Cable Failures

The following types of failure and quality faults are amongst those seen in recent years.

Arc Tracking

Airworthiness Notice No. 12 Appendix No. 32 has drawn the attention of Industry to the problem of wet arc tracking of damaged cables subjected to fluid contamination. Observation of this Appendix and the actions of cable manufacturers should resolve the problem, but the greatest need is to ensure that hot stamp printing is properly controlled. ‘Inter-connect’ and ‘Equipment Wires’ should not be hot stamp printed.

Minyvin

Some batches of Minyvin have shown a tendency to shed the outer nylon sheath because of splitting along a flow line inadvertently introduced during manufacture. In dry areas of aircraft, replacement of such cable is not a matter of urgency. But if moisture is present, then cable must be replaced. In areas which are exposed and prone to fluid contamination such as undercarriage bays, modifications to introduce a more suitable cable have been raised on some aircraft types.

BMS 13-28

Larger sizes of this mineral-filled PTFE cable (especially those used on Boeing 707, 727 and 737 aircraft) tend to experience complete insulation failure due to a longitudinal splitting of the total dielectric. Replacement by BMS13-58 or EFGLAS to BS G222 under modification action is desirable.

Abrasion

Some types of cable have shown a tendency to ‘wear through’ the insulation at a point where the cable rubs on cable or cable rubs on structure. Areas of high vibration induce this failure mechanism and it may be supposed that the stiffer construction of some cables tends to produce a greater contact force and transmit vibration where previously it was damped. Careful cable loom tying and clipping is necessary to alleviate this problem (see Airworthiness Notice No. 12 Appendix No. 42).

Conductor ‘Knuckling Through'

Some earlier cable constructions tended to exhibit ‘knuckling of conductors which could be severe enough to penetrate the insulation. This was induced by applying excessive pull through forces and care should be taken not to put cables under tension. FEPSIL to BS G206, which is now ‘obsolescent’, requires particular care in manufacture and installation to avoid this defect.

Red Plague

Cables with silver plated conductors can exhibit the aptly named ‘Red Plague’ if the plating has been damaged and then exposed to moisture. Consequently, silver plated conductors are generally unsuitable for use in unpressurised areas.

Glycol Fires

If de-icing fluid contaminates silver plated conductors, an electrical fire can result. Accordingly, silver plated conductors should not be employed in areas where de-icing fluid can be present.

Poor Solderability

It should be recognised that the quantity of free tin or plated conductors rapidly reduces with time. The replacement of soldered connections during aircraft maintenance will probably require that conductors are ‘tinned’ as part of the process. The loss of free tin starts as the cable is manufactured and thus prolonged storage should be avoided.