Par 803 Page 162 attachment, and which provide a path for current flow through or across the aircraft from entry to exit points.
(2) Design Goals. One goal of designing for lightning protection is to provide large conductive areas of structure where high concentrations of current may dissipate rapidly. Small metallic parts, such as control surface actuators, or thin structural members separated from larger metallic areas may be vulnerable to damage. Protrusions such as antennas, probes, and light assembly projections are particularly vulnerable to lightning strike attachment. Discontinuities or nonconductive areas may provide points of entry to the interior of the aircraft. It is important to keep the high current out of sensitive electronics, flammable areas such as fuel tanks, controls, and away from personnel. Nonmetallic materials used in aircraft, such as Kevlar, fiberglass, graphite epoxy, Plexiglas, etc., are of particular concern because they do not conduct lightning current-like metallic materials. Direct effects of lightning may include puncture or burning of these materials. In addition, the high current will seek another, more conductive path. Adjacent wiring or electronics subjected to this high current may be damaged or vaporized. Some methods of protecting aircraft from the effects of lightning include providing filters or transient suppression for critical circuits; installing diverters to provide a path for current in areas where nonmetallic materials are used; and bonding of access doors, panels, light fixtures, antennas, probes, landing gear, fuel dumps and fuel vent lines, and electronic components (bonding should be low resistance with high current carrying capability for short duration).
d. HIRF. Ground-based, shipboard, and airborne RF transmitters may create high fields where aircraft operate. The intense fields created by high power transmitters, including some radars, radio transmitters, and satellite communication links, are used to define the HIRF environment. These intense RF fields can cause circuit upset and damage in unprotected aircraft systems. Aircraft electrical and electronic systems that perform flight-critical functions are protected against the effects of HIRF. Aircraft structure shielding, wire shielding, and equipment circuit design are all used to provide the required HIRF protection.
e. Precipitation Static. Precipitation static, or P-static, is the triboelectric charging of the aircraft due to flight through dust, ice, rain, sleet, hail, or snow. When precipitation or dust particles contact the metallic or dielectric surfaces, charge separation occurs. This electric charge can cause radio interference through corona discharge from trailing surfaces, and streaming from dielectric surfaces or from impact charging itself. Discharges can occur between metallic parts that are not electrically bonded together. In addition to bonding, P-static discharges are often installed on trailing edges and tail surfaces. Static discharges decouple corona discharge currents from antenna fields and permit noiseless discharge to occur. Static discharges should have a low resistance bond to the airframe and a high resistance (between 6 and 200 mega-ohms, depending on placement) from tip to base.
804. TYPES OF EMI PROTECTION.
a. Structure Shielding. Structure shielding is a method of protecting susceptible circuits inside the aircraft from lightning, HIRF, and EMI. Metal structures provide a low-impedance path for currents generated by EMI, so that these currents will be minimized on systems and wiring. In addition, enclosed structures, such as the fuselage, provide some shielding for radiated fields. The principle of shielding is derived from the fact that the total charge completely
Par 804 Page 163 enclosed by a conductive surface will be zero, regardless of electromagnetic fields external to the surface. The completely closed conductive surface is often called a Faraday cage, named after Michael Faraday, the English physicist and chemist who provided experimental data proving the concept. Of course, no aircraft can be a perfect Faraday cage since there must be openings, doors, windows, vents, etc. The goal of the structure shielding is to seal the cracks or holes in the fuselage to make it as close to a Faraday cage as possible with respect to external EMI that may disrupt internal circuits of the aircraft. It is becoming more common to see aircraft structure shielding in new aircraft designs. This is due to several factors, including the increased sensitivity and number of electronic components within the aircraft and aircraft designs that incorporate composite materials in the structure. These lead to greater susceptibility to EMI, especially lightning and HIRF (radar, radio transmitters, etc.). Composites are not as conductive as metal and do not provide the same level of shielding, particularly for lightning. The requirement for structure shielding is dependent upon the external electromagnetic environment and the level of protection designed into the wiring and electronic equipment. If structure shielding is required in the aircraft design, it should consider the effects of seams and joints, such as around avionics bays and between the structure and quick access or removable doors, louvers, and vents. Gaskets and spring fingers may be avoided during aircraft design because of the maintenance required for these features. The following provides identification of some common types of shielding:
(1) Gaskets. Conductive gaskets may be used to seal access doors and removable panels from EMI intrusion. They should be electrically conductive, fit snugly between the two surfaces of the joint, and make good electrical contact between the mating conductive surfaces.
