Page:Aviation Accident Report, Western Air Lines Flight 1.pdf/14

 expected under reasonably unfavorable flight conditions, including the loads due both to gusts and to control forces. It is possible, however, for the aircraft under abnormal circumstances to be subjected to a combination of high loads, as a result of which stresses of unusual severity are developed.

The air load distribution over the stabilizer and elevator is dependent not only upon the prevailing flight condition, but also, to some extendextent [sic], upon the shape of the surfaces and their location with respect to other parts of the airplane. As a consequence, even for the some flight condition, the air load distribution will vary with different aircraft. For design purposes, simplified distributions are conservatively assumed. It is conceivable, however, that the assumed distribution may not be conservative for every airplane and every possible flight condition. Unforeseen circumstances may result in a variation of load distribution on the surfaces, sufficient to reduce the apparent margin of safety by a noticeable amount without increasing the total load beyond that for which the structure was designed. The location of the diagonal failure in the stabilizer indicates that a considerable down load was acting near the tip of the elevator, or more probably, on the outboard portion of the stabilizer along the rear spar.

Whether the stresses which caused the failure of the horizontal surfaces were due to "high speed buffeting", a coincidence of high loads, an unusual air load distribution, or combination of any of these, the Board is not able to state definitely at the present time. In any case, it seems apparent that the stabilizer failed under flight conditions which cannot be expected to occur except in very isolated cases. A study of this problem is being continued by the Board with the hope of obtaining more definite information.

The testimony of Lt. Gardner regarding the events which interrupted normal flight of Western #1 is in close agreement with other facts disclosed during the investigation. His description of the violent pull-up is especially valuable evidence because it confirms the conditions existing at the time the wings and the horizontal surfaces failed. Obviously, the important question is the reason for the pull—up, which, in the opinion of Lt. Gardner, was abrupt enough to develop loads in excess of 4G. The airplane either entered this maneuver as a result of some action beyond the control of the crew, or the pull-updue directly to the operation of the flight controls by the crew.

All detailed examination of the control system did not indicate that it had been malfunctioning at any time during the flight prior to the accident. Nothing was found which would lead to the belief that there had been, from within the aircraft, any interference with its normal operation. There were no signs in the wreckage of collision with a bird or with any other foreign object.

Investigation of the records of civil and military traffic control agencies revealed no evidence of any other aircraft, or free or captive balloons in the vicinity of Fairfield. Despite the fact that a diligent search revealed no such evidence, the Board nevertheless is fully cognizant of the possibility that there might have been an unrecorded aircraft, derelict balloon, or perhaps a large bird, the collision with which the pilot was seeking to avoid by the pull-up.