This is the most difficult blog that I have written concerning F-16 wiring harnesses. It is difficult because it is a technical subject that needs to be understandable to the average reader of InterConnect’s articles.
InterConnect has written blogs on the benefits of Kapton wiring as well as the problems of Kapton wiring. InterConnect has written blogs on what breeches in insulation of wires are as well as arc tracking. This is the first blog where InterConnect has broached the subject of “inherent viscosity”.
Before we discuss “inherent viscosity”, let’s talk about what “viscosity” is. The layman’s way to think of viscosity is how “sticky” a fluid is. For instance, a cup of normal drinking water has a very low viscosity. It is not sticky at all. When you pour a cup of water it moves quickly. Compare that to a cup of honey. Honey has a much higher viscosity. It sticks together much more than a cup of water. So, when you think of Kapton insulation and inherent viscosity think of how much the Kapton is clinging to itself (how sticky the Kapton is). New Kapton is sticky. Once Kapton ages it is less sticky and is not as good an insulator as new. Kapton insulation loses its good properties over time.
Simply put, in terms of F-16 wires, Inherent Viscosity is a measurement of the relative age of the Kapton insulation. If inherent viscosity is high, that is good. The insulation of the wire is conforming to design parameters and thus is doing its job. If inherent viscosity is low, that is bad. The insulation is not doing what it is supposed to do, which is to protect the inner metal conductor.
Inherent viscosity for new Kapton wiring is generally about 1.8. If a batch of Kapton is made and its inherent viscosity is 0.9 or less, it is rejected and regarded as non-conforming. For the F-16, InterConnect uses 0.9 as the cut-off point of when replacement of the wiring on the F-16 should strongly be considered. If the inherent viscosity is 0.9 or less the Kapton has lost much of its good properties and is suspect.
Since 2006, InterConnect has sent old F-16 wiring harnesses to a professional laboratory to have inherent viscosity measured. In order to measure it, the laboratory must conduct a destructive test on the Kapton insulation. As one would expect, inherent viscosity of the Kapton wire varies throughout the F-16 due to the environment/where the F-16 wiring harness is installed. In some areas, the Kapton is like new. In other areas it has seriously degraded. This table shows the results (i.e. the lowest values found in each area tested) of laboratory studies of F-16 Kapton wiring. Notice that it varies depending on the environment where the wiring harness is installed.
The bottom line of the laboratory results is that when aged, the Kapton wiring has degraded beyond the recommended cutoff value of 0.9 in some areas of the F-16; hence, the Kapton is a problem, should be considered a safety of flight issue, and should be replaced. For more information about Kapton inherent viscosity or if you would like to comment on this blog, please go to our Facebook page or contact us at firstname.lastname@example.org.