Airplane Icing

When ice builds up on an aircraft's wings, it increases drag, decreases lift, and can become a significant hazard. Icing has the potential to slow a plane in flight or even cause it to lose altitude. If ice forms on an aircraft on the ground, it can interfere with takeoff.

In-flight icing has caused a number of fatal accidents, including the 1959 crash that took the lives of rock 'n' roll legends Buddy Holly, Ritchie Valens, and The Big Bopper (J.P. Richardson). In 1994, an American Eagle ATR-72 went into a high-speed dive and crashed near Roselawn , Indiana , killing all 68 people on board. While the plane circled for a half hour waiting to land in Chicago , ice had formed on the wings, causing the aircraft to plummet out of control.

Pilots can encounter icing in any season, anywhere in the country, at altitudes up to 18,000 feet, and sometimes higher. Small aircraft, including commuter planes, are most vulnerable because they fly at lower altitudes, where ice is more prevalent. They also lack mechanisms common on jet aircraft that prevent ice buildup by heating the front edges of wings.

Icing can occur in clouds any time that the temperature drops below freezing and supercooled droplets build up on an airplane and freeze. (Supercooled droplets are still liquid even though the temperature is below 32°F, or 0°C.)

Cancellations and delays due to icy weather can cost airlines millions of dollars in a single day. For example, on March 20, 2000 , icing conditions at Denver International Airport forced United Airlines to cancel 159 outbound and 140 inbound flights.

To reduce this hazard, scientists have worked toward better detection of potential icing areas and improved computer forecasts. In 2002, airlines got a new tool, called CIP (Current Icing Potential). Developed by NCAR, the system provides an online display of high-precision maps and plots, updated hourly, identifying areas of potential aircraft icing produced by cloud droplets, freezing rain, and drizzle. The online display is derived from surface observations, numerical models, satellite and radar data, and pilot reports. NCAR is developing a companion tool to provide potential icing information up to 12 hours ahead. Based solely on numerical model output, Forecast Icing Potential, or FIP, is already available on the CIP Web site.

Another system being developed at NCAR may help spy out water droplets in clouds that cause icing. Finding these cloud water droplets has long posed a challenge. The droplets are typically 50 microns or less in diameter, or just one-tenth the size of raindrops. Existing radar often cannot detect the droplets (which can adhere to a plane and freeze) if they are surrounded by larger raindrops or snow. Even if small cloud particles are actually detected, a radar signal cannot indicate whether they are water droplets or ice crystals. The new system, known as S-Polka , combines two radars that use different wavelengths to pinpoint the tiny water droplets that are difficult to distinguish using either radar alone.

NCAR researchers are also looking into efficient ways to prevent ice buildup on aircraft waiting for takeoff . As little as 0.8 millimeter of ice on the upper wing surface increases drag and reduces airplane lift by 25%. With deicing fluids ranging from $2 to $4 a gallon, battling ice buildup can cost airlines tens of thousands of dollars on a single snowy day, on top of the expense of flight cancellations and delays.

Research at NCAR has found that the potential of snow to form ice on an airplane's wings and fuselage corresponds to the amount of water in the snow rather than to visibility through falling snow, which has traditionally determined deicing and takeoff decisions. Large, dry snowflakes that hamper visibility are less of a threat than small, dense flakes holding more water. Accordingly, NCAR is working with other organizations to measure the actual liquid content of snow , as well as monitor other conditions, such as wind and humidity, that can spur icing.

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