Page:Aviation Accident Report, Western Air Lines Flight 1.pdf/13

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An analysis of the weather prevailing in the vicinity of Fairfield at the time of the accident does not indicate conditions conducive to turbulence. Except for local ground fog near Salt Lake City, the ceiling and visibility were unlimited along the flight path. The air mass was very stable and precludes any probability of free convective currents. Only mild mechanical turbulence could have resulted from the rough terrain as the wind velocity was very light up to the height of the surrounding mountains. There is no evidence of wind shear turbulence. Considering all of the meteorological factors, there appears to be no reason to believe that the airplane could have been subjected even to a single isolated gust of sufficient severity to cause damage to the structure.

The problem of forced vibrations of the tail surfaces, similar in origin to the phenomenon commonly termed "buffeting", was given careful study. Reference was made to the British Report on the notable accident at Meapham, England (July, 1930), and to the various subsequent technical studies of that case. According to these studies, "buffeting", which occurs on most airplanes near the stalled attitude and results in a highly disturbed flow of air aft of the wings' trialing edge, may be of such magnitude as to impose appreciable vibratory deflections on the tail surfaces.

The DC3 is known to develop normal "buffeting" when the airplane is placed in a nearly stalled attitude. It has also been established that under certain conditions this "buffeting" may become quite pronounced. However, in no case known to the board have the vibrations been of sufficient magnitude to cause serious damage to the aircraft.

The normal "buffeting" condition, being directly connected with the airplane's stalled attitude, occurs at comparatively slow speeds, and therefore the vibrations of the tail are confined to reasonable small amplitudes which, in turn, do not impose unduly severe stresses on the structure. Furthermore, at slow speeds the usual maneuvering loads on the tail are relatively small, so that the combination of the two can be expected to be well within the strength limits of the structure. However, if an airplane while flying at, on near, its cruising speed is suddenly placed in a stalled attitude, the "buffeting" might be so accentuated as to introduce stresses in the tail structure, which when acting simultaneously with the maneuvering stresses due to gust, or pull-up, would cause failure. Such "high speed buffeting" is not necessarily an inherent feature of any particular model airplane, but may possibly occur in isolated cases under an unusual combination of flight conditions. Such conditions could have existed at a moment of pull-up during the subject accident. The fact that some vibration was present during the pull-up is supported by the testimony of Lt. Gardner, who described the action of the airplane as, ". . . . beginning to shake a little bit, . . . ."

If the unusually high stresses on the horizontal tall surfaces during the pull-up were not due to a so-called "high speed buffeting" condition, it is still possible that some other combination of air loads existed which resulted in a condition severe enough to cause failure of the stabilizer. The horizontal tail structure is designed to withstand the air loads that would be