Par 702 Page 153 TABLE 7-2. HARDWARE FOR NUT PLATE BONDING OR GROUNDING TO FLAT
SURFACE Structure Screw or Bolt and Nut Plate Rivet Lockwasher Washer A Washer B
ALUMINUM TERMINALS AND JUMPER Aluminum Alloys Cadmium-Plated Steel Aluminum Alloy Cadmium- Plated Steel Cadmium-Plated Steel Aluminum Alloy Magnesium Alloys Cadmium-Plated Steel Aluminum Alloy Cadmium- Plated Steel Cadmium-Plated Steel Magnesium* Alloy Steel, Cadmium-Plated Cadmium-Plated Steel Corrosion- Resistant Steel Cadmium- Plated Steel Cadmium-Plated Steel Aluminum Alloy Steel, Corrosion- Resistant Corrosion- Resistant Steel Corrosion- Resistant Steel Cadmium- Plated Steel Cadmium-Plated Steel or Aluminum Alloy Cadmium-Plated Steel
TINNED COPPER TERMINALS AND JUMPER Aluminum Alloys Cadmium-Plated Steel Aluminum Alloy Cadmium- Plated Steel Cadmium-Plated Steel Aluminum Alloy Magnesium Alloys Caution: Do not connect copper to magnesium Steel, Cadmium-Plated Cadmium-Plated Steel Corrosion- Resistant Steel Cadmium- Plated Steel Cadmium-Plated Steel Cadmium-Plated Steel Steel, Corrosion- Resistant Corrosion- Resistant Steel Corrosion- Resistant Steel Cadmium- Plated Steel Cadmium-Plated Steel Corrosion- Plated Steel or Corrosion- Resistant Steel *When not available, use aluminum alloy.
Par 702 Page 154 FIGURE 7-3. BOLT AND NUT BONDING OR GROUNDING No tes:
*See Table 7-3 for specific materials.
†Limited to a quantity of four (4).
Par 702 Page 155 FIGURE 7-4. COPPER JUMPER CONNECTOR TO TUBULAR STRUCTURE Notes:
*Inside surface of clamp (conduit) is cylindrical.
Par 702 Page 156 TABLE 7-3. HARDWARE FOR BOLT-AND-NUT Structure Screw or Bolt and Nut Plate Rivet Lockwasher Washer A Washer B
ALUMINUM TERMINALS AND JUMPER Aluminum Alloys Cadmium-Plated Steel Cadmium-Plated Steel Cadmium-Plated Steel or Aluminum Alloy None Cadmium-Plated Steel or Aluminum Alloy Magnesium Alloys Cadmium-Plated Steel Chromium- Plated Steel Magnesium Alloys Magnesium* Alloys Cadmium-Plated Steel or Aluminum Alloy Steel, Cadmium-Plated Cadmium-Plated Steel Cadmium-Plated Steel Cadmium-Plated Steel Cadmium-Plated Steel Cadmium-Plated Steel* Steel, Corrosion- Resistant Corrosion- Resistant Steel Corrosion- Resistant Steel Cadmium-Plated Steel Cadmium-Plated Steel Cadmium-Plated Steel
TINNED COPPER TERMINALS AND JUMPER Aluminum Alloys Cadmium-Plated Steel Cadmium-Plated Steel Cadmium-Plated Steel Aluminum Alloy Cadmium-Plated Steel Magnesium Alloys Caution: Do not connect copper to magnesium Steel, Cadmium-Plated Cadmium-Plated Steel Cadmium-Plated Steel Cadmium-Plated Steel Cadmium-Plated Steel Cadmium-Plated Steel Steel, Corrosion- Resistant Corrosion- Resistant Steel or Cadmium- Plated Steel Cadmium-Plated Steel Cadmium-Plated Steel Cadmium-Plated Steel Cadmium-Plated Steel or Corrosion- Resistant Steel *When not available, use aluminum alloy.
FIGURE 7-5. BONDING CONDUIT TO STRUCTURE
CADMIUM PLATED AN 735 CLAMP* STEEL SCREW ALUMINUM WASHER
STRUCTURE
- ALUMINUM WASHER
CADMIUM PLATED
STEEL LOCKWASHER
CADMIUM PLATED
STEEL LOCKNUT NOTE: Aluminum alloy or corrosion-resistant steel conduit (inside surface).