Gaskets should be used very selectively, because of the likelihood of corrosion of the conductive gasket material or the mating metal surfaces. Some commonly used EMI gaskets are: elastomers filled with silver or nickel plated aluminum particles; neoprene; silicone rubber bulb seals filled with stainless steel particles and wrapped with stainless steel or Inconel wire mesh; and beryllium copper spiral gaskets. An example is shown in Figure 8-2. Gaskets are used most often where frequent access is not required, since repeated compression and decompression of the gasket may result in permanent deformation.
Par 804 Page 164 FIGURE 8-2. TYPICAL METAL PARTICLE FILLED ELASTOMER GASKET
INSTALLATION (
2) Imbedded Metal Strips. Aircraft panels made of composite materials may require an occurred metallic plate which makes electrical contact with the embedded conductive composite of the door. Another method uses a bonded foil strip or metallic tape to form a capacitance couple with the conductive composite fibers. Corrosion between the imbedded metal and graphite composites is a particular concern if this technique is used.
(3) Contact Strips and Spring Fingers. Beryllium copper contact strips and spring fingers are used to seal joints between doors and structure in areas where frequent access is required. See Figure 8-3. The strips have evenly spaced fingers which are mechanically and electrically fastened (metal-to-metal or capacitance coupled) to the door or structure and pressed against the mating structure or door (metal-to-metal contact) to provide electrical conductivity across the joint. As with gaskets, contact strips used in joints between composite materials may require a concurred metal plate, or conductive foil or tape, for contact with the embedded conductive fibers of the composite material. Again, corrosion between the imbedded metal, spring fingers, and graphite composites is a particular concern if this technique is used.
Par 804 Page 165 FIGURE 8-3. EMI SPRING FINGER INSTALLATION ON A DOOR (
4) Screens. Screens which cover vents and louvers may be designed to prevent EMI intrusion. The screen mesh should be the correct size to prevent wavelengths of expected EMI from passing through. The screen should also be electrically bonded to the aircraft structure around the entire periphery of the screen.
b. Bonding. Bonding is the process of establishing a low-impedance (good electrical contact) path between two metal surfaces. The purpose of the bond is to allow radio frequency or lightning current to flow between metallic components, preventing a potential difference or voltage which may result in EMI.
(1) Structural bonding of all parts of the aircraft structure is essential for controlling the conducting path currents associated with lightning, HIRF and EMP. In addition, structural bonding is important to eliminating static charge buildup, which can couple into communication systems. Discontinuities in the aircraft skin (skin joints, access doors, etc.) can create a high-impedance boundary (poor electrical contact) across the joint. Therefore, all discontinuities in the aircraft structure should be designed to provide electrical bonding. Since a low-impedance path is the goal of a bond, the best bond is direct metal-to-metal contact covering a relatively
Par 804 Page 166 large surface area and as close as possible to the two surfaces to be bonded. For hinged areas, such as leading or trailing edge flaps, a conductive flexible strap or cable is the best bond that can be provided. The bond areas should be clean and unpainted, and the strap should be as short as possible to keep its impedance low.
(2) Bonding of the outer cases of avionics equipment to the aircraft structure is required to ensure maximum operational stability of the equipment and correct functioning of EMI-reducing circuit components, such as filters and shielding. As with the bonding of structural components, the best bond incorporates metal-to-metal contact covering a relatively large surface area as close to the two surfaces as possible. Beryllium copper pressure-wiper fingers with relatively large surface areas are often used between the equipment and aircraft structure.
Figure 8-4 shows a typical metal-to-metal contact using beryllium copper pressure-wiper fingers.
Flexible conductive straps between avionics equipment and structure may also be used, but the impedance of the bond will be higher.
FIGURE 8-4. BONDING USING BERYLLIUM COPPER PRESSURE-WIPER FINGERS (
3) Aircraft external lighting, antennas, probes, fuel vents, and fuel cells are susceptible to lightning, and should be bonded to the aircraft structure. Internal circuitry for any of these may require special isolation or high current carrying design requirements.
