Page:CAB Accident Report, United Airlines Flight 21.pdf/56

 of the aileron control combined with a tendency of the airplane to wobble, without any tail buffeting being present. Inasmuch as Captain Thompson was primarily interested in finding out whether ice had the effect of greatly increasing the stalling speed, he did not care to carry the test through to a full stall, due to bad vision from the ice on his windshield and darkness.

Although pilots do not agree as to the effect of ice on the DC-3, it may be concluded that ice does raise the stalling speed to some unpredictable extent and that the effect cannot be stated in terms of the amount of ice alone, but is dependent upon both the amount of accumulation and its location on the wing.

Effect of Uneven Engine Response

Shortly after the accident, Captain Thompson, accompanied by representatives of the Board and of the Civil Aeronautics Administration, conducted several tests on a DC-3, one of which was to determine the performance characteristics of the ship without ice under stall conditions with power. With wheels up, three-quarter flap position and 400 feet per minute descent, the ship stalled at 68 miles per hour indicated air speed; with wheels, down, three-quarter flap position, 400 foot per minute descent, the stall developed at 64 miles per hour indicated air speed; with wheels down, three-quarter flap position, manifold pressure of 17 inches, the ship trimmed for normal approach and brought to the point of stall, the left throttle closed and the right throttle jammed open, a roll of approximately 45 degrees resulted, combined with a necessary uncontrollable yaw of 20 degrees and 300 to 500 feet required for recovery.