Examination of damaged wing leading edge wiring:
A selection of individual bundles of twisted pair electrical wires (figure 1) removed from the right wing leading edge of an A320 aircraft were received for the characterisation of damage to the wiring insulation.
Figure 1: Wiring Bundle
While numerous wiring bundles were received, only five were identified with respect to their installed location. For the purposes of correlating damage characteristics to the wire path, only those identified bundles were subject to study. Diagrams and photographs showing the wire and damage locations were presented to assist with the investigation.
The wiring bundles examined were as follows (all from raceway 1S):
Figure 2: Damage locations
The damaged areas of each wire strand previously identified were studied under the low power stereomicroscope.
2793 4537 Blue
This wire displayed a broad area of flat, tapering damage, consistent with rubbing against a flat surface at a shallow angle. The damage extended through to the inner insulator (figure 3). No related damage to the adjacent red wire was noted, however there was a small sliver of shaved or lifted insulation approximately two millimetres away.
Figure 3: Damage extending to the inner insulator
2793 4537 Red
Both blue and red wires at this location showed sharp, angular notches consistent with pressure against a rigid corner or similar (figure 4). Some shaving and lifting of the red wire outer insulation was shown, as was a set of parallel, much less extensive indentations (arrowed) roughly 3mm from the main damage marks.
Figure 4: Sharp angular notches
2793 4535 Red
This wire showed another extended and tapering flat area of damage in this case extending for around 3mm. The inner insulating material had been exposed for around half of this distance (figure 5). No related marks were noted on the blue wire.
Figure 5: Exposed inner insulating material
2793 4535 Blue
Again, this wire showed the characteristic angled edge rubbing features - in this case just contacting the inner insulation (figure 6). A profile view showed the acute angle of the indentation and the radius around the corner of the indenting object (figure 7).
Figure 6: Angled edge rubbing damage
Figure 7: Acute angle of the indentation (arrowed)
2793 4533 Blue
This wire was the most severely damaged of the selection studied, with exposure of the copper conductor over roughly half a millimetre (figure 8). The damage profile was similar to most previously examined wires, with the shallow angular face clearly shown. Some flattening of the red wire insulation was also noted around ten millimetres from the main damage point (figure 9).
Figure 8: Exposed copper conductor
Figure 9: Flattening of the red wire insulation
2793 4549 Red
These two areas were alongside to each other and were similar in terms of their sharp intrusive profile. While not the type of cuts produced by a knife or similar, these notches were clearly produced by a sharp rigid edge (figures 10 & 11). With the red wire, some lifting of the insulation had also occurred, disclosing the inner sheath. Around 15mm away, a similar set of notches was also shown by both red and blue wires, however these were less significant and had not exposed the inner insulator (figure 12).
Figure 10: Notches due to sharp rigid edge
Figure 11: Notches due to sharp rigid edge
Figure 12: Similar less extensive damage - 15 mm away from the main area
Studying an unused harpoon tie of the type used to secure the wiring within the raceways showed both the central tongue and outer plate to have rigid flat surfaces with rounded edges. The form and dimension of these features was consistent with the flat, rubbing type damage experienced by some of the wires.
Figure 13:Typical harpoon tie assembly used for securing wiring within raceways