Par 804 Page 167
c. Electrical Circuits, Avionics Design, and Aircraft Wiring. Most EMI protection is associated specifically with electrical circuits in avionics equipment and the associated aircraft wiring.
(1) The design of aircraft wire routing attempts to avoid both intersystem and intrasystem EMI problems. However, in some cases, physically separating susceptible wiring from other wiring to protect against intrasystem EMI cannot be accomplished. In addition, some wiring in wheel wells, engine bays, cockpit areas, and other open areas may be exposed to lightning, HIRF, or intersystem EMI. In these cases, the use of metal overbraid over the electrical wiring or wire bundle may be the only method to protect the susceptible wiring.
Materials for metal overbraid vary. Corrosion is a serious problem for overbraid made of ferrous material, particularly in high moisture areas such as wheel wells. Overbraid of tin-coated copper or Inconel is more commonly used in these areas. Cable overbraid shielding is only effective if the entire cable is completely shielded and both ends have low-impedance terminations. Splice areas and the area where the overbraid terminates at the connector are the most likely to degrade since these areas frequently require maintenance. Most aircraft wire shields are terminated using pigtails, which may be attached to ground studs or to connector backshells. Another method, illustrated in Figure 8-5, which ensures complete shielding uses a tightly knitted wire mesh conductive tape which overlaps the overbraid and contacts the connector all the way around the connector circumference. The wire mesh tape is soldered to the overbraid where it overlaps.
When wiring must be repaired at the connector, the knitted wire mesh tape can be replaced easily. The connector itself can also be a problem if it incorporates an impedance discontinuity with the wire mesh tape. Connectors used to terminate shields should have good electrical contact between the shield-terminating backshell, the main connector plug, and the mating receptacle. Connectors specifically designed to ensure EMI protection may incorporate conductive contacts between the backshell, main shell, and receptacle around their entire circumference. An EMI connector made of a composite material is available and has the advantages of being lightweight and noncorrosive, however the conductive plating material is still subject to corrosion.
FIGURE 8-5. CABLE OVERBRAID AND KNITTED WIRE MESH TAPE
Par 804 Page 168
(2) Grounds. A ground is a common reference for potential in circuit design. In aircraft, the ground for circuits is usually the aircraft structure. Grounding studs are installed in the aircraft near each subsystem to connect ground wires. Similar functions may be grounded on the same grounding stud. Ideally, the ground reference has zero impedance, is at zero potential, and conducts zero current. Since no ground plane is ideal, some potential always exists between ground points, with the possibility of undesirable ground currents coupling into and disturbing the circuits. A good aircraft design minimizes ground currents by keeping ground wires as short as possible, balancing circuits, using twisted signal and return wires to cancel unwanted signals, using coaxial or biaxial lines for RF circuits, and employing a number of other techniques. A ground reference also serves to prevent shock hazard and static charge buildup. Good circuit design relies upon dedicated signal and power return wires, so that the aircraft structure is not used for the return circuit, particularly for highly critical or sensitive systems.
(3) Filtering. Inputs and outputs from avionics equipment usually require filters. These filters may provide EMI noise, lightning transient, and HIRF suppression. This is best accomplished with filters at the connector internal to the equipment. The purpose of a filter is to exclude unwanted frequencies while allowing transmission of the desired signal frequencies.
Capacitance or resistor/capacitor filters are often adequate for high-impedance circuits, while inductive filters are needed for low-impedance circuits. In some cases, filters may be grouped on a ground plane behind the interface connector and enclosed in a shielded area. Also, in some cases, filter pin connectors can be used, providing significant weight savings and filtering.
(4) Avionics Enclosures. The avionics enclosures may be designed to minimize conducted and radiated EMI from entering the avionics, and to minimize EMI emissions from the avionics. Conductive gaskets may be used in the avionics enclosures, particularly between the enclosure and the connector receptacle installed on the enclosure. In addition, conductive gaskets may be used between access panels on the avionics enclosure, or between ventilation hole screens and the enclosure. The enclosures may also be designed to segregate the filters for input and output wiring from circuit boards and sensitive electronics within the avionics. The enclosures should also provide a means for bonding the enclosure to the avionics rack or structure. This should not be provided by the power or signal returns.
