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Three other recent cases are known wherein the wings of similar models failed at the same location. However, none of these was in scheduled airline operation and they seemed under circumstances which would not be expected in such operation. In at least one of these cases the speed of the airplane at which failure occurred was in excess of normal. In each instance the pilot was able to control the airplane sufficiently to effect a safe landing. From these experiences and from analysis it appears that such failure of the structure in this type aircraft may result either from air loads occasioned in encouraging gusts of extraordinary intensity, or from violent use of the controls. In one of these three instances, the stabilizer also sustained slight damage, insufficient however to render the surfaces inoperative. This damage was evidenced by wrinkling of the bottom skin along a line almost identical to the diagonal line of failure found on the stabilizer of the subject aircraft. This is considered quite significant inasmuch as it identifies this type failure with air loads rather than with impact loads, thus supporting further the contention of stabilizer failure in the air. It also indicates that the line of least strength extends from the trailing edge of the stabilizer at its root to the leading edge near the tip.
In studying the failures in the horizontal surfaces, it appears much more probable that the initial failure, or failures, occurred in the stabilizer, and that the failures in the right elevator were the result thereof. As described previously, both sides of the stabilizer sustained breaks running approximately chordwise and located adjacent to the fuselage. The break on the right side of the stabilizer was more pronounced and this side also sustained an additional major fracture running more or less diagonally, starting at the inboard portion of the rear spar and ending at a point outboard near the leading edge. The appearance of the two described breaks on the right side of the stabilizer is consistent with the assumption that the stabilizer first failed along the diagonal fracture in the downward direction, and that a subsequent failure occurred in the upward direction along the chordwise fracture. Such a sequence of events can be expected during a pull-up[1] maneuver, sufficiently severe to impose loads in excess of the design.
The failure of both the left wing tip and the right stabilizer are of a character consistent with air loads imposed during a severe pull-up. The coincidence of two separate elements of the airplane's structure failing at the same time is unusual, therefore consideration was given to the possibility of the failure of one of them causing the failure of the other. However, analysis of the probable sequence of events in such a case seems to preclude that supposition. It is difficult to imagine how a failure of one, or even both wing tips, in itself could cause subsequent failure of any part of the horizontal tail surfaces, since the loss of the wing tip area would not be expected to have any appreciable tendency to alter the longitudinal balance of forces of the airplane. It is also highly improbable that the failure of the horizontal surfaces could have influenced to any marked degree the failure of the wing tips. The diagonal fracture of the right stabilizer apparently resulted from maneuvering down loads on the outboard portion of the elevator and the rear portion of the stabilizer near the tip. Consequently the distortion of the failed stabilizer would have the effect of an increased angle of
- ↑ A "pull-up", as used herein, refers to the maneuver of the aircraft, regardless of whether it is the result of operation of the controls by the pilot, or whether it is caused by some external forces acting on the aircraft.
