Combating noise and other effects
By Jim Sparks
Almost everyone involved in aviation at one time or another has had an encounter with a static discharger or static wick. You know the device that protrudes from the corner of the wing or some other surface and frequently gets in your path and may on occasion tear your clothes or inflict some wound and often gets broken while positioning the aircraft just prior to a flight.
The main purpose of the static discharger is to improve dispersal of accumulated aircraft static charges in an effort to reduce radio interference.
Location on the airframe enables them to serve a role as a lightning conductor and provide protection against arcing for surrounding aircraft structure.
Static dischargers are replaceable and may be mounted on supports that are attached to the aircraft structure in such a way to ensure adequate electrical contact.
In the early days of World War II, aircraft navigation and communication systems were, at times, unreliable in some weather environments. Ways to develop a significantly reduced precipitation static noise on aircraft was forthcoming and led to the invention of the first static discharger. This new concept was far superior to any other methods of the day and precipitation static (P-static) noise reduction came to be.
Different types of static dischargers are used to alleviate different problems on various types of aircraft. This means a small general aviation aircraft flying at lower speeds will not use the same type device as a commercial airliner or business jet. Aircraft manufacturers or companies that specialize in aircraft static dissipation should always be consulted when selecting an appropriate device.
Static dischargers work on the principal of creating a relatively easy path for dissipating charges that develop on the aircraft by using a device with fine metal points, carbon coated rods, or carbon wicks. Rather than wait until a large charge is developed and discharged off the trailing edges of the aircraft a static wick when working properly will allow a small but constant stream of electrons to flow to the surrounding air. This process offers various decibels (db) levels of static noise reduction, which can be adapted to different aircraft types to eliminate interference in avionics equipment.
Some common symptoms of excessive precipitation static are:
- Complete loss or weakening of VHF communications
- Erroneous magnetic compass readings
- High pitched squeal on audio
- Static noise in audio
- Loss of all equipment in clouds
- VLF navigation system intermittent
- Erratic instruments
- 'St. Elmo's Fire' on windshield
Aircraft charging will occur as an airplane flies through freezing rain, ice crystals, dust, sand, or snow. Contact with these particles leaves a charge on the airframe and 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 may be created on aircraft flying in clear air beneath a charged cloud layer. The magnitude of the charge will be determined by the potential difference of the cloud to ground plus the speed of the aircraft. This condition is expressed mathematically by: I = QCVA.
I = Charging currentTypes of static discharge
Q = Charge per particle
C = Particle density
V = Aircraft velocity
A = Aircraft frontal area
Static discharge occurs in several forms referred to as streamering, corona, and arcing.
Streamering. This type noise is generated by nonconductive devices such as radomes, fiberglass winglets, and other nonmetal panels positioned on front impact areas of the aircraft. As particles strike, they deposit an electron on the dielectric surface. As more particles impact, the voltage increases until it reaches the flash over point. When the charge flashes over the surface of the dielectric material, it generates broadband radio frequency noise.Procedures to aid in maintaining static dischargers
This phenomenon 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 often be solved by coating the nonconductive surface with special conductive paint. Such paint quietly bleeds the charged particles to the aircraft fuselage.
Corona noise occurs when the aircraft accumulates sufficient charge due to aircraft charging and ionizes the air around wing tips, vertical and horizontal stabilizers, and other protrusions. More than 500,000 volts have been measured on general aviation aircraft in flight. As current bleeds from trailing edges radio frequencies are produced that sound like loud hissing in aircraft receivers. The charging may also cause 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 radio may go perfectly quiet. The pilot assumes no one is calling, but in reality, corona current has shut down the receiver. When the aircraft state of charge is reduced and antenna corona current stops, receivers return to normal and communications can continue. The crew in some cases is unaware of what happened. When communications are reestablished, ATC may assume that the pilot was not paying attention.
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. This can greatly lessen the effect of environmentally induced noise while in flight.
Identify the problem and then cause
Before solving an electrostatic problem a clear understanding of the situation is required.
The flight crew, most likely, will have identified an electrostatic problem. When possible try to obtain as much information as possible. Some important questions include:
- Does it only occur when the aircraft is flying in a precipitation environment or in close proximity to charged clouds or around an electrical storm? (If the interference or degradation to the communication or navigation systems takes place in clear air or on the ground, it is not a P-static induced problem.)
- Which systems are affected?
- Is only one system affected, or are other systems affected as well?
- If the problem is related to radio noise, ask if it is continuous, or if it is like a fast clicking. Continuous noise corresponds to a streamering corona discharge and a clicking noise corresponds to an ARC/surface discharge.
Once the problem is clearly identified contacting the manufacturer of the equipment in question might provide insight to the cause.
Contact the manufacturer of the aircraft and ask for a drawing of the aircraft showing: antenna installation (type, position), composite parts with the type of composite clearly marked, the areas protected by anti-static paint (if any), the placement of dischargers (if any), and the position/construction of access panels.
A continuous noise observed in very low frequency navigation systems (VLF), automatic direction finding (ADF), very high frequency (VHF) communications, and very high omni directional range (VOR) systems is often a result of a corona effect and may be caused by defective antennas, improper paint treatment including paint thickness on nonconductive panels, and improper bonding on windshields.
The clicking noise will most often impact VHF communications and VOR. This is most often attributed to improper bonding and may be aggravated by composite panels, defective static dischargers, or nonfunctional lightning diverter strips.
Verification of airframe surfaces using an electrostatic diagnostic tester will often pinpoint the culprit enabling corrective measures to be initiated.
Aircraft skin mapping is one method used in the identification of P-static problems.
The equipment consists of a voltage generator and an array of specially designed probes.
The voltage generator can build up significant potentials that often can be of levels encountered in electrical storms. Special probes including a corona ball, collector dish, and ion streamer are used to specifically identify problem areas.
A corona ball is a metal sphere that is used over the vast majority of the aircraft surface, while the collector dish is effective for locating problems with antennas. The ion streamer is the tool of choice for testing radomes, windshields, and other nonconductive surfaces. As this equipment produces very strong electrical charges it should only be used by someone knowledgeable about the device and adequate safety precautions should be employed.
The base used to hold the static discharger to the airframe should also be considered as an electrical conductor. If the connection of the base to the airframe is not a good path for electron flow the discharger will be ineffective.
Dayton-Granger is a manufacturer of static dischargers and aircraft antennas. It has published the procedures to facilitate testing of the electrical properties of their equipment on various aircraft (see page 30).
In all cases aircraft manufacturers maintenance documents should be used to determine the airworthiness of all P-static protective devices.
It is true that some aircraft are less prone to lightning strikes. Size, shape, and speed are all aircraft-specific variables, which determine an aircraft's susceptibility to a lightning strike. However, it is also true that all aircraft are potential targets. Aircraft damage varies with aircraft type. Careful aircraft design can minimize lightning damage. However, all surfaces are susceptible to lightning strikes, and all unprotected systems can be affected. Proper care and maintenance of external airframe surfaces including bonding checks and verification of static dissipation are essential to making an aircraft a lesser target for lightning strike or P-static.
In conclusion 'Don't take any static'!