Static Charging and Its Effects on Avionic Systems

Static electrification of the airframe can often cause electromagnetic interference on aircraft radios. This paper reviews the causes and possible effects of this phenomenon. Sources of static electrification include frictional charging by precipitation particles, engine exhaust non-neutrality (at low altitudes), and exogenous electric fields (between oppositely charged cloud regions). The potential of an aircraft increases until a threshold is reached at which electrical discharges occur, generating electromagnetic noise that interferes with both navigation and communications. Noise-producing discharge mechanisms include 1) corona-discharge breakdown of the air, 2) streamer discharges across insulating dielectric surfaces to the metal airframe, 3) sparks between inadvertently unbonded adjacent metal sections of the aircraft, and 4) the passage of charged particles past an antenna. Corona discharges and streamer discharges are discussed for both narrowband and broadband systems. The electromagnetic fields generated by discharges raise the local equivalent noise field above the atmospheric noise level, causing interference in narrowband systems. Pulse discharges can be misinterpreted on broadband digital systems as operating signals. Aircraft charging parameters have been measured over time (after takeoff) and altitude for several aircraft types, including a jet fighter, a 707, a Titan III rocket, and a helicopter.