Page:CAB Accident Report, United Airlines Flight 21.pdf/53

 The increase in weight added by the weight of the ice itself is of relatively little importance. The important effect of the accumulation of ice is aerodynamic. Since the flight characteristics of the wing are very sensitive to the precise contour of the wing section, the accumulation of ice may be expected to have an does have pronounced effects.

Although pilots are not in agreement as to the degree of effect of ice upon the performance of the aircraft, they all agree that ice does raise the stalling speed. Wind tunnel tests under simulated icing conditions confirm this conclusion, which also follows from a consideration of the basic theory of wing action. The stall is not accompanied by the usual visual or mechanical warnings and may occur at an attitude which is not expected by the pilot. Tail buffeting is often decreased or entirely eliminated. The longitudinal stability of the ship is not ordinarily affected by ice, nor does the use of flaps in icing conditions appear to affect the stall characteristics adversely. The drag of the airplane is increased and the maximum lift decreased. The rate and angle of descent of the aircraft in a glide is also increased.

Several pilots who landed at Chicago on the afternoon of the crash after remaining in the icing conditions for various periods, reported an increase in the landing or stalling speed and increased drag of the aircraft, requiring the use of considerable power. The