Mixing wires of different types in the same bundle has been shown to be detrimental to wire life, because the harder coating on one wire can cut through the other when the wire bundle is subject to vibration. Wheel wells, engine compartments, and areas near the air-conditioning packs all have different vibration cycles, and yet the current regulatory and industry approaches to wiring do not take those differences into account. As mentioned previously, moisture is another major contributor to wire aging. Most insulation material is a very complex long-chain polymer, and moisture accelerates changes to this complex polymer. These changes decrease the insulating qualities, and can occur over a very short period of time.
Temperature is another factor affecting the aging rate of wire insulation.
Wiring insulation is subjected to heat generated by the wire itself, as well as the heat from the surrounding equipment. The configuration of the installation of the wire can also affect wire aging rate. Each aircraft is slightly different, and it is common for the same wire runs on different aircraft to differ slightly from one another. Most insulation types are unable to withstand tight radius bends. During our examinations of aircraft wiring, we have seen wires with bend radii exceeding the industry bend radius standards.