Aircraft Charging and Discharging
Date: Summer 2012
How does an aircraft, while flying through clouds, get charged; and how is it discharged? What type of charge it gets i.e positive or negative?
Aircraft can build up a static charge, which is a result of an excess or lack of electrons. Electrons can be transferred to/from the aircraft by the air itself, by raindrops, dust or snow, or it can build up when the aircraft flies near clouds that have built up a high charge (i.e., towering cumulonimbus clouds or thunderstorms). This is known in aviation as "precipitation static". Helicopters are known for building up a static charge very quickly, upwards of 50,000 volts.
Without a way to dissipate this charge, an airplane could build up a very large static charge. If the charge builds up too high, it can interfere with radio communication, radio navigation, magnetic compass reading, and it can even result in St. Elmo's fire. So to keep the charge from building up this high, an airplane is fitted with what are called static wicks or rods. Since static charge tends to accumulate around sharp points, a static wick or rod will build up to a high static voltage before any other part of the airframe. The local static voltage on the wick will build up high enough to result in a small local corona discharge, allowing a small but constant stream of electrons to flow to the surrounding air. The static wicks or rods are usually attached to the trailing edges of the wings, horizontal stabilizers and fins. They are conductive but are designed to be high-resistance to limit the current flow, and are usually made of a conductive wire core and carbon fibers or carbon coating.
Static charging on airplanes tends to be negative, that is, electrons are transferred to the airplane and it becomes negatively charged. However, if an airplane travels through a region of high positive charge (e.g., exhaust smokestacks tend to generate positively charged exhaust gases) it could pick up a positive charge by flying through it.
John C. Strong
When an aircraft is in the air, flying, its engines drive (provide mechanical energy to) electrical generators that provide the electricity for the instruments, radios, and other systems such as the motors for ailerons, elevators, flaps, hydraulics and landing gear. The electrical system uses the fuselage of the aircraft as the electrical ground, so the voltage the generators provide, provides voltages above the level of the fuselage.
When an airplane flies through air that is charged, that particular air mass may be positively or negatively charged, the whole aircraft is raised or dropped in electrical potential relative to the earth, but the generators, using the airframe fuselage as ground maintains the same potential difference relative to the airframe for operating the electrical equipment on the aircraft.
Problems arise when the aircraft whose airframe fuselage is charged to a voltage level higher than the earth lands. On some aircraft, static discharge wicks are provided on the trailing edge of wings to help dissipate some of that fuselage charge, but when the plane lands, when it first touches the ground, a current is generated to equalize the voltage of the fuselage to the voltage of the earth. This is a special problem when helicopters hover over ships and transfer people or stuff to the deck of the ship. The ship crewman first let the cable transferring the people or equipment touch the deck of the ship first to equalize the voltage difference between the ship and the helicopter. Airplanes can use special electrical discharge wires on the landing gear to dissipate the potential difference when landing.
I hope this answers your question.
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Update: June 2012