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tank with negative results. The guidance and assistance of recognized authorities in the field of lightning physics and research were available and used in this phase of the investigation. Particular attention was devoted to fuel quantity probes, discontinuities in the structure which form the fuel tanks bonding, fuel tank access plates, filler cap assemblies, fuel vent outlets and also tank wall surfaces. Information obtained by analyses, research utilizing laboratory produced electrical discharges and observations of natural lightning damage was utilized in determining where and how to make the detailed examination.
Although physical evidence of the means by which ignition occurred was not found, the likelihood of ignition by lightning is not ruled out. The correlation of time, place and damage cannot be dismissed. Certain phenomena associated with lightning discharges, some well defined, others more subtle and of which we have less finite information and which leave no physical evidence can ignite flammable mixtures. Probably most significant in the former category is the development of streamers from extremities and/or surface discontinuities as a lightning stroke develops in the step leader stage. These streamers have been found to be in the duration and energy range required to ignite flammable mixtures. Other phenomena, of which less is known because of generation and measuring difficulties, that are considered potential ignition sources are plasma, shock waves and sparking due to induced voltages from extremely high current rise rates that occur. Although much has been learned about lightning and its effects through research and study, many questions are still unanswered and the upper limits of voltage, current and total energy that may be associated with lightning are not conclusively defined. The apparent capricious nature of lightning is evidenced by the range of extremes as well as, in some cases, the unique nature of the damage that results. Thus, to predict in the absence of physical evidence the exact mechanism of ignition in this particular case is not possible. By the same reasoning, in view of the known facts of this accident, there is no logical explanation for ignition of the flammable vapors other than some effect stemming directly from the lightning strike.
The vent system interconnects all fuel tanks and the vent outlets through passages capable of transmitting flame when filled with flammable mixtures; thus initial ignition could have occurred inside the left reserve tank, inside the left surge tank, or at the left vent outlet.
Based on evidence furnished the Board, it is reasonable to conclude that the state of the art was fully exploited within the limits imposed by basic airplane structural and configuration requirements in the design of the fuel system, including the vent outlets. Application of known lightning protection information as well as specific testing was applied during the design stages. The lightning protection requirements of the Federal Aviation Agency were satisfied.
The Federal Aviation Agency sponsored research, initiated immediately subsequent to the accident, with the intent of improving protection from ignition of fuel by lightning strikes to aircraft. The Board participated in this program by providing a member on the Committee organized to advise in the program planning and execution as well as to evaluate the results. The Committee was informed currently on the investigative results and, conversely, persons responsible for the investigation were advised as information evolved from the research activities to the mutual benefit of all responsible parties. Although the circumstances and facts of the accident were applied in the planning of the research program, its scope and intent was to provide information that could be applied industry-wide and thus was
