Weather Support to Deicing Decision Making

The FAA and NCAR are working to solve the Aircraft Ground Icing Problem.

The accumulation of ice on aircraft prior to take off has long been recognized as one of the most significant safety hazards affecting the aviation industry today. As little as 0.08 mm of ice on a wing surface can increase drag and reduce airplane lift by 25%. Acutely aware of the impacts these icing hazards can have on aviation, the Federal Aviation Administration (FAA) began supporting ground de-icing research at the National Center for Atmospheric Research (NCAR)* in 1991. As a direct result of this FAA program, scientists at the Research Applications Program (the principal division of NCAR responsible for aviation weather projects) have developed a state-of-the art, integrated display system that depicts accurate, real-time nowcasts of snowfall rate, plus current temperature, humidity, wind speed and direction.

NCAR's Weather Support to De-Icing Decision Making System requires minimal training to operate, and no special meteorological knowledge to interpret. The display provides a maximum amount of weather information at a glance.

Who benefits from the WSDDM system

Designed specifically for airport decision-makers, graphic displays are strategically located at airline station control, dispatch, and deicing facilities, airline and city snow desks, and FAA air traffic manager positions. The snowfall and weather information are used by ground personnel conducting aircraft de-icing operations, airline station control managers coordinating flights, airport managers coordinating runway plowing activities, and air traffic controllers involved in gate-hold program planning. The information allows decision makers to anticipate both the onset and termination of snow at the airport and surrounding regions.

WSDDM's Data Sources

The principal sources of data for the system are regional area Doppler radars (National Weather Service WSR-88Ds and FAA Terminal Doppler Weather Radars), surface weather stations, and snow gauges situated within the terminal area which accurately measure the amount of water in the snow (i.e., the melted liquid-equivalent snowfall rate).

The Problem: Visibility is a poor measure of the Danger

Research indicates that the icing hazard for aircraft directly corresponds to the amount of water in the snow, rather than visibility. It is the latter that has traditionally been used to determine de-icing and take off decisions. Results from field tests of de-icing fluids have identified the liquid-equivalent snowfall rate as the most important factor determining the holdover time (time until a fluid fails to protect against further ice build-up).

Aircraft ground icing danger is found to be greatest when the temperature is between 25 and 31 degrees Fahrenheit, and the wind speed is about 9 to 15 mph. After studying the weighing snowgauge data for five major take off crashes in the US attributable to inadequate de-icing/anti-icing prior to take off, NCAR scientists determined that all five accidents occurred when the liquid-equivalent precipitation rate reached the danger zone of a minimum of 0.08 inches/hour.

Analyses also indicated that visibility at the time of the accidents was suggestive of only light to moderate snow, indicating that the current National Weather Service snowfall rate categories, which are based on visibility alone, are not always adequate for effective ground de-icing operations. Since the de-icing fluid depends critically on liquid-equivalent snowfall rate, visibility alone can give pilots and ground crews a false sense of security.

The Solution: A Weather Display Specifically Tailored to Airport Operations

NCAR's integrated display consists of one large plan view showing radar reflectivity of the storm in the main window. Geographic references, such as the terminal runways, taxiways, and airport concourses are overlaid on top of the gridded data. Yellow and green storm motion vectors are shown overlaid on the radar reflectivity of the storm, and indicate the expected 30 minute motion of the snowbands. Smaller windows on the right side of the same screen show current surface weather conditions from National Weather Service ASOS stations in the surrounding region in a simple, easy to read decoded form, WSDDM snowgauge and surface weather data updated every minute at the major airports, a time line graph of liquid equivalent snow intensity, temperature, wind speed and direction, and humidity for the last two hours updated every minute from specific locations within the terminal area. Also shown in the lower right is a 30 minute forecast of snowfall intensity (light, moderate, heavy) based on a nowcast of snowband motion and a real-time calibration between the snow intensity and radar return.

Snow and rain bands passing over LaGuardia, John F. Kennedy International, and Newark International airports are depicted.

How the Information is Used