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.