Technical articles about electrostatic charging

ELECTROSTATIC CHARGING IN FLIGHT

By Jay D. Cline, Dayton-Granger Inc.

(First Published in Avionics Magazine August. 1988) Updated January 1989

 It is widely known that electrostatic charging of aircraft in flight generates radio frequency noise, which disrupts navigation and communication. Such charging results from several sources. For example; flight through precipitation, electric cross-fields and engine produced ionization. Streamer currents on the plastic frontal area of an aircraft also generate RF noise during precipitation encounters, and corona discharge between airframe members. Looking at the phenomena in the laboratory reveals significant broadband RF noise generated in a band extending from direct current (DC) up through 1,000 MHz.

 This can affect almost all aircraft, from General Aviation through airline transport and military supersonic. At ground speeds of two to ten nautical miles per minute, loss of navigation or communication due to streamering, corona or arcing noise can be serious, especially while maneuvering near airports in instrument conditions.

 Aircraft Charging. These effects occur as an airplane flies through freezing rain, ice crystals, dust, sand or snow. Contact with these particles leaves a positive or negative charge on the airframe, and can be expressed mathematically as: 

I=qCVA where

i=

Charging Current
q= Charge imparted per particle
C= Particle density
V= Aircraft Velocity
A= Projected aircraft frontal area

As the aircraft charge builds, a potential is reached where the charge leaks off the aircraft and antennas, generating broadband radio frequency noise. This interferes with ADF, HF, as well as VHF and VOR receivers.

 Cross-field currents are generated on aircraft flying in clear air beneath a charged cloud layer. The magnitude is a function of the potential of the cloud with reference to ground and the speed of the aircraft.

 Streamering. This noise source is generated over dielectric surfaces such as radomes, fiberglass winglets and other fiberglass panels positioned on frontal impact areas of the aircraft. As particles strike, they deposit an electron on the dielectric surface. As more particles impact this isolated pool, the voltage increases until it reaches the flash over point. When the pool of charge flashes over the surface of the dielectric material, it generates broadband radio frequency noise. 

This phenomena is also observed over metal surfaces painted with a high dielectric strength paint, or paint buffed to a high polish. In this case, charges accumulating on the paint generate streamers to a rivet head or screw fastener. Streamering can be solved buy coating the non-conductive surface with high resistance paint. Such paint quietly bleeds the charged particles to the aircraft fuselage.

 Corona Noise. This occurs when the aircraft accumulates sufficient charge due to aircraft charging and / or cross-fields to ionize air around wing tips, vertical and horizontal stabilizers and other protrusions. Over 500,000 volts have been measured on General Aviation aircraft in flight. As current bleeds trailing edge, it generates radio frequencies that sound like loud hissing in aircraft receivers. The charging also causes antennas to go into corona (bleeding off charge). When this happens, the noise appears like a strong signal to the receiver. In some cases, the automatic gain control circuit, sensing noise as a strong signal, desensitizes the receiver to the point where the receiver may go perfectly quiet. The pilot assumes no one is calling, but in reality, corona current has, for all practical purposes, shut down the receiver. When aircraft voltage lessens and antenna corona current stops, receivers AGC returns to normal and communications can continue. The pilot is seldom aware of what happened. When communications are reestablished, ATC may assume that the pilot was not paying attention to his radio.

 Solutions to corona noise include antennas that are insulated from space, and static dischargers positioned where the aircraft is most likely to go into corona; wing tips, vertical and horizontal stabilizers are examples. Static dischargers bleed off charge quietly. They lower aircraft voltage below a level where antennas go into corona.

 Arcing Noise. This interference is generated by an isolated piece of metal situated on an aircraft where, as the aircraft charges, it reaches a potential at which a spark jumps the gap from aircraft structure to isolated metal. The spark can produce broadband noise extending through 1,000 MHz. The cure is to locate the isolated metal and bond it to the aircraft structure with a grounding strap. To locate this problem the aircraft can be probed with an Electrostatic Test Set while monitoring aircraft receivers for arcing noise. When the noise area is identified, physical identification can isolate the piece of metal. These and other solutions can greatly lessen the effect of environmentally induced noise while in flight.

 

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