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Statistics indicate that the majority of lightning strikes to aircraft occur at ambient temperatures near the freezing level. This correlates with thunderstorm electrification theories that charge separation occurs about the freezing level. N709PA was at or near the freezing level just prior to the accident.
ANALYSIS
The flight crew of N709PA was properly certificated and qualified for this flight. The flight was properly dispatched and cleared, and conformed to the air traffic control rules. The history of this aircraft reveals no item that stands in casual relationship with the accident.
No evidence of any flight crew incapacitation, prior to impact, was found.
Air carrier aircraft operated into and out of Philadelphia immediately before and after the accident. There were scattered thunderstorms and areas of severe turbulence in the Philadelphia area. However, the turbulence in the area where N709PA was operating was light to moderate and was not of the strength normally associated with a loss of control or structural failure. Even though the aircraft was operating in precipitation at or near the freezing level, icing is not considered a factor.
The weather documentation and briefing furnished the crew at San Juan was satisfactory. The weather service provided the crew at Baltimore was not complete, however, all the weather data available indicated that the cold front would be past Philadelphia and the weather would be improving upon their arrival. By definition, lightning is always present in thunderstorms. At the time and place of the accident, a thunderstorm was present and lightning was observed and scattered thunderstorms were forecast for the area.
The record indicates that the flight was normal until 32:15 minutes after take-off from Baltimore. The aberrations appearing in the traces of N709PA's readout at 32:15 after takeoff indicates the time of the explosion in the left wing. The pilot was able to maintain control only a few seconds after that and then the aircraft dove into the ground.
There was no evidence of any powerplant or system malfunction that could have caused the inflight explosion. However, due to the inflight fire damage in the left inboard wing trailing edge, the left horizontal tail and the left side of the vertical tail and aft fuselage, the possibility of a fuel leak in the trailing edge of the left inboard wing was explored in connection with initiation of the accident. Possible sources of such leaks are the dump chute assembly, the reserve tank transfer tube, or a rear spar crack.
Fuel released into the trailing edge could be ignited and the cavities in the wing trailing edge would act as flame holders making continued burning likely. Although the dihedral angle of the wing in normal flight would tend to cause an inboard flow of some of the released fuel along the rear spar lower edge, various obstructions in the trailing edge and chordwise discontinuities in the lower surface of the trailing edge would tend to localize any fire to the general area of the leak. As a result any leak appreciably inboard of the reserve tank transfer tube could
