This discussion has frequently recommended sealing, and at the same time stated drainage holes were essential. This apparent paradox is explained by a very basic design objective: seal the top and sides of all units to be protected and provide low point drainage for the range of attitudes expected for the unit, including in-flight and at-rest (static) conditions. Specific suggested configuration features follow:
Design on the assumption that moisture will get into the equipment! Except for hermetically sealed or pressurized equipment, this means protection from moisture effects must be provided inside of the equipment, and provisions must be made to let the water drain out of the equipment.
HERMETIC SEALING OF EQUIPMENT
Ideally, maximum resistance to C/F/M is achieved by the hermetic sealing (solder or glass fusion) of an equipment or component. The hermetic sealing must include a dry, inert gas atmosphere within the sealed box. This configuration results in a minimum of fleet maintenance on the equipment. Cooling requirements, size (rigidity), container penetrations for wiring and similar considerations generally preclude hermetic sealing.
PRESSURIZATION OF EQUIPMENT
The next greatest resistance to C/F/M is obtained through the use of a pressurized equipment housing or compartment. The introduction of pressurized dry air into a semi-sealed (organic seals vice solder or glass fusion) area greatly minimizes moisture intrusion while there is a positive pressure differential over ambient. During periods when positive pressurization is not available, however, there will be increased diffusion of ambient air into the housing or compartment. While this is a very effective means to minimize moisture intrusion, the additional weight required to achieve the necessary housing rigidity and pressure tight sealing has largely precluded the use of this configuration.
SEALING OF EQUIPMENT
For equipment with neither hermetic sealing nor
pressurization, protection from fluid intrusion is achieved by the application
of a polysulfide or an RTV (Room Temperature Vulcanizing) sealant.
Polysulfide is most resistant to fluid attack, but one part noncorrosive RTV
per MIL-A-46146 is easier to apply and remove while still providing fairly
good sealing of small areas. Clear types of RTV allow visual inspection
of the sealed surface; however, they are not as resistant to temperature and
operational fluids. Figure 5-2 is an example of a leaky equipment
housing that was sealed by RTV, only to have an operational fluid damage the
RTV along the top cover-to-housing mating surfaces. If an RTV has a
vinegar odor-DO NOT APPLY IT TO AVIONIC EQUIPMENT. This odor indicates
the RTV is corrosive. Table 5-1 lists RTVs to avoid (corrosive), as well as those known to be noncorrosive. Two part RTVs generally are used in deep cure applications (potting and encapsulating).
TABLE 5-1. CORROSIVE SILICONE SEALANTS, ADHESIVES AND COATINGS
ONE PART CORROSIVE RTVs
RTV 1890 RTV
RTV92-007 RTV 118
92-009 RTV 157
96-081 RTV Q4-2817
ONE PART NON-CORROSIVE
RTV 161 RTV 189 RTV
RTV 162 RTV 738 RTV 3145
TWO PART NON CORROSIVE RTVs
RTV 8112 with cat. 9858 & primer
RTV 8262 with cat. 9858 & primer SS4004
with cat. 9891 & primer SS4004
RTV 170A and B with primer
RTV 182 with SYLGARD
RTV 184 with SYLGARD
When access to equipment is through the top, use a "shoe box" type of lid with no penetrations through the top of the lid. Specifically, avoid countersunk or dimpled fastener holes. Fasteners through the skirt of the shoe box lid can provide security without collecting moisture. If fasteners through the top of the lid are unavoidable, ensure the area over each fastener is covered with a sealant.
