I remember an old, but low-time Cessna 180 that spent most of its life as a hangar queen. The tin hangar was like a bake oven in the summer, so I agreed to do the annual in the cool of the evening. This was the first time that I had ever looked at the airplane. While I was taking the seats out, I observed that there was a white power on the rug under rudder pedals.
At the time, I didn't know what to make of the white dust. When I finally lay down on the floor and looked up under the instrument panel, I realized that the rubber insulation on the original wiring was cracked and literally falling off. The airplane had three different radio packages so far in its life time. All the original wiring for systems, lighting, the "coffee grinder" VHF, low frequency radio and flare arming system were still there along with all the added wiring. The end result was removing every wire in the airplane. When that was done there were three bushel baskets full of problems.
Aging is two-pronged problem
We have a two-pronged problem. The aircraft are aging and the wiring is aging. The civilian and military fleets have shown the same experience. Aircraft range from new to 40 years. Of a small random sample of civilian and military aircraft the average age of this "active fleet" was 21.2 years. Even with new aircraft being added the average has grown from 13 to 21 years according to my math. Another small sample of civil corporate aircraft showed an increase from 10 to 11 years average aircraft age. These are my numbers. They may indicate that considering aging wiring in each model may need to be done because of the impact on readiness and operating and support costs.
The challenges of aging aircraft wiring are generally found as a result of something in the aircraft not working. Another indicator of problems found by inspection is that "the wiring just looks bad." Pilots will tell us that some equipment is "INOP" or "intermittent." When we trouble shoot the system, it checks out good (no fault found). We should keep in mind that an aircraft in flight is in constant vibration and that the fuselage is lengthening and shrinking with temperature changes. Too often wire failures are not appreciated for their scope, impact, and severity. We fix the immediate problem but don't address the reliability and maintainability concerns. We often don't know the actual costs of wiring maintenance on an airplane because the costs are lumped with other maintenance costs. Only a very few aircraft in a particular model will never see an intensive, wire integrity program to address wiring faults. Very few aircraft are tracked for trends in wiring failures.
Sometimes, wiring problems were literally built into the airplane during the original manufacturing process when spools of wire are converted into installed "bundles." We can consider the probability of wire failure as a function of time. My experience is that there is significant probability that wires will fail early in the life of the aircraft from weak splices, cold solder joints, bundle ties that are way too tight and no strain relief on unsupported wiring runs. These problems may be built into a new airplane from poor design, poor practices, and/or poor quality control of the manufacturing process.