Primarily, environmental design characteristics have been defined in terms of shock, vibration, relative humidity and temperature requirements. Laboratory testing is utilized during the full scale development phase on prototype equipment and during the production phase on equipment to demonstrate compliance with such requirements. The actual maintenance environment to which the equipment will be subjected during fleet service includes a variety of fluids as well as numerous sources of gross fluid intrusion into (and on) the avionic equipment and components. Complexity of accurate laboratory simulation of the attacks posed by these various fluids, especially if consideration is given to the inevitable loss of seal effectiveness, has made it impractical, to date, to provide a laboratory analog of the maintenance environment relative to fluid induced deterioration and failures. This maintenance environment is being described so that the design engineer may recognize and prevent reliability problems that are otherwise not described by specifications and laboratory testing.
NON-MISSION CAPABLE PERIODS
Most often because of shortages of spare parts, it is not unusual for an aircraft to be non-flyable for up to sixty days at a time while awaiting parts. Such an aircraft is subject to controlled cannibalization, and generally power cannot be applied to the aircraft for the purpose of operating the avionic systems and equipment. As a result, moisture collecting in a system or equipment from rain, ocean spray, or diurnal condensation may be very slow to dissipate. Such static periods in the characteristically damp and saline naval environment provide the potential for significant undetected corrosion. Normally, OMA corrosion control teams will put considerable effort on aircraft that are in a non-flying status. These teams basically inspect for evidence of corrosion, then repair and preserve as appropriate. If a corrosion condition is not visible to an external visual inspection it will not be corrected. OMA avionic maintenance technicians are not authorized to open a WRA (Weapons Replaceable Assembly) and internal corrosion in such a unit always will go undetected until failure forces removal and transfer of the WRA to the IMA for repair.
Normally, the time deployed on a ship constitutes the minority of the service life of an aircraft and its installed equipment. However, some aircraft are based for years on shore stations such as Guam or Key West where the environment is only slightly less severe than that for a ship based aircraft. There are very few major naval air bases that are not located in a marine atmosphere. Even the six Naval Air Rework Facilities (depots) are all located immediately adjacent to salt water. In addition, four of these depots are located close to cities with significant industrial gas emissions. An equipment design must both minimize susceptibility to fluid intrusion as well as limit corrosion in the presence of the industrial and marine atmospheric contamination in order to achieve increased reliability and reduced cost of ownership.
GROUND EQUIPMENT OPERATION
The amount of time equipment is operated on the ground increases the vulnerability of the system to the elements. Ground operation time varies widely between types of equipment depending upon reliability and troubleshooting time. It is not unusual for ground operating time to exceed airborne operating time. The following are potential problems associated with increased ground operation time:
a. The involved systems will be opened (radomes up, equipment bay doors open, canopies raised, etc.) for extended periods.
b. Maintenance can damage seals, the locking integrity of fasteners, scratch protective finishes and otherwise impart a form of maintenance wear deterioration to the system.
Ideally, all uninstalled avionic equipment is stored and handled in shock and moisture resistant packaging complete with an active desiccant. The following are normal conditions:
a. Equipment with a high replacement rate is kept relatively unprotected on the flight deck, (aft of the island) available for quick installation.
b. Equipment may be moved several miles on an open flat bed vehicle between the OMA (removing/installing activity) and the IMA repair shops.
c. Equipment sits on pallets exposed to the environment awaiting transfer to the IMA or depot activity.
At the depots all MTR (Mandatory Turn-in Repairables) are immediately screened upon receipt and repackaged, as necessary, to prevent further deterioration while awaiting induction for repair. Depending upon demand, equipment may await induction at a depot for a year or more. It has been estimated by depot engineering personnel that approximately 30% of the equipment returned for repair arrives in such bad condition that it is beyond economical repair. Corrosion and handling damage are about equal as causes for the destruction of the returned equipment. Some of the same design features that support corrosion susceptibility in equipment installed in aircraft also lead to destructive corrosion in inadequately packaged retrograde units. Design with unsealed bimetallic couples is an example. Not only is readiness adversely affected by the loss of these repairables, but the cost of replacement equipment is a multi-million dollar annual expense.
