The Effects of Old Age: On wiring systems
By Jim Sparks
Aging is a fact of life. Although the aging process begins at birth the effects are usually not recognized until the onset of middle age. The realization that what we do while in the early stages of life may have a significant impact on future operation of various body parts is something that has caught many of us off guard. Aircraft are in the same boat.Routing and separation of wire bundles may be influenced by what the wires carry.
Historically, wiring in aircraft has been installed without much thought given to the overall life. The concept has been one of ?fit and forget.? Various failure modes and their severity may not have been considered during the design or modification process. Maintenance programs often did not address the impact of insulation breakdown and resulting arcing, nor was consideration given to the probability of splices developing high resistance or increasing the strain on a wire bundle. Service history also indicates that Foreign Object Damage (FOD) such as drill shavings or caustic liquids will cause wiring degradation.
Within the U.S. federal government, the Aging Transport Systems Rulemaking Advisory Committee known as ATSRAC has been formed. One of the projects undertaken by this group was to perform inspections on electrical installations on four transport aircraft that had recently been decommissioned. Nearly 1,000 questionable conditions were observed using visual inspection. Nondestructive testing (NDT) and laboratory testing resulted in many additional findings. Most of the discrepancies could be classified as installation damage or trauma induced during maintenance. There was also degradation on wire, connectors, and terminals.
As a result of these investigations the focus has been placed on six categories:
- Deteriorating wire repairs or splices
- Damage from overheating
- Vibration damage or chafing
- Cracked insulation
- Insulation failures
The following factors can have detrimental impacts on wiring.
Vibration ? High vibration areas tend to accelerate failures, and often result in intermittent problems. High vibration can also cause wire bundle securing devices such as tie wraps or clamps to damage insulation. Simply having a wire resting on a metal panel can result in wear of the insulation due to the resulting chafe.
Moisture ? High moisture areas promote corrosion of electrical connections. Seals and protective varnishes or other coatings should always be inspected to ensure adequate protection is available.
Maintenance ? Maintenance activities can contribute to long-term problems and wiring deterioration. Removal or opening of access panels and doors that have wire bundles attached will often require movement or flexing of the wiring. This simple action over time may be responsible for failures.
Metal shavings and debris accumulating within wire bundles can also have a significant impact on wire longevity. Care should be taken to protect wire bundles and connectors during modification work, and to ensure no foreign objects are introduced.
Wiring that is undisturbed has less degradation than wiring that is exposed to external factors.
Chemical contamination ? Chemicals such as hydraulic fluid, battery electrolytes, fuel, corrosion inhibiting compounds, waste system chemicals, cleaning agents, deicing fluids, paint, and soft drinks can contribute to failures. Some fluids may be damaging to connectors, grommets or seals, and wire bundle clamps. This may lead to damage such as arcing and chafing.
Heat ? Wiring exposed to high heat can accelerate degradation, insulation dryness, and cracking. Direct contact with a high heat source can quickly damage insulation. Even low levels of heat can break down wiring over long periods of time. This type of malfunction is sometimes seen on engines, in galleys, and behind lights.
Indirect damage ? Events such as pneumatic duct ruptures can cause damage that, while not initially evident, can later cause wiring problems. When such an event has occurred, surrounding wire should be carefully inspected to ensure no damage is evident.
Installation ? Wiring not installed properly can further increase the likelihood of failure. Improper routing, clamping, and terminating can lead to damage.Excessive wire bends may result in failure.
Certain specific criteria need to be reviewed when determining the correct wire for the job. Wire size selection takes into account the mechanical strength of the conductor along with the cross section area to determine minimal voltage drop and the ability to carry adequate amperage. Stranded conductors are used more frequently than single strand wires primarily to minimize fatigue failures.
When smaller diameter wires are used particular attention should be given to the mechanical strength, vibration, flexing, and termination. Wire containing less than 19 conductor strands must not be used.
Consideration should be given to the use of high-strength alloy conductors in small gauge wires to increase mechanical strength.
As a general practice, wires smaller than #20 should be provided with additional clamps and be grouped with at least three other wires.
They also should have additional support at terminations, such as connector grommets, strain relief clamps, shrink coatings, or telescoping bushings.
Small diameter wires shouldn?t be used in applications where they will be subjected to excessive vibration, repeated bending, or frequent disconnection from screw termination.
When determining adequate wire size Advisory Circular 43.13-1b, Section 5: tables and figures provide in many cases an acceptable reference. The current ratings established here were determined for the wire only and do not take into account termination devices.
Testing and maintenance
Most aircraft wire designs are required to pass rigorous testing before they are approved. Aircraft manufacturers who maintain their own wire specifications exercise close control of their approved sources. Therefore, it is important to review the aircraft maintenance manual or contact the aircraft manufacturer when wire repair, replacement, or substitution are necessary. Factors to consider include splicing instructions, compatible replacement wire types, pertinent clamping and routing aspects, and shielding ground specifications.
The use of circuit breakers as switches can often result in premature failure. In some cases a latent failure will occur where the breaker will not open during a high current draw condition. This too is being considered by ATSRAC and periodic testing may be recommended to verify proper function.
Current ATSRAC recommendations include a ?clean-as-you-go? philosophy which includes wiring general visual inspections (WGVI), nondestructive testing (NDT) equipment, preemptive repair of splices and/or replacement of wire.Terminal strips should be checked for security, cleanliness, and proper stacking.
Testing methods that are effective for determining proper operation of wiring may include using a power supply and applying a known electric load. Then by measuring the voltage drop in the wire a decision can be made about the system?s functionality. Testing equipment such as meggers are good for validating the integrity of wire insulation. Caution should always be used as meggers can produce very high voltages and are not suited for testing in all types of circuits.
Protect wiring during maintenance and clean it periodically (vacuum, light brushing, etc.) when hidden areas are exposed.
When you?re inspecting wires visually focus on clamping, improper installation, wire damage, clamp cushion migration, or other deterioration of restraints.
Inspect connectors for worn seals, loose connectors, cleanliness and corrosion, lack of strain relief, and tight wire bends.
Wiring exposed to the atmosphere should also be regularly evaluated
for fatigue failure and may be found in areas such as engine/APU/pylon/nacelle,
landing gear/wheel wells, and wings and stabilizers.
Other areas requiring attention include high maintenance activity areas as wiring may be disturbed by frequent intrusions. And after extended storage aircraft may need an extra check as they may have been exposed to damage from animals.
One ATSRAC recommendation is to include wiring inspections within the continued airworthiness programs of various transport aircraft. It is desirable to become familiar with installation recommendations from manufacturers or installation specific design agencies prior to installing or replacing wires. Methods of securing conductors may also impact proper system operation. Over-tightening a clamp on a coax antenna cable or a digital bus wire may change the electrical characteristic of the wire and introduce either degraded operation or a failure.
The formation of ATSRAC and the observance of its recommendations, which will probably include educating technicians on what to look for while conducting routine maintenance will no doubt have a positive influence on future safe operation. In fact an information program has already been created and is available on line at http://www.academy.jccbi.gov/airdl/wiringcourse.
Hopefully the policies of the airworthiness authorities regarding aging aircraft will always apply to aircraft and not aging aircraft technicians. I don?t care for NDT when it is performed on me.