805. EMI PROTECTION MAINTENANCE.
a. Common Failure Modes. Common failure modes for EMI protection features include breakage, deformation, and corrosion.
(1) Breakage. Aluminum foil and mesh used on composite structure panel may be torn or cut. Bonding straps, particularly attached to moveable surfaces, may break from flexing or aerodynamic forces. Shield-terminating pigtails may be broken during connector mating/dismating. Spring finger contact strips, which are typically 0.005 inch thick, seal joints between panels and structure and are easily broken when panels are removed or during equipment removal and installation. Beryllium copper spiral gaskets, which seal joints between panels and structures, and beryllium copper pressure-wiper fingers, which bond electronic equipment to aircraft structures, are also easily broken if too much pressure is applied to them.
Light weight screens and the stainless steel and Inconel mesh around bulb seal gaskets are easily
Par 805 Page 169 torn if care is not taken with them. Fortunately, most vent screens are heavy stainless steel and not easily damaged.
(2) Deformation. Connector receptacles attached to structure and brackets may be deformed, or the brackets deformed, if the mounting screws are over-tightened, or if the bracket is not thick enough. Beryllium copper spring fingers can also be bent or deformed such that they do not contact the structure with enough pressure to seal the joint against EMI. This can also be true for bulb seals and conductive elastomer gaskets if they are pressed past the point where they can spring back (compression set).
(3) Corrosion. Corrosion is one of the most common problems associated with EMI protection for two reasons: finishes over metal are often removed during bonding preparation to ensure good electrical conductivity, and many conductive materials used for EMI protection system are dissimilar to the aircraft structure. The two types of corrosion which can occur are electrolytic and galvanic. Chapter 2 provides a discussion on these types of corrosion.
Electrolytic corrosion occurs when a net direct current (DC) is applied between metals in the presence of a conducting fluid (electrolyte). The rate of corrosion depends upon the amount of current and the nature of the electrolyte. Galvanic corrosion occurs when a potential difference exists between two dissimilar metals in the presence of an electrolyte causing a current to flow.
The current is made up of electrons sacrificed from one of the metals. The rate of corrosion depends on the electrochemical potential between the two metals and the conditions under which contact is made. Most corrosion associated with EMI protection is galvanic since dissimilar metals are used. Table 2-3 lists materials with respect to galvanic corrosion potential. The closer together the materials are on the list, the less potential difference can develop between them when they are placed together with an electrolyte. Consequently, corrosion will occur slowly.
Materials at the extremes of the list will develop a high potential difference when placed together. The materials at the top of the list will be the ones to sacrifice electrons and will exhibit corrosion. Unfortunately, aluminum, which is used most often in aircraft structure, is higher on the list than materials used in EMI protection devices. Consequently, it is important to inspect areas where EMI protection is installed to ensure structural components are not corroding. In some cases, to protect the aluminum aircraft structure from corrosion, a sacrificial material such as tin/zinc is applied to the structural side of the EMI joint. The EMI gasket or spring fingers contacting this material make the required metal-to-metal contact for EMI protection and at the same time protection is provided to the aluminum structure. Consequently, corrosion at the EMI joint will be slower and the sacrificial material can be replaced before corrosion attacks the aircraft structure. Because an electrolyte is required for corrosion to occur, environmental seals are often used in conjunction with EMI protection to prevent moisture from contacting the metal-to-metal EMI joint. Conductive coatings on aluminum, such as alodine coatings, should be used, instead of nonconductive anodize coating.
b. Inspection Procedures. Inspection procedures for EMI protection devices and associated corrosion include visual inspections and EMI testing.
(1) Visual Inspections. The condition of avionics enclosures, bonding straps, shields, shield terminations, structural joints, gaskets, spring fingers, and conductive coatings on composites may be assessed during visual inspections. Wherever EMI protection is installed, it is imperative that periodic visual corrosion inspections be performed. Where sacrificial coatings are
Par 805 Page 170 used, inspection and re-application will be necessary on a periodic basis as well. It may be necessary to remove some EMI protection to inspect aircraft structure for corrosion if there is a history of it or if the aircraft has been exposed to salt spray. Electronic equipment bonding pressure-wiper fingers and bonding straps should be visually inspected when the equipment is removed and replaced. Broken or damaged bonding devices should be replaced. Metal overbraid on wiring in external areas such as wheel wells should be periodically inspected for corrosion.
