A Review on the Impact of Icing on Aircraft Stability and Control.

Abstract

Several years of earlier research was conducted for the U.S. Air Force, related to the impact that warhead-induced damage had on the aeroelastic integrity of lifting surfaces and in turn the resulting upset of the complete aircraft. This prompted us to look at how similar aeroelastic events and aircraft upsets might be triggered by ice accumulation on specific parts of the aircraft. Although seldom studied, icing can also significantly impact the aircraft's aeroelastic stability, and hence the overall aircraft stability and control, and can finally result in irreversible upset events. In this latter context, classical flutter events of the lifting surfaces and controls can occur due to ice- induced mass unbalance or control hinge moments and force reversals. Also, a loss of control effectiveness caused by limit cycle oscillations of the controls and lifting surfaces may appear, due to significant time-dependent drag forces introduced by separated flow conditions caused by the ice accumulation. A review is presented in this article on the mechanisms that initiate these ice-induced upset events when considering the class of small general aviation aircraft. The review is based on literature and earlier experimental work performed at The University of Texas at Austin. Two commonly observed ice-induced aircraft stability and control upset scenarios were selected to investigate. The first upset scenario that is presented involves an elevator limit cycle oscillation and a resulting loss of elevator control effectiveness. The second upset is related to a violent wing rock or an unstable Dutch Roll event. Ice accumulation on aircraft, and the resulting aerodynamic unsteadiness and possible stability upsets of the entire aircraft, are undesired. Research on these upsets is of high importance, since ice protection systems are not able to com- pletely eliminate the presence of ice accumulation on aircraft. Structural ice formation on leading edges of wings and control surfaces initiate significant re- gions of unsteady flow (1). This change in the performance of the lifting surfaces can result in a major change in handling of the aircraft; the aircraft may stall at higher speeds, the stall angle of attack may decrease and irreversible upset events can be initiated. In the period of 1990 - 2000, a total of 3,230 aircraft accidents were recorded by the Air Safety Foundation. Twelve percent of those were related to icing (2). In one type of aircraft, most of the icing accidents occurred during the approach and landing phases (3, 4, 5), when the aircraft was flying at a higher angle of attack when compared to cruise flight. Studies on ice-related accidents of small general aviation aircraft have revealed that in many cases even the most experienced pilots have less than 5 to 8 minutes to escape the harmful icing conditions before their aircraft experiences violent upsets. This suggests that in cruise the accumulation of ice, and its effect on the stability of the aircraft, remain mostly unobserved. Upon changing the attitude of the aircraft, the formation of ice induces unsteady flow phenomena capable of upsetting the aircraft in a catastrophic manner.