Corrosion is an age old enemy of the aviation industry. As the worldwide fleet of commercial and private aircraft matures, it is up to those of us in the support field to ensure continued airworthiness. Most learn early on that the effects of exposure of bare metal to the elements results in some form of degeneration. This fact has stimulated many different programs addressing inspection of aging aircraft and countering the effects of metal fatigue and corrosion.
Well, what about the effects of time and elements on avionics equipment? A common misconception is that “black boxes” are most often sealed and therefore don’t require the surveillance and scrutiny associated with aircraft structural components. Many technicians are familiar with the U.S. Federal Aviation Administration Advisory Circular (AC) 43 – 4 which addresses inspection and corrosion in aging aircraft. There does not currently appear to be the same familiarity with AC 43 – 206 which is titled “Inspection, Prevention, Control, and Repair of Corrosion on Avionics Equipment.”
The proper care of avionics equipment requires a thorough understanding of aircraft electrical/electronic systems corrosion control. This knowledge includes the definitions and descriptions of the situations that cause problems in avionics equipment. Corrosion is the natural process of materials returning to their natural state. A good explanation of this process is where a chemical or electrochemical deterioration of a material, usually a metal, occurs because of a reaction with its environment. This deterioration can be complex and needs to be investigated prior to applying any remedy. The nature of the different individual types of corrosion, simultaneous attack by several types of corrosion, and the design characteristics and maintenance factors that make avionics systems susceptible to molecular deterioration require specific resolution.
Corrosion can cause intermittent malfunctions, undesirable changes in electrical characteristics, or complete equipment failures. Avionics equipment does not have to be installed, operated, or located in a particularly harsh environment to be affected by corrosion. Some forms of corrosion will be active in near ideal environments. Avionics maintenance personnel should recognize that corrosion never sleeps and once started, will continue to attack 24 hours a day, 365 days a year. Inadequate prevention and control will ultimately affect the equipment in cost, downtime and overall system reliability.
Most aircraft electrical/electronic equipment should be opened and inspected for evidence of internal moisture and corrosion on a scheduled basis. When anomalies are detected, prompt corrective action is required and should include cleaning, removal, treatment, application of protective finish, and preservation where required. Maintenance personnel should always use the mildest method of cleaning and removal.
There are many design decisions and compromises to be made in the course of developing avionics equipment. The design specifications leave room for a wide range of engineering practices to meet not only the performance, cost, and schedule, but also the reliability and maintainability requirements. Each piece of avionics equipment is designed to withstand its intended operational environment. However, some design compromises have to be made to provide the unique electrical, mechanical, and thermal characteristics of the equipment. These compromises can cause the equipment to be vulnerable to corrosion, especially during inoperative periods. According to AC 43-206 good design practices include:
Shoe box type lid construction. When access to the equipment is from the top, use a shoe box type lid construction. Fasteners securing the shoe box lid should be from the sides and not through the top.