Icing Still Poses Threat To Small Planes Despite Redesign Of Deicing 'Boot'

November 07, 1996

CHAMPAIGN, Ill. -- In severe conditions, the formation of ice on the wings of small, commuter aircraft can compromise the safety of passengers and crew, a University of Illinois researcher says.

"Certain types of icing conditions can result in the formation of a ridge of ice on the wings that can severely affect the pilot's ability to control the aircraft," said Michael Bragg, professor of aeronautical and astronautical engineering at the U. of I. "Some aircraft may not be adequately protected against this threat."

When ice forms on the wings, the airflow is disrupted, resulting in reduced pressure on the top of the ailerons, Bragg said. (Ailerons are the movable flaps mounted on the trailing edges of the wings that provide lateral control.) In a severe case, known as "aileron snatch," an aileron can be pulled swiftly upward, thereby yanking the controls from an unprepared pilot.

Bragg believes a form of aileron snatch caused the crash of an American Eagle ATR-72 commuter aircraft Oct. 31, 1994, near Roselawn, Ind., in which 64 passengers and four crew members died.

Bragg speculates the following scenario occurred, based in part on information from the flight-data recorder: "When the plane encountered large drops of freezing drizzle, a ridge of ice formed on the wings. Because the auto-pilot was engaged, the pilot was unaware of the changes occurring in the lateral control forces due to the accretion of ice. When the auto-pilot disengaged, the right aileron rotated up violently and the aircraft rolled rapidly to the right. As the pilot fought to regain control, the aircraft rolled further and crashed into a field."

Large jetliners use hot air from the engines to prevent ice from forming on the wings. "But smaller, propeller-driven aircraft lack the power required for these systems," Bragg said. "Instead, pilots of commuter aircraft rely upon inflatable rubber boots along the wings' leading edges to 'pop' off accumulated ice. In the case of the ATR-72 accident, because the ridge of ice formed aft of the rubber boot, inflating the boot had no effect."

Although the ATR-72 deicing boot has been redesigned and now extends farther back on the wings, this is only a partial solution, Bragg said. The potential accretion of ice remains a problem for other aircraft and other icing conditions.

Bragg hopes to minimize this risk by better understanding the effects of icing. In wind-tunnel experiments, Bragg and his students are examining aerodynamic problems caused by simulated ice accretions.

"In particular, we are trying to understand the phenomena of large droplet ice accretions and the effect they have on the aerodynamics and control of the aircraft," Bragg said. "By understanding the relationship between aircraft design and sensitivity to icing, we'll learn how to build safer airplanes."

Bragg presented his findings on the ATR-72 accident to the American Institute of Aeronautics and Astronautics earlier this year.

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University of Illinois at Urbana-Champaign

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