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Lessons learned from analysis of in-service icing incidents are described. The airfoil and wing design factors that define an aircraft's natural stall characteristics are explored, including the aerodynamic effects of contamination. Special attention is given to contamination in the form of “roughness” along wing leading edges typical of frost. In addition, the key aerodynamic effects of ground proximity and sideslip/crosswind during the take-off rotation are described. An empirical method, that can be used to predict a wing's sensitivity to wing leading edge roughness, is demonstrated. The paper explores the in-service differences of aircraft that incorporate “hard”, “supercritical” and “slatted” wings. The paper attempts to explain why the statistical evidence appears to favor the slatted wing for winter operations.

The Learjet 85 is a business jet with an unpowered manual elevator control and is designed for a maximum dive Mach number of 0.89. During the early design, it was found that the stick force required for a 1.5g pull-up from a dive would exceed the limit set by FAA regulations. A design improvement of the tailplane was initiated, using 2D and 3D Navier-Stokes CFD codes. It was discovered that a small amount of positive camber could reduce the elevator hinge moment for the same tail download at high Mach numbers. This was the result of the stabilizer forebody carrying more of the tail download and the elevator carrying less. Consequently, the elevator hinge-moment during recovery from a high-speed dive was lower than for the original tail. Horizontal tails are conventionally designed with zero or negative camber since a positive camber can have adverse effects on tail stall and drag.