Influence of Angle of Attack on Drag

The drag resulting from ice contamination is very sensitive to angle of attack. As AoA is increased, the drag rise can be overwhelming. This is a particularly pernicious effect, as it degrades airspeed, leading to greater angle of attack when the pilot attempts to maintain altitude. This is a very significant threat to aircraft operating near the service ceiling over high terrain; they have little excess power, and any attempts to maintain altitude result in an even greater angle of attack, rapidly increasing drag, furthering airspeed decay and, often quickly, leading to a stall and loss of control. Pilots should be prepared to surrender altitude early, as soon as it becomes clear that the airspeed decay cannot be managed. Flights over particularly high terrain should be planned accordingly, and not even attempted if ice is a likely threat. 

It is also a very insidious effect during the landing. Approximately one third of icing-related accidents and incidents are hard landings, with damage varying from none to serious. The important point during the landing approach is to recognize that the initial drag increase when the ice is accreted may seem small, or perhaps not even be noticed. But when the angle of attack is increased during the round-out and flare, the drag increases rapidly and quite substantially. This can best be managed with a shallow flare and delaying the power reduction, even to the point of landing with power. 

The drag rise from ice accreted at a lower angle of attack may be similar to that from ice accreted at a higher angle of attack, as shown in the top and bottom profiles at left. It is the change in angle of attack that leads to a potentially catastrophic rise in drag.