MIL-HDBK-274A
4.4.2.3.4.1 RF arcing categories. RF arcing can be separated into two categories: RF glow discharges and RF arc discharges. In RF glow discharges, a high minimum voltage is necessary to maintain the discharge. The voltage rises to an ignition voltage of 350 V to 500 V, and then decreases slightly after the current flow begins. The current flow ceases when the field can no longer sustain the high voltage. Even at very small separations between elements, a minimum voltage of 275 V is necessary to sustain the glow. Conversely, RF arc discharges are sustained at relatively low voltages. The events leading to the formation of the arc discharge are similar to those of glow discharge. However, once the arc forms, the voltage can usually drop to as low as 30 V without the arc extinguishing.
4.4.2.4 Lightning. Lightning is a discharge of atmospheric electricity from one cloud to another, within a cloud, or from a cloud to Earth. Table I provides a listing of lightning characteristics. The cloud to Earth or ground strike is the type of discharge that produces the direct lightning strike. Due to its large amount of energy and potential for destruction, the direct lightning strike is one source of energy that can be very dangerous to personnel and aircraft. A direct lightning strike can damage an aircraft and its equipment. Voltages as high as 100 MV and currents to 650 kiloamperes (kA) have been known to occur. When a charge of sufficient magnitude builds up in a cloud, preliminary breakdown occurs and a stepped leader is initiated. As the stepped leader nears the Earth, upward-moving discharges may be produced from local high points that are at or near ground potential. For this reason, the tail of an exposed aircraft often provides the primary attachment point for a stepped leader. As the leader completes its
path to Earth, a return stroke is generated. The currents in return strokes typically reach levels of
20 kA, with peaks of up to 650 kA recorded. During the life of an aircraft, it may be struck many times by lightning, but only on rare occasions is significant damage sustained. Little can be done to prevent a lightning strike to an aircraft parked on a concrete/asphalt apron outside a hangar. A low resistance ground will reduce the amount of time needed to bleed-off the induced charge due to nearby lightning, and reduce the chance of side-flashes. Of course, no ground will totally protect personnel working on the exterior of an aircraft near an electrical storm. If weather conditions are such that lightning strikes are imminent, all operations involving refueling, ordnance handling, or maintenance should be suspended. These high levels could
puncture the aircraft if the skin is not sufficiently thick or could possibly cause structural damage or localized melting. Damage to the aircraft's electronic equipment or ignition of the fuel tank could also occur, depending on the location of the strike. A person in contact with the ungrounded aircraft during a direct lightning strike risks severe injury or death. For personnel who wear communication headsets the probability of occurrence and the severity of injury increases.
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