The limits for corrosion on EMI protection features is very dependent on the aircraft and system design, and should be specified in the aircraft and component maintenance manuals.
(2) EMI Testing. Some EMI protection failures cannot be detected through visual inspection. An example is wire overbraid which is covered with an opaque jacket. Shield corrosion under the jacket cannot be detected visually. Compression set may occur in conductive elastomer gaskets, on avionics enclosures, where the gasket has been deformed to the point that it no longer seals against EMI. Structural bonding for P-static and lightning protection cannot be visually inspected without major aircraft disassembly. High-impedance bonds between electronic equipment and aircraft structure also cannot be visually detected. Circuit components such as filters should be tested to ensure correct operation. Electromagnetic vulnerability testing, in which the aircraft is radiated with EMI and aircraft electronic systems are monitored for failures, is costly and unlikely to be conducted unless major problems are suspected. Even simpler EMI testing requires some specialized equipment. Consequently, unless in-flight EMI problems are reported, testing will probably not be conducted. The following describes some types of testing which may be conducted.
(a) DC Resistance Test. Bonding of electronic equipment to aircraft structure is commonly verified by measuring the DC resistance between the equipment case and the aircraft structure with a low resistance ohmmeter. In order to make an accurate measurement paint must be removed from the equipment case structure where the measurement is taken. These areas must then be refinished. DC resistance is not an accurate indication of how well the bond performs at alternating current (AC) frequencies. Rather than measuring DC resistance to verify a bond, it is recommended that the bond be cleaned of contaminants or corrosion, which may cause high resistance on a regular basis.
(b) Shield Continuity Tests. DC resistance measurements may be used to measure the resistance and continuity of wire shields and shield terminations. Again, in order to make an accurate measurement, the insulation or finishes over the shields and terminations must be pierced or removed. DC resistance measurements may not effectively detect high resistance shields or terminations if there are multiple conductive paths for the shield. This is particularly true for shielded wire bundles with multiple branches and connectors.
(c) Interfering Signal Analysis. For EMI emissions, many times the most effective means of determining the source of the emissions is by using a spectrum analyzer and appropriate probes. The spectrum analyzer is used to measure the frequency and amplitude of the interference. Clamp-on current probes may be used to detect conducted emissions on wires and wire bundles. Small receiving antennas may also be used to detect radiated emissions. The spectrum analyzer may also be connected to the antenna port at the susceptible RF receiver rack connector to measure the level and frequency of the interfering signal. Training is required to
Par 805 Page 171 properly set up and monitor the spectrum analyzer, and to pick out the EMI interference from normal internal and external electromagnetic fields.
(d) P-Static Test. Structural bonding degradation is usually suspected when there is static in communication systems. A P-static test can be conducted to determine where the degradation has occurred. This test deposits a simulated P-static charge on the aircraft using a handheld ion discharge wand. As the wand is moved slowly across the structure, a receiver detects the noise associated with degraded bonds. Once the area of the bond degradation is located and isolated by visual inspection, a reason for the problem should be determined. The P-static test is a high voltage test which requires special training to ensure aircraft and personnel safety.
(e) Transient Anomalies. Electronic equipment EMI protection circuit component failures usually cannot be isolated on the aircraft and may be transparent to equipment operation if no EMI is present. Periodic anomalies associated with electronic equipment operation may be EMI related, and the equipment should be replaced and sent for diagnostic testing and repair.
c. Repair Procedures. Repair procedures related to EMI protection and associated corrosion depend on the type of EMI protection involved, and the degree and type of corrosion.
