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.
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."
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.
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.
Environmental effects are usually due to weather and geography. Wire ages at different rates in warm and humid conditions vs. cold and dry or any other combination. Temperature, humidity, and sun UV (ultraviolet) exposure can premature aging. The impact of environment can be different for different locations in the aircraft.
Physical abuse can immediately break the copper conductor or insulation. Stepping on a wire bundle can ruin your day and the wires. Unsupported wires can flex or rub on the structure. Wires with missing or broken wire clamps held in place with very tight wrap ties can crush the wires. Cutting the wrong wire in a bundle or removing/breaking the wrong connector pin is worth one "Awe ----!"
Improving the various processes for wiring system analysis requires establishing processes based on available technology, acquiring proper tools, and performing preventive repairs. These activities may be addressed through good maintenance and diagnostic practices; appropriate training; technically sound repair and modification techniques; and adequate documentation. These issues could be a study in themselves for another time. Let me end by saying, we don't make money if they don't fly so "Keep 'em Flying."
Fred Workley is the president of Workley Aircraft and Maintenance Inc. in Alexandria, VA, Indianapolis, IN, and San Jose, CA. He holds an A&P certificate with an Inspection Authorization, general radio telephone license, a technician plus license, ATP, FE, CFI-I, and advance and instrument ground instructor licenses.
Economic Values for Evaluation of FAA Investment and Regulatory Decisions, FAA-APO-98-8, June 1998, US DOT, FAA
Review of Federal Programs for Wire System Safety, November 2000, National Science and Technology Council
Transport Aircraft Intrusive Inspection Project, December 2000, Aging Transport Systems ARAC.
"Wire Integrity Programs," Brown, March 2001, GRC International Inc.
Typical wire failure modes
The typical wire failure modes for aircraft are:
- Chafed wire insulation leading to short circuit and /or arching
- Short circuit, unspecified cause (includes arcing incidents)
- Broken wires
- Connector failure
- Unspecified failure
- Failure due to corrosion
- Loose connection
- Insulation failure
- Circuit breaker failure