THE AFFECTS OF ICING ON AIRCRAFT ACCIDENTS

The Affects Of Icing On Aircraft Accidents

The Affects Of Icing On Aircraft Accidents

Introduction

For years, airframe icing has been recognized as a significant aviation hazard. Icing encounters can lead to increased aerodynamic drag and weight, along with a reduction in lift and thrust. Together, these factors result in a higher stall speed and degradation in overall aircraft performance. To maintain altitude and counter the effects of drag during flight in icing conditions, the angle of attack is generally increased and power is applied to the engine(s). This can further expose unprotected regions of the aircraft to ice accretions. If exposure is prolonged, the aircraft will lose the ability to continue stable flight. Of equal importance is ice that accumulates on aircraft surfaces prior to takeoff. One of the first jet air transport category accidents linked to airframe icing occurred on December 27, 1968. A Douglas DC-9, operated by Ozark Air Lines, Inc., crashed shortly after takeoff. In this case, the aircraft suffered substantial performance penalties when it was subjected to freezing drizzle before takeoff. Considerable progress has been made in understanding the meteorological conditions associated with airframe icing (Sand et al. 1984; Cober et al. 1995; A substantial amount of interest and research into icing, with attention to supercooled large droplets (SLD), was generated when an ATR-72 was destroyed after it experienced an uncommanded departure from controlled flight and crashed near Roselawn, Indiana (1994). A ridge of ice that accreted behind the deice boots contributed to an unanticipated aileron hinge moment reversal and an abrupt loss of control. The accident raised awareness about the hazards of operating in SLD conditions, which are not accounted for in 14 Code of Federal Regulations (CFR) Part 25, Appendix C. Because supercooled large droplets can run back and freeze on surfaces behind an airplane's deicing boots, it is extremely hazardous. In recent years, icing research has translated into applied technologies aimed at diagnosing and forecasting icing hazards for both ground and in-flight aviation operations (McDonough and Bernstein 1999; Rasmussen et al. 2001; McDonough et al. 2004). Continued development and improvement of such technologies, along with training initiatives, will aid in reducing the number of icing related accidents.

Kinds of Ice and Their Effects on Flight

Structural ice is the stuff that sticks to the outside of the airplane. It is described as rime, clear (sometimes called glaze), or mixed. • Rime ice has a rough, milky white appearance, and generally follows the contours of the surface. Much of it can be removed by deice systems or prevented by anti-ice. • Clear (or glaze) ice is sometimes clear and smooth, but usually contains some air pockets that result in a lumpy, translucent appearance. The larger the accretion, the less glaze ice conforms to the shape of the wing; the shape is often characterized by the presence of upper and lower “horns.” Clear ice is denser, harder, and more transparent than rime ice, and is generally hard to ...