In order to combat airframe corrosion, the salt spray deposits that are characteristic of the marine environment are washed off of naval aircraft frequently. Under normal conditions, washing with a mixture of Cleaning Compound, (MIL-C-436 16), and fresh water is performed every 14 days. The pre-wash procedures include taping of the vents, air inlets and other potentential sources of water intrusion during the washing process. The following are:
a. Some of the oils, fuel residue, and other surface contaminations act to preclude good adhesion of the tape, and prior cleaning of these areas often is not performed.
b. Many of the doors in service will have deteriorated or damaged seals and, since such a condition is not evident to an outside observer, the leaky doors are not taped.
c. The airframe will be subject to leaks past some fasteners and at some seams.
d. Surfactants in the washing compound reduce surface tension and allow high penetration by the washing solution throughout the aircraft.
Due to limitations on fresh water aboard ships it is not always possible to wash deployed aircraft with the same frequency as when shore-based. Because of the limited space and lack of special washing facilities, there is more hand washing (with less taping) of the aircraft. Another special aircraft washing situation involves those aircraft which are shore-based but per form frequent low level flights over salt water. These aircraft may go through a "birdbath" following such flights. This type of washing involves pressure hosing from all angles, including below. While this provides thorough external rinsing, it also tends to introduce water into the interior of the aircraft by penetration of normal drain holes by the upward directed jets, as well as penetration past any marginal compartment seals and through vents. This form of washing removes external dirt and salts from the airframe surfaces, but can act to drive moisture and contamination deeper into internal recesses.
GROUND SUPPORT EQUIPMENT AIR CONDITIONERS
It is not unusual for a GSE (Ground Support Equipment) air conditioning unit to contain residual condensate prior to use on an aircraft system. The unit is supposed to be purged of all moisture prior to being connected to the aircraft, but, if this step is omitted in the rush to complete maintenance or due to inadequate training, the residual moisture will be injected into the aircraft cooling system. Avionic systems whose design contain water traps are difficult to clear of the gross amounts of water which can be ingested in this manner. Another problem related to the use of GSE air conditioning units involves the concurrent cooling of equipment not undergoing maintenance while work is being done on other items cooled from the same system. The non-operating equipment can be cooled to the point that condensation will take place.
LACK OF CORROSION INFORMATION IN EQUIPMENT PUBLICATIONS
Adequate maintenance and repair analyses related to control of corrosion effects have not been performed, nor will they be, until the full nature of the corrosive environment is recognized and considered. Only when the development activity includes complete and realistic corrosion control guidance in the maintenance publications will the fleet actions required to minimize repeating reliability and maintainability weaknesses be accomplished.
The individual equipment publications, such as the NAVAIR 16-30-XX, NAVAIR 16-35-XX or NAVAIR 16-45-XX, contain very little information concerning corrosion repair and preventive measures to be applied on various parts of a unit. General guidance on the identification, repair, and prevention of corrosion in avionics systems and equipment is contained in the NAVAIR 16-1-540, Avionic Cleaning and Corrosion Prevention/ Control" Manual. While this publication identifies the materials and procedures to be used in a maintenance program to minimize avionics corrosion, currently there are very few requirements in the specific aircraft or equipment publications that specify the application of these materials and procedures to the particular equipment. Since most of the maintenance manuals pre-date NAVAIR 16-1-540, it is likely that materials and procedures specified in an individual manual may be in conflict with some provisions of the general manual. A NAVAIR policy statement in NAVAIR 16- 1-540 resolves such conflicts: "This manual shall be used in conjunction with, and in support of, the appropriate MIM (Maintenance Instruction Manual) and SIM (Service Instruction Manual). However, in the event of conflict between this manual and the materials and procedures specified for a specific system, the materials and procedures contained in this manual shall take precedence."