Beryllium copper gaskets, spring fingers, and bonding pressure-wiper fingers that are broken or damaged beyond tolerable limits should be replaced. Damaged bulb seals and ground straps should also be replaced. Since metal-to-metal contact is required, surfaces should be cleaned carefully to remove any primer, paint, grease, or corrosion prior to replacement. In some applications, after the replacement of EMI protection, an environmental seal is installed to protect the metal-to-metal joint from corrosion. Torn screens may be repaired by stitching them together as long as none of the metal mesh is missing. Conductive elastomers may have damaged sections replaced without replacing the entire gasket. Sections of metal wiring overbraid can be replaced if corroded or damaged. Maintenance personnel should refer to the applicable Original Equipment Manufacturer’s (OEM) service directives for specific repair information. The following general instructions apply when corrosion is noted:
(1) When corrosion is observed, disassemble or remove only the affected area;
(2) Remove corrosion using the mildest available method; and
(3) Apply protective finishes, and assemble as required.
NOTE: See Chapter 4 for cleaning and surface preparation; Chapter 5 for corrosion removal, surface treatment, painting, and sealing; and Chapter 6 for treatment of specific installations.
806 through 900. RESERVED.
Par 901 Page 172 CHAPTER 9. EFFECT AND TREATMENT OF CORROSION ON ELECTROSTATIC
DISCHARGE SENSITIVE EQUIPMENT
901. DEFINITION/DESCRIPTION OF ELECTROSTATIC DISCHARGE (ESD). ESD is a transfer of an electrical charge between bodies of different electrostatic potentials. ESD is caused by direct contact or induced by an electrostatic field. Static electricity is a potential electrical charge at rest. Static electrical charges can accumulate on electrical insulators and ungrounded conductors. A static electrical charge will build up as a result of triboelectric (frictional) activity which transfers electrons from one material to another. When two materials are physically rubbed together or pass close to each other, or where one material flows relative to another (such as a gas or liquid over a solid), electrons are transferred between the materials. One of the materials gains electrons and becomes negatively charged while the other loses electrons and becomes positively charged. A potential difference exists between the negatively and positively charged objects and between the charged objects and ground. This potential difference may increase due to a change in capacitance as the objects are moved. Capacitance is defined as the ratio of the magnitudes of the total charge on either conductor to the potential differences between conductors. The equation, Charge = Capacitance x Potential Difference, illustrates this relationship. Since the charge is constant, the potential difference will increase as the capacitance decreases. For example, when a polyethylene bag is rubbed, the potential difference between it and another charged object or ground may be only a few hundred volts while it is laying on a bench. However, when it is picked up the potential difference may increase to several thousand volts due to a decrease in capacitance. As the capacitance of a charged object decreases, the potential difference between it and another object will increase until an electrostatic discharge occurs via an arc. Static charges discharged near sensitive electronic components may cause damage. The electrical potential generated during discharge can be as high as 25,000 volts; more than 1,000 times the minimum voltage required to damage the most sensitive electronic components. The threshold of sensitivity for a human to feel a static discharge is approximately 3,500 volts. Therefore, ESD-sensitive components can be damaged by maintenance personnel without their knowledge.
902. SOURCES OF STATIC CHARGE.
a. Triboelectric Series. A triboelectric series, as seen in Table 9-1, is a list of materials and substances in order of positive to negative charging as a result of the triboelectric effect. A substance listed higher on the triboelectric series list develops a positive charge when rubbed with the lower listed substance. This happens because the higher listed substances have more free electrons compared to lower listed substances. Electrons from those substances positioned higher on the list may be transferred to substances positioned lower on the list when the charge is high enough. The order of ranking in a triboelectric series is not always constant or repetitive and the degree of separation of the two substances in the triboelectric series does not necessarily indicate the magnitude of the charges created by triboelectric effect. The magnitude of the charge is dependent upon numerous properties and the nature of the material or substance. These properties can also be modified by other factors such as surface cleanliness, ambient environmental conditions, contact pressure, speed of rubbing or separation, lubrication, and the amount of surface area over which the rubbing occurs. Table 9-2 lists some common sources of static electricity.
Par 902 Page 173 TABLE 9-1. TRIBOELECTRIC SERIES (PARTIAL) Most Positively Charged Air Human hands Glass Mica Human Hair Nylon Wool Fur Lead Silk Aluminum Paper Cotton Steel Wood Hard Rubber Nickel Copper Brass Silver Gold Platinum Acetate Rayon Polyester Celluloid Orlon Polyurethane Polyethylene Polypropylene PVC (vinyl) Silicon Teflon More Negatively Charged