FIGURE 7-6. ALUMINUM JUMPER CONNECTOR TO TUBULAR STRUCTURE
CADMUIM PLATED
STEEL SCREW AN 735 CLAMP* ALUMINUM WASHER
ALUMINUM TERMINAL
ALUMINUM WASHER
CADMIUM PLATED
STEEL LOCKWASHER
CADMIUM PLATED
STEEL LOCKNUT NOTE: Inside surface of clamp (conduit) is cylindrical.
Par 702 Page 157
Par 702 Page 158
b. Preservation. Preservation provides a nonconductive film to protect the bonding or grounding connection from corrosion by excluding moisture. Three preservation methods are discussed. See Chapter 4, paragraph 407, and Table 4-4 for more information.
(1) This application is intended for bonding or grounding connections that require protection for up to a year when the connection is covered and protected from the outside environment. Apply a thin film of Water-Displacing Corrosion Preventive Compound to the bonding or grounding connection, conforming to MIL-DTL-85054.
(2) This application is intended for bonding or grounding connections that may require fairly frequent disassembly. Apply a thin film of Water-Displacing Corrosion Preventive Compound to the bonding or grounding connection, conforming to MIL-PRF-81309, Type III, followed by a coating of Corrosion Preventive Compound, conforming to MIL-PRF-16173, Grade 4.
(3) This application is intended for bonding or grounding connections that seldom require disassembly. Using a spatula and Sealing Compound Type II, fully encapsulate the bonding or grounding connection, conforming to MIL-PRF-81733.
c. Electronic Equipment Shock Mount Bonding and Preservation. This type of electrical bonding uses a bonding wire (jumper assembly) or strips of aluminum or copper. The following corrosion prevention method applies:
(1) Clean base of shock mount and bonding wire (jumper assembly) or strips of aluminum or copper by wiping with a clean cloth or cheesecloth dampened with Isopropyl Alcohol, TT-I-735. Allow components to air dry.
(2) After assembly of the shock mount and bonding wire (jumper assembly) or strips of aluminum or copper, apply a thin film of Water-Displacing Corrosion Preventive Compound over the shock mount and jumper assembly attach area, conforming to MIL-PRF-81309, Type II.
For protection of up to a year and for shock mount and bonding wire assemblies that do not require frequent disassembly, apply an additional coat of Water-Displacing Corrosion Preventive Compound over the shock mount and jumper assembly attach area, conforming to MIL-DTL-85054.
703 through 800. RESERVED.
Par 801 Page 159 CHAPTER 8. EFFECT AND TREATMENT OF CORROSION ON
ELECTROMAGNETIC INTERFERENCE SHIELDING DEVICES
801. DEFINITION/DESCRIPTION OF ELECTROMAGNETIC INTERFERENCE (EMI).
a. EMI is the presence of undesirable voltages or currents which appear in a circuit as a result of the operation of another electrical source. EMI includes effects from lightning, external radiated radio frequency (RF) fields, or conducted and radiated electromagnetic interference between systems in the aircraft. In this section, the term EMI will include all of these effects.
Some examples of EMI-related aircraft system malfunctions are microprocessor bit errors, computer memory loss, audio tones on communication systems, and false indications
(i.e., alarms, lights, readouts, or power loss). The results of such malfunctions can severely impact system or subsystem operation. EMI may be radiated or conducted. Typical sources of radiated emissions are radio and radar transmitters, power supplies, generators, and transformers.
The way in which external EMI intrudes into a circuit is called the coupling mode. Radiated EMI propagates through the air from the source to the victim circuit. An antenna, or a cable which acts as an antenna, couples the EMI to the victim circuit. Conducted EMI is coupled from the source to the victim circuit between common connections, either wiring or metallic structure.
b. Due to the increase in electronic systems installed in modern aircraft, the importance of many of these systems for flight safety, and the decrease in power levels required to upset them, EMI, lightning, and high intensity radiated field (HIRF) protection has become an essential part of aircraft design. The use of electrical and electronic systems for full-authority aircraft flight and engine controls has been a significant factor in increasing the importance of aircraft EMI protection. The aircraft system EMI protection will involve the structure around the system, wire routing, shields over the wires, shield terminations, filters inside and outside the equipment, circuit and equipment grounding, and circuit design. Most of these protection features rely upon low-resistance and low-impedance electrical bonds for wires, shields and structure, which often include dissimilar metals. For this reason, an understanding of the purpose of these devices, where they may be located, and corrosion-control processes are necessary knowledge for the aircraft maintainer.
