Why not perform certification test on aged aircraft wire?

Before a wire is first brought onto an aircraft, it must first go through a series of tests that validate the quality of the finished wire. Most of these tests are conducted on new wires that have not been subjected to accelerated aging. This raises the question of whether subjecting an aged aircraft wire (one that has been on an aircraft for a number of years) to the original new wires tests would give a good picture of the wire condition.

First, it is important to consider what needs to be determined from the testing of an aged wire. Ideally, the testing should determine the following:

1. What is the current condition of the wire?
2. Can the wire remain on the aircraft?
3. If the wire can remain on the aircraft, how much longer should it stay on the aircraft?
4. Should the wire be removed now?

These questions are at the heart of any aircraft’s (or any vehicle’s) life extension effort. But how do you answer these questions? Would selective sampling and testing of wires from the aircraft to new wire specifications answer any of the last three questions?

Directly, the answer is ‘no’, but it would certainly provide data to help make decisions. Some engineering decisions are needed at the initiation of such a project because the specification testing can take months to complete. The questions that immediately come to mind when considering whether to perform certification testing on aged wires include:

1. Should all the tests be performed, or only a subset?
2. What if the wire does not pass one spec test? Does that mean the wire is not good enough to be on the aircraft?
3. If a wire does not pass one or more spec tests, how much below the spec baseline can the wire perform to make it acceptable for it to remain on the aircraft?
4. Are there any existing guidelines to determine how much longer the new or questionable wires should remain on the aircraft?

These are not simple questions to answer. Ideally, this would be an excellent way to evaluate a wire if there was a defined degradation age data for each test spec. At this time, the level effort necessary to generate sufficient data is beyond the capabilities of most organizations. Additionally, there would need to be data from multiple manufacturers to make the model applicable.

From a practical and engineering perspective of focusing on simplified quantifiable values representing wire degradation, Lectromec recommends the use of wire degradation analysis technologies. These technologies are designed to examine a common physical wire insulation property that has been thoroughly tested under multiple conditions and analyzed as part of a complete dataset. From this research, wire life degradation models are developed directly applicable to most commonly found environments.

If you are interested in aerospace wiring, read Lectromec’s Hazards of Hot Stamping for Aircraft Wiring article.