2.2 Design and Test Specification Interpretations
Design of avionic equipment requires a large number of
bimetallic (galvanic) couples. In many cases one of the metals in the
galvanic couple may be a very corrosion prone active metal. Table 2-1 lists
the metals most commonly used in avionic equipment, and Table 2-2 lists metals in order of their relative activity in a sea water environment. Recognizing that the dissimilar metal couples are common in avionic equipment, it is essential that the design engineer apply a very critical interpretation of the Military Specifications and Standards requirements. Where design options are based upon an expected degree of environmental severity, any question should be resolved on the side of the more severe environment.
Table 2.1 Metals
Most Commonly Used in Avionic Systems
|Gallium||Stainless Steel (CRES)|
|* Usually considered corrosion-resistant|
A design assumption as to the expected environmental severity leads to the specific limitations that are placed on the selection and use of dissimilar metals. The Requirement 16, "Dissimilar Metals" of Standard General Requirement for Electronic Equipment" MIL-STD-454,3 provides for the selection of metals for use in electronic equipment to be in accordance with the requirements of "Dissimilar Metals", MIL-STD-889,4 Interpretations are necessary in applying MIL-STD-889 to non-hermetically sealed avionic equipment since this Military Standard is applicable to all military equipment parts, components and assemblies. An assumption that cockpits, equipment bays and cabins are "protected environments" is not valid in the design of avionic equipment to be installed in these areas of a naval aircraft under the definitions of MIL-STD-889.
Recognition that the conditions in fleet aircraft do not
meet the "protected environments" criteria of MIL-STD-889 leads to the valid
requirement that "...dissimilar metal contacts should be treated as if
protection were required against the worst environment." Similarly, in the
interpretation of MIL-STD-889, the design engineer should never assume the "...
absence of wet, saline or acidic conditions" or that avionic equipment will be
"... in mild atmospheres in absence of saline, alkaline or acidic
conditions." In summary, design of avionic equipment should be based on
the assumption that moisture will get in and on all non-hermetically
sealed avionic equipment in fleet service!
LABORATORY TEST ASSUMPTIONS
There is a continuing effort to develop better accelerated laboratory tests that would provide measures of susceptibility to corrosion that could be correlated with the corrosion that occurs in fleet deployed equipment. This is a very complex problem due to the number of factors that can contribute to the corrosion process. Only limited acceleration of the process currently appears feasible. Thus, the destructive process that takes place in service, especially during the ambient static ( non-operating period which is approximately 90% of the equipment life, currently is not subject to valid and economical (short term) simulation in the laboratory. Humidity and salt fog tests, such as in "Environmental Test Methods", MIL-STD-810,5 provide means of evaluating the uniformity and relative degree of corrosion resistance of various types of protective coatings, but, as noted in the Military Standard, do not duplicate a marine environment nor guarantee a test item will survive under fleet conditions.
Since the current corrosion related environmental tests do not, in fact, prove resistance to the fleet environment, it is important that equipment not be designed with the primary objective of merely passing contractually required laboratory tests or reliability demonstrations that are not representative of fleet conditions. Of course these tests must be passed as a minimal measure of resistance to a particular form of corrosion, but the equipment design must include features that will make the assembly resistant to the real, even though not laboratory testable, conditions in fleet service. Most laboratory tests used to demonstrate compliance with "Electronic Equipment, Airborne, General Specification for", MIL-E-5400,6 and "Standard General Requirements for Electronic Equipment", MIL-STD-454, are considered successful per a test specification such as "Testing, Environmental, Airborne Electronic and Associated Equipment", MIL-T-5422,7 if the equipment operates within the performance requirements when power is applied at the end of the test. Equipment undergoing such tests also should be analyzed for evidence of success of the various design features intended to minimize the inevitable fluid intrusion. The design should control fluid migration, provide drainage, seal bimetallic couples, be free of moisture traps, etc. While not definitive, such additional analyzes can provide indications of potential design weaknesses when the equipment is ultimately deployed into the very wet and intrusive fleet environment.