Page:CAB Aircraft Accident Report, Pan Am Flight 214.pdf/9

 between the limits of either fuel examined separately. These limits vary according to the temperature and pressure. Flammability is also affected by misting or foaming of the fuel. Expert testimony at the hearing indicated that the fuel vapors in the air spaces of the tanks were well within the flammable range at the time of the accident.

Pan American World Airways conducted a flight test in a Boeing 707 to determine if fuel would discharge through the tank vent system ram air inlet. There was no visible discharge of fuel at any time during the test. There was evidence that fuel entered the vent system, collected in the surge tanks, and returned to the proper fuel tanks.

To clarify the subject of lightning, a U. S. Weather Bureau witness was called to testify at the hearing and various technical reports were reviewed. These sources indicate that a lightning stroke begins when the air's resistance to the passage of electricity breaks down. At that time a faintly luminous stepped leader advances toward and area of opposite potential, the earth in the case of cloud to ground lightning. The difference in electrical potential between a cloud and the ground may be in the order of ten to one hundred million volts and discharge current may exceed 100,000 amperes with 10,000 amperes per micro-second rate of current rise.

The stepped leader advances toward the ground in a series of discrete branching movements, forming an ionized path down from the cloud. As a branch of the stepped leader is approaching the ground, the intensified electric field causes an upward moving streamer to form at a ground projection and advance toward the stepped leader. As the oppositely charged leaders meet, completing the ionized channel, an avalanche of electron flow follows, discharging the cloud to the ground. This entire sequence is accomplished in approximately one millisecond. Additional charge cells in the cloud may then successively discharge through this main ionized channel as a single flickering flash which may last as long as one second. The electron flow suddenly heats the ionized channel to about 15,000°C, expanding the air supersonically outward with a thunderclap. The discharge can also occur between oppositely charged regions within a cloud, or in different clouds.

If the stepped leader of a stroke approaches a flying aircraft, the intense electric field induces streamers from the extremities of the aircraft out toward the approaching stroke. Th stepped leader contacts one of these aircraft streamers completing the ionized channel to the aircraft and raising he potential of the aircraft to the order of 100 megavolts. This high potential produces streamers from all extremities and high gradient points of the aircraft. These streamers can have sufficient energy to ignite fuel vapors. Meanwhile, the stroke leader continues on from the aircraft to another cloud or to the ground to complete the ionized channel for the electron avalanche.