Corrosion Prevention and AC 43-206 Part 2
Introduction
In a previous Lectromec article, we discussed the preventative measures and identification methods for corrosion on avionics equipment identified in AC 43-206. This time, we address the advisory circular’s recommendations for corrosion maintenance and cleaning. A third article discussing some of the cleaning equipment/ tools, materials, and potential hazards identified in AC 43-206 will be posted in the near future.
As stated in Lectromec’s last article, corrosion is a natural and unavoidable process within the aircraft environment that can become destructive, expensive, and hazardous over time. Knowing this, measures to prevent such damage should be considered crucial to general aircraft maintenance.
General
After initial inspection and identification of corrosion in avionics systems, the next logical step is to perform cleaning procedures to reduce/eliminate the risks from corrosion. Inspection and cleaning procedures should be performed on a regularly scheduled basis, as well as immediately following any situation(s) in which the avionics components are exposed to probable sources of contamination (weather, salt spray, spilled fluid(s), cleaners used for nearby systems, any remnants from repair).
The primary reference to inform proper cleaning methods should be either the component or OEM instructional manuals. Such manuals should identify proper cleaning methods, equipment, and materials for the respective equipment. Any information from the applicable instruction manual regarding the frequency of cleaning for an avionics component should be integrated into the preventative maintenance program. Where possible, scheduling should align with other maintenance activities to reduce the total time needed and limit the number of times equipment might have to be removed for access.
All avionics components should be inspected and cleaned both externally and internally; this will typically require a maintenance technician to open avionics equipment in accordance with its instruction manual
Procedures
For any piece of avionics equipment, AC 43-206 recommends the mildest method capable of sufficiently removing corrosion and corrosive deposits. Thus, unnecessary risk of damage to the component is avoided. Such methods should be identified in the component or OEM manual and incorporated into the preventative maintenance program. Prior to any cleaning actions, considerations must be made regarding the appropriate equipment, materials, and solvents applicable to each piece of equipment to be cleaned.
In general, the cleaning procedure will consist of the following steps:
- Paint or preservative removal (if applicable)
- Cleaning
- Corrosion removal
- Treatment
- (Re-)application of preservative/paint
Note that it is common for layers of paint and/or preservatives to be removed and reapplied as part of typical maintenance; these freshly applied layers offer renewed protection against corrosive deposits. Adhesion of the re-applied coating material depends on the thorough cleanliness of the surface itself.
One of the primary methods necessary for adequate corrosion prevention and maintenance for avionics equipment is hand cleaning. Certain aircraft components are extremely sensitive to harsher cleaning methods and require specific hand cleaning methods to avoid additional damage during the cleaning process. Hand cleaning may also be appropriate when other methods are not available.
Additional cleaning methods often utilized for avionics equipment include ultrasonic scrubbing, abrasion of corrosive deposits, and water spray. Each of these methods is accomplished with machines or equipment specifically designed for its associated cleaning function. The next article in this series will provide further details of such equipment
| Component | Problem | Solution |
|---|---|---|
| APC connections (microwave) | Shock damage to center conductor | Seal and hand clean only |
| Crystal detectors | Heat damage from ovens | Dry at 130°F (54°C) maximum |
| Delay lines | Trapped solution in housing | Seal or remove |
| Fan motors | Trapped solution in housing | Seal or remove |
| Gyroscopes | Trapped solution in housing | Seal |
| Klystron cavity | Trapped solution in sockets | Remove tube and seal |
| Meters and instrument gauges | Trapped solution through open housing | Seal |
| Paper capacitors | Disintegrate | Seal |
| Potentiometers | Trapped solution in open housing | Seal |
| Printed circuit | Trapped solution (when installed) | Remove (clean separately) |
| Rotary switches | Trapped solution in open housing | Seal |
| Sliding attenuators (RF) | Trapped solution in slide housing | Seal or remove |
| Sliding cam switches | Shock damage to cam only | Remove or hand clean |
| Synchros and servos | Lubricant removed from bearing | Seal or remove |
| Transformers | Trapped solution in housing | Seal |
| Tunable cavities | Trapped solution in cavity area | Seal or remove |
| Vacuum tubes | Shock damage | Remove |
| Variable attenuators (microwave) | Trapped solution in housing | Seal or remove |
| Wave guide (microwave) | Trapped solution in housing (when installed) | Seal or remove |
| Wire wrap connections | Shock damage | Hand clean only |
Part of any cleaning procedure includes taking precautionary measures to protect equipment from unintentional damage. Sensitive areas that may be prone to entrap moisture or solvents during cleaning must be adequately protected from such entrapment. Table 4-3 in the AC is dedicated to identifying several components, damage to which each is susceptible, and recommended means of avoiding damage during cleaning.
Another key part of the cleaning procedure is drying after the component has been sufficiently cleaned; equipment such as hot air blowers and ovens are often utilized for this task. Cleaning instructions for each piece of equipment should include acceptable means of drying. Typically, one can expect factors such as the complexity of the avionics equipment and the humidity of the surrounding environment to impact the drying time.
Conclusion
There is a multitude of complex pieces of avionics equipment on a single aircraft and the knowledge of how to maintain each is crucial to the longevity of the aircraft. Thus, extensive in-depth procedures provided by AC 42-206 are an excellent resource for aircraft maintainers and for OEMs or component manufacturers working to develop maintenance instructions.
Contact Lectromec today for further information on how we can help with your corrosion assessment and prevention needs.