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attack of the horizontal tail surfaces. The resulting up loads acting together with damping forces might explain the failure in the up direction of the stabilizer at the fuselage. The latter failure would cause the horizontal surfaces to lose their effectiveness, thus upsetting the longitudinal static balance and inherent stability of the airplane. According to the loading records for the subject flight the center of gravity was located at 2412% of the MAC. In this condition it is highly improbable that loss of the tail surfaces would cause the development of additional up loads on the wings, but rather the reverse. The center of pressure on the wings being aft of the c. g. at all normal angles of attack, a diving rather than a stalling tendency may be expected. From the above discussion it is concluded that the wing tip and the horizontal tail surface failures occurred during the same pull-up maneuver (mentioned by Lt. Gardner), and were not directly dependent upon one another.
The design of the DC3 and similar model wings reveals that their strength is sufficient to withstand the maximum air loads that the aircraft can normally be expected to encounter, but that the first to fail if the design loads are exceeded will be the structure just outboard, of the wing tip connections, i. e., at the point where the subject left wing actually failed.
The condition of the tail surfaces after the accident seems to indicate failure in the air at the time of the severe pull-up. The fact that the airplane went out of control and remained so after the pull—up, contrary to the safe landings experienced in the other three cases, also suggests a loss of control, which would naturally result from a failure of some part of the tail. Furthermore the use of flaps during the descent suggests that normal longitudinal control of the airplane was no longer available. It is possible that the crew endeavored to restore longitudinal equilibrium by the use of flaps.
Although in view of all the evidence it appears highly probable that the stabilizer and elevator did fail in flight, the exact cause of the failure is not clear. The structure of the horizontal surfaces of the DC3 and similar models has been designed for loads substantially in excess of those which the Regulations require this type of civil airplane to sustain. Specifically, the horizontal stabilizer is designed to withstand a limit load (neglecting the factor of safety) which would correspond to the exertion by the pilot of a force of 380 pounds on the control column, provided that the actual load distribution was in exact accordance with the conventional distribution used in the stress analysis. Such a load, of course, would be beyond normal strength. Therefore, the stabilizer is considered sufficiently strong to preclude failure as a result of any action on the part of pilot, providing no unusual and unexpected dynamic conditions exist at the time. Since a detailed examination of the horizontal surfaces did not reveal any weakened condition of the structure prior to the accident, it should not be expected to fail during a pull-up, even one sharp enough to cause failure of the wing tip. It is reasoned, therefore, that the stabilizer having actually failed, there existed during the pull-up forces other than, or additional to, those normally encountered in such maneuver. Various theories have been explored to explain the presence of such forces.
