1987 and the Washington National Airport crash of 13 January 1982), the RAP/United team found generally similar weather conditions: temperatures just below freezing, winds between 10 and 15 miles per hour, and snowfall. But visibility ranged dramatically, from 1/4 mile to 2 miles.

The most significant index turned out to be the snowfall rate in equivalent water content. This was between 0.08 and 0.10 inches per hour in each case, despite the wide range of visibility. Snowflake density seems to make the difference, according to Rasmussen and colleagues. Large, fluffy flakes obscure visibility much more than dense, compact flakes, even when the amount of water deposited is the same. Thus, pilots and controllers may be misled by scenarios where visibility remains fairly good, yet dense, heavy snow is accumulating on wings. "An accumulation of as little as 0.03 inch of ice on the upper wing surface can result in a 25% loss of lift and increase in drag during takeoff rotation," says Rasmussen.

The RAP/United team also found an exacerbation of the problem due to aircraft wing geometry. Planes typically taxi downwind to the takeoff runway in order to take off into the wind. When rain or snow is falling, any wind during the taxi to takeoff will blow the particles more directly onto the upper surface of the wing, which is typically angled around 10 degrees from true horizontal. The team found that the accumulation rate of snow on a wing can nearly double if a plane is pointed in the same direction as a 1015 mph wind while taxiing to the takeoff runway.

In September, Roy went to Toronto, Ontario, Canada, to brief the Society of Automotive Engineers' Ground De-Icing Subcommittee on Holdover Time. This month, he presents his findings at the American Meteorological Society's 75th annual meeting in Dallas, Texas, and at the 33rd Aerospace Sciences Meeting and Exhibit in Reno, Nevada. Major airlines already have been notified of the results and encouraged not to rely on visibility as an index of snowfall rate or icing threat. USAir and United Airlines are now implementing some of this information into their winter weather training materials.

Location       Date, time  Precip. rate       Visibility

Newark, NJ     11/27/78    0.095 inches/hour  0.5 mile
(Newark)       11:50 a.m.

Boston, MA     2/18/80     0.08 in/hr         2.0 miles
(Logan)        2:08 p.m.

Washington, DC 1/13/82     0.09 in/hr         0.25 mile
(National)     4:00 p.m.

Denver, CO     11/15/87    0.10 in/hr         0.5 mile
(Stapleton)    2:15 p.m.

Flushing, NY   3/22/92    0.10 in/hr          0.75 mile
(LaGuardia)    9:35 p.m.

The table above shows the location of each of the five icing-related airline accidents studied by RAP, the date and time (local standard) of each accident, the average liquid-equivalent precipitation rate during the hour of the accident, and the most recent visibility report preceding each accident. A more complete table is included in the paper by Rasmussen et al., "Meteorological Conditions Associated with Ground De-Icing Accidents and the Effects of Wind on Snowfall Accumulation on Aircraft."

Commerce Bronze Medal Goes to NOAA Staff at NCAR

Three scientists housed in the Mesoscale and Microscale Meteorology Division (MMM) earned NOAA's highest honor this past November. Dave Jorgenson, Tom Matejka, and Brad Smull received the U.S. Department of Commerce Bronze Medal for their work in planning and executing flight strategies for TOGA COARE, the Tropical Ocean and Global Atmosphere Program's Coupled Ocean-Atmosphere Response Experiment. Bronze medals were awarded to around 120 Commerce staff in all during a ceremony at Greenbelt, Maryland.

Dave, Tom, and Brad are part of an observational group from NOAA's National Severe Storms Laboratory that took up residence in FL3 two years ago, allowing for easy collaboration with MMM, while other NSSL staff went to Norman, Oklahoma. Their work for TOGA COARE involved "both design and participation," according to Tom. The team came up with innovative ways to use the dual-Doppler capacities of the NOAA P-3 aircraft and the NCAR Electra Doppler radar (ELDORA), which debuted in TOGA COARE. "In some cases we combined the radar data into four- and six-beam configurations, which led to many more analysis possibilities. We also integrated the Doppler data with other weather data. It was a ton of work."

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Edited by Bob Henson, bhenson@ucar.edu
Last revised: Wed Mar 29 12:32:53 MST 2000