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Response to Aircraft Question by Ben Baumgartner Aircraft can become charged from several sources. Air does not charge the aircraft, but rain drops and ice crystals in the upper atmosphere can charge the aircraft. The charging of an aircraft is called "precipitation charging." The jet engines, because of the burning gases, can also charge the aircraft. On military aircraft, the after burners charge the aircraft very quickly when they are turned on. They can take the aircraft to 40,000 volts. The people inside the aircraft do not get charged. The charge is all on the outside of the aircraft. The charge that is built up on the aircraft is reduced by the discharges on the wing and tail. The dischargers are not called instruments. The dischargers are passive devices. These types of dischargers can be called self dischargers. Technically, the correct identification is "ortho mode discharger." Otho means perpendicular in this case. There are no active elements. The discharge consists of a high resistance resistor that is electrically connected to the metal surface of the aircraft. The end of the resistor has two sharp needle points. The needle points are perpendicular to the resistor and face in opposite directions. There is a reason for the needle point facing in the opposite direction. The charges leaving the needle points in the opposite direction tend to cancel the electrical noise (static) that is generated when the discharger is operating. See the figure below. The discharger works like a self discharger. A self discharger has lot of thin metal foil strips that are connected to ground. Self discharges are used to discharge paper in printing operations. Thin metal strips are used to discharge paper that comes off of a computer printer or other high speed paper printer. Perhaps you have seen them on old computer printers. Self discharges start to operate when the electric field becomes high. A high electric field causes a corona discharge from the needle point. The needle points are very sharp and therefore a plastic guard is placed around the point to keep people from being injured. Each needle point can have a current up to about 4 microamps when the voltage is high. The discharge stops when the voltage drops, probably about at the 3,000 Volt level. I believe that most aircraft have conductive rubber tires, therefore when the aircraft lands, any charge that still is on the aircraft is discharged to ground when the wheels touch.
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