802. EMI STANDARDS, TEST REQUIREMENTS, AND DOCUMENTS. Electronic components are designed to various specifications, depending on their application and the electromagnetic environments in which they are expected to operate. Most specifications include susceptibility requirements, the level of EMI in which the device is expected to operate successfully, and emission requirements, the limits of EMI that the device may generate. Aircraft and systems manufacturers may have their own EMI standards and requirements, but they are based on the following:
a. RTCA DO-160, Environmental Conditions and Test Procedures for Airborne Equipment.
b. AC 20-136, Protection of Aircraft Electrical/Electronic Systems Against the Indirect Effects of Lightning.
Par 802 Page 160
c. AC 20-53, Protection of Aircraft Fuel Systems Against Fuel Vapor Ignition Caused by Lightning.
d. DOT/FAA/CT-83/3, User’s Manual for AC 20-53A, Protection of Airplane Fuel Systems Against Fuel Vapor Ignition Due to Lightning.
e. AC 20-158, The Certification of Aircraft Electrical and Electronic Systems for Operation in the High-Intensity Radiated Fields (HIRF) Environment.
f. MIL-STD-464, Electromagnetic Environmental Effects Requirements for Systems.
g. MIL-HDBK-235, Military Operational Electromagnetic Environment Profiles.
h. DOT/FAA/CT-86/40, Aircraft Electromagnetic Compatibility.
803. AIRCRAFT SYSTEM EMI PROTECTION REQUIREMENTS. Although electronic and electrical equipment are usually tested individually for lightning, HIRF, radiated and conducted EMI, and EMI susceptibility and emissions, aircraft manufacturers should still ensure that the integrated electronic systems on the aircraft will operate successfully in the electromagnetic environment to which the aircraft will be exposed. Figure 8-1 and the following paragraphs describe some of the aircraft-specific EMI issues that should be addressed.
a. Intrasystem EMI Requirements. Intrasystem EMI refers to electrical or electronic subsystems within an aircraft interfering with one another. Even subsystems designed to similar emission and susceptibility requirements may have EMI problems when integrated in an aircraft.
This may be due to the location of the equipment on the aircraft, frequencies involved, cable routing, or bonding and grounding techniques. All of these factors should be addressed by the aircraft manufacturer in order to ensure a compatible design. In a worst-case situation, when intrasystem EMI cannot be avoided, operation of one system may not be possible while another is operating. Changes to electronic systems or installation of new systems can impact intrasystem electromagnetic compatibility (EMC) as well.
b. Intersystem EMI Requirements. Intersystem EMI refers to electrical or electronic systems external to the aircraft interfering with aircraft systems. These external systems emit various frequencies and power levels which are not always easily predictable. Protection from intersystem EMI can include various types of shielding for aircraft openings and wiring.
Par 803 Page 161 FIGURE 8-1. TYPES OF ELECTROMAGNETIC INTERFERENCE
c. L ightning.
(1) Effects of Lightning Strikes. The effects of lightning strikes on aircraft may be classified as either direct or indirect. The direct effects are largely structural. They are the burning, eroding, blasting, and structural deformation caused by lightning arc attachment, high pressure shock waves, and magnetic forces produced by the associated high currents. Indirect effects predominantly result from the interaction of lightning fields and currents with electrical equipment or wiring. Fuel ignition may result from either direct arc ignition, or indirect effects creating arcs or sparks. For the purpose of studying lightning effects, aircraft are typically divided into three major lightning attachment zones: direct attachment, swept stroke, and conducted currents or transfer. Testing with simulated high voltage strikes on aircraft scale models using various aspect angles can provide confidence in zone assignment. Zone 1 (direct attachment) includes areas where lightning current enters or exits the aircraft. Zone 2 (swept stroke) includes areas directly behind the direct attach points where the established ionized lightning channel is swept back over the aircraft surface as it flies through or away from the channel. Zone 3 (current transfer) includes areas where there is a low probability of direct
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.