Aircraft flying in clouds or precipitation will accumulate electrical charges due to frictional contact with liquid or other solid particles in the atmosphere. These charges cause interference in radio communication and navigation, so we need to eliminate this.
Radio interference can threaten the safety of aircraft because it can interfere with aircraft communications and navigation equipment.
Almost every component of the aircraft is a possible source of radio interference.
Radio interference deteriorates the performance and reliability of the radio and electronic components.
Aircraft Static Discharger helps to reduce interference in the communications and navigation systems.
Radio Interference
Radio interference is a disturbance in the radio frequency spectrum generated by an external source that affects electrical circuits by electromagnetic induction.
This disturbance may degrade the performance of the circuit or even stop it from functioning.
Types of static discharge
Radio interference is most commonly caused by three types of static discharge:
- Corona,
- Streamering, and
- Arcing.
Corona
A corona discharge is luminous and can be audible. Corona is created by an electrical discharge that causes ionization of the air in the presence of a strong electrical field. The corona discharge on nonmetallic composite radomes, winglets, windshields, and propellers, for example, is also known as St. Elmo’s fire.
When the charge on an aircraft reaches 100,000 to 200,000 volts, the electrical fields on the aircraft become concentrated on its extremities – wing tips, tail surfaces, etc. Air is ionized at the sharpest points on these surfaces and current discharges into the surrounding air, resulting in radio interference that causes a hissing noise in a radio receiver.
Streamering
Streamering is an electrical discharge that may trail from the aircraft, including dielectric surfaces such as composite radomes and composite winglets, as well as glass and ceramics. The discharge may stream (jump) several inches from composite material to a metal airframe or into the air. Or a charge can build up on a painted surface before it streams to a nearby unpainted screw or rivet head. Streamering can be reduced by coating non-conductive areas with a high resistance paint that allows the charge to be bled to the airframe.
Arcing
Arcing can create a brilliant luminous glow, similar to streamering, but it is usually limited to jumping point to point in distances of an inch or less. Often it is barely visible, but when observable it has the appearance of a curved line. Arc discharges are almost always the result of a part of the aircraft being electrically insulated from the main structure so that when a charge accumulates on the isolated part, it arcs to the main structure.
Arcing is usually caused by the failure of some device installed to reduce precipitation static noise, such as broken bonding straps at control surface hinge points, gear doors, access panels, fuselage cargo, and entrance doors. Cracked radome lightning diverter straps and static discharger bases cause arcing. (Bonding straps at control surface hinge points are critical.
Reducing Radio Interference
Radio interference is reduced by preventing the uncontrolled discharge of static electricity from the aircraft to the surrounding air. The charge is blended into the air mass around the aircraft at a controlled rate by using a discharge device.
Suppression of unwanted electromagnetic fields and electrostatic interference is essential on all aircraft.
In radio communication, this can be noticed as unwanted noise. In other components, this poses a threat to proper operation.
Many components of an aircraft are possible sources of electrical interference which can deteriorate the performance and reliability of avionics components.
To prevent the build-up of electrical potential, all electrical components should also be bonded to the aircraft structure (ground).
Bonding
Bonding provides the necessary electrical connection between metallic parts of an aircraft.
Bonding will not eliminate the static charge but will equalize the potential between the parts bonded so that a spark will not occur between them.
Static wicks discharge the static electricity.
Aircraft become highly charged with static electricity while in flight. If the metal parts of the aircraft are improperly bonded, there will be a difference in charge (voltage). Where different voltage potentials exist, arcing can occur between them due to vibration or control surface movement causing noise (crackling) in the radio receiver.
Bonding jumpers, leads, and clamps are the normal methods of equalizing voltage potential throughout the airframe.
Bonding also helps to provide the low resistance return path for single-wire electrical systems. All avionic equipment is bonded to the airframe to provide a low impedance path for both static voltage and induced voltages to be discharged to the ground, thereby reducing radio interference.
Bonding Jumpers
- Make sure bonding jumpers should be as short as possible.
- Be sure finishes are removed in the contact area of a bonding device so that metal-to-metal contact exists.
- Resistance should not exceed .003 ohm.
- When a jumper is used only to reduce radio frequency noise and is not for current carrying purposes, a resistance of 0.01 ohm is satisfactory.
Static Dischargers
Static dischargers, or wicks, are installed on aircraft to reduce radio receiver interference. This interference is caused by corona discharge emitted from the aircraft as a result of precipitation static.
Static dischargers are normally mounted on the trailing edges of the control surfaces, wingtips, and the vertical stabilizer.
A resistance measurement from the mount to the airframe should not exceed 0.1 ohms. Inspect the condition of all static dischargers in accordance with the manufacturer’s instructions.
Static dischargers (wicks) are installed on aircraft to reduce the buildup of static charge on the airframe and radio receiver interference.
During the flight, the aircraft becomes charged with static electricity. If the discharge of the static electricity is not controlled, it causes interference in the communications and navigation systems. To decrease the effect of this interference, static dischargers are installed.
Each discharger has a carbon fiber tip at the end of a slender rod. The rod is a resistive (conducting) material and attaches to a metal base. The base attaches and bonds to the airplane surface.
Static dischargers are normally mounted on the trailing edges of the control surfaces, wingtips, and vertical stabilizer.
There are trailing edge and tip dischargers. The tip dischargers are smaller than the trailing edge dischargers.
Purpose of static dischargers
- Decrease the voltage level necessary to start corona discharge
- Make regions of very low radio-frequency field-strength and thus cause the discharge to occur in these regions
- Discharge the static electricity charge
How it works
As an aircraft moves through the air, friction generates that can cause a buildup of static electricity.
The static dischargers (also called wicks) which resemble a wire, transfer that electricity back into the atmosphere to prevent it from interfering with radio communication and navigation.
The static wicks would work to dissipate the energy from that lightning strike.
