- Aircraft wiring does age.
- There are methods to determine the current health vis-a-vis the original condition.
- Lectromec assessment methods make it possible to predict the remaining service life.
As aircraft age, it is natural to contemplate the aircraft condition as to how much longer the equipment can be maintained. Typically, after the engines, structures, environmental control system (ECS), navigation, landing gear, interiors, flight controls, and fuel system are considered, the question will ultimately appear, “do I need to replace my wires that are 15/20/25/30 years old?” While it would be nice if there were a simple answer, like all engineering, it comes down to the evaluation of the system. Here, we answer this question.
How Does Wire Age?
Before we can talk about if there is a need to replace wiring, it is important to understand how wiring ages. Wiring will age in one of two ways either degradation of the conductor or the degradation of insulation.
The wire insulation of installed aircraft wires and cables are designed to endure the environmental conditions, but the wiring system is not an ageless component. Aircraft wire insulation is often a polymer and with thermal cycling, environmental exposure, UV exposure, and/or fluid exposure, the materials will degrade over time. Further, the insulation is more likely to fail in applications that are static i.e. do not require flexing.
How this materializes in service is the progressive weakening of insulation that makes the insulation more susceptible to cracking or damage with any thermal/mechanical shock.
For conductors, repeated flexing can wear down the conductor and cause a breach of the conductor strands. Also, temperature exposure can cause the plating material to wear down, particularly in the case of tin-plated copper constructions. The evaluation of conductors should not only be limited to primary conductors but also shielding for cables. The shielding is much more susceptible to corrosion and degradation. This is a significant impact on signal performance primarily signal relation and susceptibility to EMI.
…Do I Need to Replace It?
In Lectromec’s experience, we have never come across the need for an entire aircraft wire system replacement due to aging. Some areas of the aircraft have more harsh environments. These harsh conditions are sure to take a toll on the wire insulation and conductor health and progressively create degradation. Testing can quantify the degradation and the current health can be assessed by comparing the test results with the wire/cable’s original and worst-acceptable performance. Using the quantified performance as inputs, the remaining reliable service life can be predicted for each aircraft zone eliminating the need for aircraft-wide wiring replacement [See Lectromec’s July 2018 Webinar on “Lessons learned from EWIS evaluation of aging aircraft.”].
The issue is that it is impossible to visually inspect a wire and say if there has been material aging. Wire insulation does a great job of hiding its age. A visual inspection of an installed wire might find discoloration, but if that wire has been installed on the aircraft for more than 20 years, chances are the discoloration is dirt, grime, or fluid contamination.
To determine if wiring needs replacement, sampling and evaluation are required. The sample in an evaluation of the wiring is a vetted process that has been applied to dozens of fleets. Just a few wire samples from locations within the aircraft make it possible to:
- Identify the current condition of the aircraft wiring, and
- Predict the future performance and remaining reliable service life Similar to how surface scanning technologies are used to evaluate structure health and determine crack growth, this technology is used to predict remaining wiring system performance and best means to answer the question “do I need to replace the wiring system?”.
The Justification for EWIS Evaluation
For those in the military, the justification to support this type of assessment can be found in military standard MIL-STD-1798. This document is often part of the mechanical equipment and subsystems integrity program also known as MECSIP. While the bulk of the document focuses on other systems, there is a specific call out for evaluation of the wiring system (EWIS). This seven-step process provides a thorough analysis and full engineering guidance on what can and should be done to evaluate the wiring system.
For those in the commercial segment, service life limitations on aircraft require aircraft assessment to extend its performance beyond originally established limits.
For both military or civilian applications, Lectromec has the engineering experience, lab capabilities, and software tools to expedite wire system assessment and determine what parts of your wiring system may need to be replaced and provide sufficient documentation to support our conclusions.
While it does feel as though there should be a simple answer to the question, “Do I need to replace the wiring of my X year old aircraft?”, it is complicated. Charging headfirst into a full aircraft rewire can be a substantially costly endeavor leading to the grounding of the aircraft for several months. Intelligent, focused testing makes it possible to understand the aircraft’s current wiring system condition and to plan ahead ensuring the maximum aircraft availability and reliability. Contact Lectromec to get what you need to make data-based decisions on the maintenance of your aircraft wiring system.