- In most cases, aircraft wiring is serviceable.
- To determine if the aircraft wiring is serviceable, Lectromec proposes three limiting criteria.
- Visual inspection can only do so much good in determining wiring system serviceability.
A concept employed in the civil engineering field is that of serviceability. At its core, a structure should only remain in-service as long as it is serviceable. A serviceability limit should be based on measurable performance criterion that, if exceeded, remove it from service. In limit state design, the structure fails its serviceability if the criteria of the serviceability limit state are not met during the service life with the required reliability.
This idea of serviceability is applicable to other engineering disciplines and is undoubtedly applicable to aerospace vehicles. From our perspective at Lectromec, understanding the serviceability of aircraft wiring is critical. However, to determine an aircraft wiring system serviceability state requires the limit state to be defined and assessed. Here we propose three such properties of a serviceable aircraft wiring system.
If a wire/cable is unable to perform the actions necessary to support the system to which it is attached, then it is performing below the required performance limit. As a starting point, the hope is that the correct wire/cable was put in place for the original design. Ideally, the wire/cable was selected to support a certain electrical current load (correct wire gauge) or to provide data within a certain frequency range and limit the attenuation below a certain threshold. There are other factors to be considered that are addressed in other Lectromec articles.
Often a wire/cable’s electrical performance degradation can be measured on an aircraft. This includes measuring the circuit resistance or signal attenuation and there are specific tools designed for these diagnostics. This should be progressively evaluated and recorded through the aircraft’s life; failure to do so will create maintenance questions like, “Do we know what a good value is?”.
Connectors should also be considered in the performance category as so many maintenance issues can be found at connectors. The degradation of contact/termination performance will also impact systems. Contact corrosion and/or contact wear often can be visually identified.
The second proposed limit state of aircraft wiring is its condition. Whereas the performance focuses on the conductor, this item focuses on the state of the wiring insulation. If the wiring cannot be reliably repaired (e.g. one repair generates more issues), then replacement is the only option. On an individual wire/cable/harness basis, this might be the case, but unlikely to be necessary for the entire vehicle.
Will some wiring age faster than others? Yes. Does this mean that aged wire is not serviceable? No.
While not definitive, a simple test to determine the wiring current condition is to take out a small section of wire and wrap it upon itself. See the photo for an example. If there is any viable insulation warping or cracking, then there may be problems.
As a reference point, Lectromec has tested wire from aircraft with more than 40 years of regular usage that have successfully passed this wrapback test.
The last proposed limit state is the accumulated damage to the wiring system. This asks if the wiring system endured too much damage. Have there been hundreds of repair jobs, ad hoc fixes, and collateral damage from other system failures?
There is no magical threshold after which a wiring harness/system should be replaced, but it is a contributing factor to determining sustainability. There are practical limits to how many times a single wire can be spliced. Naturally, if a wire harness has endured significant repairs, it may also require rework of a flawed design.
Is your aircraft wiring system serviceable?
Performance, condition, and accumulated damage are the three limiting factors for an aircraft wiring systems serviceability. Processes such as the US Air Force’s MIL-HDBK-525 provides thorough methods for gathering the necessary information, assessing the condition, and developing a plan of action.
If you are concerned that your wiring system has reached this point that the serviceability is a question, contact Lectromec. This is what Lectromec has been working on for 30 years and has supported numerous fleets to identify the serviceability of their wiring system through developing a proactive plan to address the wiring system. The serviceability of the wiring system should never be questionable.