EQUIPMENT HOUSING PENETRATIONS
Although sealing the sides and top of housings is a design objective, realistically there are many penetrations that are essential to the functioning and control of the enclosed equipment. The following design features can help to minimize fluid intrusion, however:
a. Use "O" rings for sealing around all control shafts that must penetrate into the enclosure.
b. Avoid the use of knurled wheels that intrude into the enclosure. Effective sealing around such wheels is extremely difficult.
c. Access plates utilizing a gasket for sealing are less subject to leakage when mounted on a vertical surface.
d. Mount electrical connectors horizontally (through the vertical sides). If it is essential to mount a connector vertically on the top of a housing, provide a slightly (at least 3/ 16") raised base for the connector mounting relative to the surrounding horizontal area.
e. Use an "L" type connector so the wiring enters horizontally into the upper portion of a vertically mounted connector.
f. For all multicontact connectors, wiring should lead upward toward the connector to prevent water from running along wire into the connector. Strain relief is essential to ensure no side loads exist on wires entering backshell grommets or potting.
ELECTRICAL CONNECTOR MOUNTING
A basic rule throughout the aircraft is to avoid mounting an electrical or coaxial connector vertically. If vertical mounting of a connector cannot be avoided, there should be a slight raised area for the connector base. A vertically mounted connector should be of the type with a right angle (elbow) on the top of the backshell so that water does not run down into the backshell of the connector. Vertically or horizontally mounted connectors should always have the cabling leading downward and away to prevent water from running along wires into the connector. This technique may require a drip loop in some cases. Failure to provide the slight additional cabling for a drip loop also denies spare cable that is needed when maintenance and/ or repair is required to the connector pins and cable. Drip loops also prevent stressing due to side loads on wiring entering the backshell. Both reliability and maintainability are enhanced through the presence of drip loops.
PRINTED WIRING BOARD MOUNTING
The PWBs, motherboards and feed-through connectors should all be located so they are well above the bottom of a housing (at least 1/2" clearance). In addition, mount PWBs vertically to prevent condensation and debris from collecting on the board. The edge connectors should be either on the vertical edges or mounted on the back of the PWB.
LOW POINT DRAINS
Provide low point drains so that water cannot collect
during normal flight attitudes or when parked on the ground. Drain holes
should be at least 1/4" diameter after the protective coating to the inside edge of the hole is applied. In the case of wing mounted avionic pods, consider the position of the pod when the wing is folded in determining the location of low points.
Avoid water traps or "bathtub" areas. Ensure that components are not shaped or mounted so that a "bathtub" area results. Frequently drain holes are impractical to correct such a condition. In this case, a polysulfide sealant such as MIL-S-8802 should be used to fill the depression. Alert design and careful inspection for residual water following introduction of water during drainage tests are required to preclude this problem. The potential for accumulation of condensate leads to water traps formed as a result of attempts to provide additional erosion or abrasion protection to wiring. Both convolute tubing and wiring boots will fill with water if adequate low point drainage is not provided.
Cooling air systems should include provisions for
removing both moisture and particulate matter (dust, dirt, lint, debris, etc.)
from the cooling air. This is of particular importance when the air
blows directly on the active electronic elements. Even with cold plate
cooling, however, blockage of the cooling air passages is a continuing problem
if air filtration is not provided. Water separators and air filters must
be located for easy access for servicing. Figures 5-3 and 5-4 are examples of plenum cooling systems that exhibit dirt and corrosion.
BONDING AND GROUNDING
Electrical bonding and grounding should be per MIL-B-5087 using straps, rather than "sliding housing-to-rack" or"tapered pin on housing-to-rack" electrical contacts. Straps should be located for easy access by maintenance personnel. Because of the bimetallic couples that are inherent with the use of straps, it is essential that a sealant be applied over each junction. If the junction is frequently disassembled Corrosion Preventive Compound, MIL-C-16173, Grade 4, provides easily applied protection. For longer term sealing or in an area subject to abrasion, erosion or external weather conditions, a polysulfide sealant such as MIL-S-81733 or MIL-S-8802 is appropriate.
A WRA installed in a compartment or an equipment bay should be mounted on a shelf or on mounting blocks of sufficient height to keep the base of the WRA well above (at least 1/2") the compartment floor.
