Page:CAB Accident Report, United Airlines Flight 21.pdf/51

 rolling motion which may be in either direction. The stall develops at the wing tip and there is little warning except for a slight "mushiness" of the controls. The slip stream effect delays the stall at the center section and also largely eliminates the tail buffeting and vibration present in the "power off" condition. In recovering, the plane usually loses a minimum of 500 feet of altitude. The speed at which the stall develops is variable depending upon many factors such as loading, flap position, and degree of bank. Pilots employed by air carriers using DC-3 equipment are given periodic flight checks which include both power on and power off stalls, so that all airline pilots using this equipment are familiar with its stall characteristics. United's procedure requires such checks of its captains every six months.

Effect of Ice

Since it has been definitely determined that some ice was present on the aircraft prior to the crash, the effect of ice accumulation on the stall characteristics must be considered. There have been few DC-3's intentionally stalled under icing conditions and the behavior of the airplane under these conditions must be largely deduced from general considerations. The results of wind tunnel tests, reports of pilots, and the general effect of ice on airplanes must all be considered in this regard.

No definite standards have been developed for describing the wide variety of types and degrees of accumulation of ice encountered under actual flight conditions. It is therefore difficult accurately to analyze the effect of various icing conditions. The accumulation of ice on aircraft depends upon atmospheric conditions which may be extremely variable. Ice may form under varying weather