As mentioned previously, the effects of static electricity are of considerable importance in the design, operation, and maintenance of aircraft. This is particularly true because modern airplanes are equipped with radio and other electronic equipment.
The pop and crackle of static is familiar to everyone who has listened to a radio receiver when static conditions are prevalent. An airplane in flight picks up static charges because of contact with rain, snow, clouds, dust, and other particles in the air. The charges thus produced in the aircraft structure result in precipitation static (p static).
The charges flow about the metal structure of the airplane as they tend to equalize, and if any part of the airplane is partially insulated from another part, the static electricity causes minute sparks as it jumps across the insulated joints. Every spark causes p-static noise in the radio communication equipment and also causes disturbances in other electronic systems.
For this reason, the parts of an airplane are bonded so that electric charges may move throughout the airplane structure without causing sparks. Bonding the parts of an airplane simply means establishing a good electrical contact between them.
Movable parts, such as ailerons, flaps, and rudders, are connected to the main structure of the airplane with flexible woven-metal leads called bonding braid. The shielding of electronic devices and wiring is also necessary to help eliminate the effects of p static on electrical equipment in the airplane.
Shields consist of metal coverings which intercept undesirable waves and prevent them from affecting sensitive electronic systems. An airplane in flight often accumulates very high electric charges, not only from precipitation, but also from the high-velocity jet-engine exhaust as it flows through the tailpipe.
When the airplane charge becomes sufficiently high, electrons will be discharged into the surrounding air from sharp or pointed sections of the airplane. The level at which this begins is called the corona threshold. Corona discharge is often visible at night, emanating from wing tips, tail sections, and other sharply pointed sections of an airplane.
The visible discharge is often called "St. Elmo's fire." Corona discharge occurs as short pulses at very high frequencies, thus producing energy fields which couple with radio antenna fields to cause severe interference. The solution to the problem is to cause the charge on the airplane to be partially dissipated in a controlled manner so that the energy level of the discharge will be reduced and the effects of the discharge will cause a minimum of interference.
In the past, static-discharge wicks were used to reduce the charge on the airplane. Because of the high speeds of modern jet aircraft and the fact that they are powered by jet engines which tend to increase static charges, it became necessary to develop static-discharge devices more effective than the wicks formerly used.
A new type of discharger has proved most successful. It is called a Null Field Discharger and is manufactured by Granger Associates. These dischargers are mounted at the trailing edges of outer ailerons, vertical stabilizers, and other points where high discharges tend to occur.
They produce a discharge field which has minimum coupling with radio antennas. Static charges must be taken into consideration when an airplane is being refueled. Gasoline or jet fuel flowing through the hose into the airplane will usually cause a static charge to develop at the nozzle of the hose unless a means is provided whereby the charge may bleed off.