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In view of the absence of persuasive evidence that the accident was caused by structural failure of the airplane, mechanical failure of its motors, fire, heavy rainfall, or sabotage, we are left with turbulence and lightning as the two major possibilities on the present record. While it has been found that the airplane was flying through turbulent air at the time of the accident, it seems highly improbable that turbulence alone could account for the loss of 5000 feet before recovery of level flight. It is possible, of course, that involuntary interference by the jump seat occupant, who may have been thrown into the cockpit, could have accounted for the inability of the pilots to regain control once it had been lost. (The Board has under consideration a regulation prescribing the technical qualifications of any person who may be permitted to occupy the jump seat.)
Especially in view of the absence of persuasive evidence indicating a other probable cause of the accident, we are greatly impressed by the evidence of the coincidence of the lightning flash seen to be in close proximity to the airplane and the immediate descent of the airplane. Nor is this impression altered by the fact that all-metal aircraft are commonly struck by lightning with no injurious results and that the character of the lightning discharge, as well as its effect upon the airplane crew, in the present instance must be regarded as an extremely unusual occurrence.
Probable Cause
Upon the basis of the foregoing findings of probable fact and the entire record in this investigation, we find that the probable cause of the accident to aircraft NC 21789, which occurred at Lovettsville, Virginia, on August 31, 1940, was the disabling of the pilots by a severe lightning discharge in the immediate neighborhood of the airplane, with resulting loss of control.
