Ageless Aircraft Wiring

The trend requires good maintenance/diagnostic practices and documentation


As maintenance providers we often assume that if the total system, as delivered new, meets the "specs for acceptance" the "as built" configuration, then at least we have a known start point for future electronic analysis. Even if we know where we started, irrespective of major modifications and retrofit activities, I contend that some wires can be expected to fail within the first two years of operation. After that, there may be a period of years that the wiring has few problems unless it is disturbed by possibly adding new wiring for new avionics or electronics modifications of systems. Maybe we need to rethink our approach to the aircraft life cycle by defining the term "aging aircraft wiring." Instead what we really need is "ageless aircraft wiring."

Avionics toys

Big boys like new toys. I bought a new four-place, low-wing aircraft in 1978 and kept it 20 years and 20 days. At one time or another, I had at least one or two of every new avionics toy. My justification is that I flew all over the country at night, single pilot IFR in the clouds and some times very bad weather. At one time or another, the airplane had an R-Nav (area navigation), DF (direction finder), LF (low frequency), loran, two ADFs (automatic direction finders for a Canadian adventure), a fuel computer, two HSIs (horizontal situation indicators with vacuum and/or electrical gyros), digital EGT (exhaust gas temperature), four different digital clocks, intercom, passenger entertainment, a wing ice detection system, and a collision avoidance system. I always wanted a storm scope as a gift. For a 90-day evaluation, I even had an in-flight phone that because of weight and balance effectively made a four-place airplane into a one and one-half seater. However, the most aggravating wiring problem was the landing gear squat switches and the aluminum power cables. This experience taught me that different locations on the airplane are affected by different environmental conditions --water, dirt, oil, runway deicing urea, vibration, big feet, and FOD (foreign object damage foreign object debris). Luckily, I never had any wire hand-holds on that airplane.

Lets think of wires as "electrical interconnect systems" that carry intelligent information both analog and digital. The condition of these systems depends on the effectiveness of maintenance based on existing directives, procedures, and inspections to prevent unsafe conditions associated with the degradation of aircraft wiring. To get out of the wordy language, we have to keep the electrons flowing from there to here and back again if it's AC.

Maintenance findings

Findings include broken wires, cracked insulation, exposed conductors and breaches/shorts through the insulation. Other wire anomalies may include reportable significant conditions (RSC) such as delaminated wire, severe embrittlement, burnt wires possible from flash-over, and "traumatic damage to wires" whatever you think that means. If we had the data to analyze trends, I expect we would find that each aircraft is unique. There probably wouldn't be any way to correlate hours, cycles, or age to the incidents of known problems.

According to a paper titled "U.S. Air Force Aging Aircraft Wiring Implementation Plan" submitted at the Conference on Aging Aircraft, by Hall and Brown in 2002, wiring integrity is decreased by the following fault categories: aging, contamination, environment, and physical abuse.

Aging refers to changes over time in the physical and chemical properties of wiring insulation and conductors. Insulation cracking and separation of insulation from the conductor may be from changes in flexibility, hardness, compressive strength, tensile strength, and torsion strength. The changes are slow and subtle. Usually small cracks show up in areas of wire bend radius and flexes but grow over time.

Contamination is anything that damages the insulation over prolonged contact. FOD like metal shavings can work into the insulation. Exposure to any fluids with Ph that changes the physical properties of the insulation is contamination. Candidates could be hydraulic fluids, fuel, washing solutions, lavatory water (Blue Water), galvanic action, and dirt trapped in corrosion prevention, corrosion control products.

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