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the tank or cause internal sparking, either from the high resistive and/or inductive voltages developed across internal discontinuities, or from possibly high voltages induced in the fuel probe wiring. In addition, flame can propagate through the vent system, from fuel vapors ignited at the vent outlet by direct strokes, streamering, or blast pressure waves, spark showers, and possible plasma penetration from direct strokes. Accelerated studies have recently been completed into these hazards to provide technical data on their probably occurrence and control. These studies have also indicated the structural damage that would be caused by the different causes of ignition. Neither blast wave compression nor induced streamer ignition would leave visible evidence of the cause of ignition. The various types of sparking could also cause ignition without leaving identifiable evidence but might leave such evidence if the sparking energy is sufficiently high to produce visible pitting or fusion of metal surfaces.
The lightning damage previously discussed was re-examined and confirmed. The detailed examination of surfaces, joints, rivets, screws, fittings, and fuel quality probes for physical evidence of ignition-producing sparks where ignitable fuel/air mixtures may normally be present disclosed no identifiable evidence of electrical arcing. Thus, the actual mechanics of ignition of the left reserve tank contents was not determined.
Metal splatters on the leading edge of the horizontal stabilizer were identified by spectrographic analysis as being formed from two different aluminum alloys. However, these alloys could not be identified since the chemical composition of the deposits did not conform to any alloy used in the airplane. The long axis of the deposits varied from 18 degrees inboard to 4 degrees outboard.
The aircraft records review indicated that N709PA was maintained in accordance with FAA and approved Pan American directives and procedures prior to the accident. These records revealed no history of fuel leaks, lightning strikes, or static discharges.
The flight recorder tape was torn and crumpled but had little fire damage. Approximately 95 percent of the tape was reassembled and read out. The tape showed an elapsed time of approximately 32 minutes and 50 seconds from lift-off at Baltimore until impact with the ground. The record of the first 32 minutes of flight was normal and indicated no severe turbulence. 32:15 minutes after takeoff abnormal excursions appear. The tape shows that the aircraft stayed at 5,000 feet for approximately 15 seconds after the beginning of the unusual excursions and then descended rapidly to ground level with little change in heading. The flight recorder tape of NAL 16[1] showed no major differences between the traces of the two aircraft while they were in the holding pattern, insofar as evidence of turbulence is concerned.
The bodies of all persons aboard the aircraft were recovered and identified. Toxicological examination of the flight crew showed no evidence of alcohol or elevated carbon monoxide levels. Carbon monoxide tests of passengers also indicated no elevated levels. The flight crew was physically qualified for flight according to FAA and Pan American records.
The flammability limits[2] of a mixture of Type A and Type B fall somewhere