DIFFICULTY IN IDENTIFYING MOISTURE SOURCE FOR CORROSION
Just as the corrosion is difficult to find, the source of the moisture that is necessary for the formation of corrosion can be difficult to locate. Moisture deposited by condensation may disappear by evaporation. A good dynamic seal can be expected to weep slightly during system operation, yet may remain completely dry during non-operating (static) periods. Thus, the specific paths followed by many of the fluids present in an aircraft may be hard to determine, but their presence and effects on seals and other parts are very real. Based on the evidence of general fluid intrusion throughout naval aircraft and the adverse effects of such fluids, it is considered essential to design using materials and techniques to minimize susceptibility to such fluids, while also attempting to minimize the movement and intrusiveness of such fluids.
As noted in the discussion of the term C/ F/ M, thin oxide, sulfate or nitrate films may result in insulative layers that can alter circuit characteristics. Damp products of corrosion can provide leakage paths, just as can moisture itself. Multicontact connectors have long been a source of system failures due to moisture effects within the connectors, and now the corrosion and films on PWB (Printed Wiring Board) edge connectors have become principal causes for WRA functional failures. The amount of corrosion or the thickness of a film required to cause such avionic failures can be microscopic in some cases. This, combined with the hidden nature of the critical areas susceptible to moisture-induced failures, makes it almost impossible to inspect and identify a specific spot of corrosion, a film, or a drop of water as the culprit in a failure. Cleaning of connectors, for example, restores a WRA or a system to an operational condition, and circumstantial evidence shows that minor corrosion or a cleanable film on one or more of the connectors was causing a failure. But seldom is the particular area or content of the damaging film identified. Unlike airframe corrosion, avionic corrosion induced failures cannot be precluded by periodic visual inspections with appropriate repair based on the timely discovery of corrosion.
CLEANING AND CORROSION CONTROL/PREVENTION MATERIALS AND TECHNIQUES
Design for maintainability, with corrosion control and prevention being but a facet, must be based on knowledge of what the maintenance technician is supposed to do, the tools and materials he uses, and his problems as related to design features. In these areas the reliability and maintainability engineers must work closely with the design engineers from the initial selection of materials and layout of components. A design decision to utilize a specific type of edge connector, for example, should include provisions in the maintenance plan for cleaning and preserving the male and female connector assembly.
There are now cleaning and corrosion removal techniques available to fleet technicians ranging from hand scraping to mini-abrasive tools to ultra-sonic cleaners. The materials, equipment and procedures are described in the NAVAIR 16-1-54.0, "Avionic Cleaning and Corrosion Prevention/Control" Manual. The designer should be aware of these materials so that the new design does not include incompatible materials. Similarly, recognizing that equipment will be cleaned in a ultra-sonic tank or detergent spray booth, the designer can avoid designs that would entrap cleaning fluids and greatly prolong an oven drying process. Components that are sensitive to the procedures described in NAVAIR 16-1-540 should be mounted where they may be easily removed or, in some cases, masked during the cleaning and drying processes. It is important to be aware that a vast majority (over 90%) of repairable failed WRAs are returned to service by the IMA technicians.
AVIONIC CORROSION TRAINING
A design activity should consider the fleet maintenance practices in the initial design of new equipment and design to be compatible with these fleet practices. Maintenance by the fleet avionics technician is not a proper remedy for poor design! The training of fleet avionic technicians relative to the identifications, removal, repair, preservation and reporting of corrosion damage is based upon NAVAIR 16-1-540. A broad one-week training course and the manual provide for initial understanding, but they do not qualify fleet avionic technicians to identify and report the more subtle C/F/M failure modes.
One of the results of this broad training is the fleet response to perceived corrosion induced equipment failures. The units are cleaned, tested, then sealed as completely as possible before being returned to service. This sealing may be quite inconsistent with the design cooling characteristics, yet the evidence of moisture intrusion and the apparent need for corrective action will dictate such a fleet response.