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type of lightning stroke which lasts an extremely short space of time but involves a very high value of current and results in a mechanical effect. The first type of lightning may set a tree on fire, whereas the latter type may splinter a tree and toss heavy pieces of wood a considerable distance away. Brief, powerful discharges set up pressure waves with characteristically steep fronts which are capable of a damaging effect at a short distance away from the path of the lightning itself. It should be noted that a single lightning discharge may involve not only mechanical but also thermal and electrical effects. Thus a severe lightning stroke involving a high value of electrical current may disintegrate a tree while also setting it on fire.
If the particular discharge of lightning with which we are concerned in this investigation were of a nature to produce a strong pressure wave, the pilots might have suffered from acoustical shock or concussion of a severity depending upon the proximity of the lightning to the cockpit and whether or not the cockpit windows were open. Experience has revealed that such pressure waves have produced severe concussion upon human beings which might result in unconsciousness. The possibility of acoustical shock or concussion as one of the basic elements in this case cannot be dismissed.
Pilots quite frequently have reported that when a flash of lightning has passed near their planes they have felt a slight "bump." This would be a result of the pressure wave, but all available data indicate that there never have been any serious effects upon the stability and flight path of an airplane from this cause. We do not believe that lightning itself produced such severe turbulence as to have caused or contributed to this accident.
The mechanical effect of lightning has been known to smash in the windows of buildings even though the discharge itself may have passed some little distance away. Although the cockpit windows were designed, installed, and