Design with an awareness of the maintenance procedures and materials that will be utilized to control C/F/M in the fleet. These maintenance procedures and materials are included in the "Avionic Cleaning and Corrosion Prevention/ Control" Manual, NAVAIR 16-1-540. This requires that equipment be easily accessible for inspection and that detergent and solvent cleaning will neither harm the equipment nor result in trapped fluids. Design equipment that will not be damaged by ultra-sonic cleaning equipment. If components sensitive to ultra-sonic energy must be used, locate so the components are easily removable prior to the ultra-sonic cleaning process.
"DO's" AND "DON'Ts"
Tables 5-2 and 5-3
list, as a summary, the design configurations discussed in this Design Guide
that require consideration relative to C/F/M in avionics equipment.
TABLE 5-2 THE "DO's" OF EQUIPMENT DESIGN
- Design on the assumption that moisture and fluid will be present
in the airframe and equipment.
- Seal all dissimilar metal (galvanic) couples.
- Use paraxylylene as a conformal coating on printed wiring boards.
- Use an easily replaceable anodic (consumable) part in assembling
grounding or bonding connections.
- Electrically isolate graphite composite materials from avionic
- Use only electrical connector boots that can be sealed with
- Carefully select a protective system for use on magnesium.
- Complete the working of aluminum (drilling, cutting, grinding)
prior to surface treatment.
- Use surface treatments (anodize and conversion coatings) on
- Carefully select the metal plating used to provide sacrificial
protection, barrier protection, as a third metal between two otherwise
incompatible metals, or as a substitute surface.
- Use a nickel strike under gold plating.
- Use solder flux with lowest possible acid content.
- Use metallic materials with the most corrosion resistant
configuration (passivated) with minimum possible residual stressing.
- Use fluorocarbon or fluorosilicone type materials for gaskets, "O"
rings and seals.
- Seal conductive and EMI gaskets against moisture/fluid intrusion.
- Use low point drains.
- Mount equipment and components at least 1/2 inch above potential
standing water level.
- Use hermetic sealing where possible.
- Use polysulfide sealants to seal non-pressurized equipment where
moisture/fluid intrusion is possible.
- Use transparent (clear) conformal coating.
- Place ambient pressure sensing components outside equipment
- Design for maintainability.
- Use shoe box lids.
- Mount PWBs vertically with the edge connectors on vertical edge or
back of board.
- Mount electrical connectors horizontally.
- Use drip loops on electrical cables.
- Use desiccant systems with visual indicators.
- Use cooling systems that remove moisture and particulate matter.
- Use "O" rings to seal around control shafts that must penetrate an
- Be aware of various airframe interior and exterior fluids.
- Use electrical connectors with cork-and-bottle interfacial seals.
- Protect against insulative films.
- Recognize the operational environment.
- Recognize the maintenance environment.
- Be aware of fleet maintenance procedures and materials.
- Get the fleet maintenance technician's input.
- Listen to feedback!
TABLE 5-3 THE "DON'Ts" OF EQUIPMENT DESIGN
- Don't use dissimilar metal (galvanic) couples if it can be
- Don't use RTV that contains acetic acid.
- Don't place graphite in contact with aluminum or any
- Don't use heat shrink (non-sealed) electrical connector
boots to stop moisture fluid intrusion.
- Don't mate magnesium to a metal more cathodic than
- Don't use acrylic, RTV or varnish type conformal coatings.
- Don't use gold over silver or copper.
- Don't use organic materials that outgas, support fungi,
absorb moisture or are degraded by maintenance and operational fluids.
- Don't use an EMI gasket without a seal on both sides of the
- Don't use top mounted lid fasteners.
- Don't mount PWBs horizontally.
- Don't mount electrical connectors (multicontact or coaxial)
- Don't place edge connectors on the bottom edge of a
- Don't create side loads or cable tension on the rear seal
of electrical connectors.
- Don't use direct air cooling on active electronic
- Don't mount WRAs less than 1/2 inch above the compartment
- Don't use hygroscopic materials.
- Don't use nickel plated electrical connector shells.
- Don't use foam cushioning materials that can deteriorate
- Don't permit the presence of water trap