Fracture Pattern Analysis
An evaluation of the fracture pattern indicated that the outboard glass ply failed due to a fracture along the bottom aft edge of the window. The conductive film interlayer, which is located directly under the outboard ply, reached a sufficiently high temperature during the arcing event. This caused the interlayer to melt and, subsequently, fracture the outboard glass ply.
Arc Escape and Containment
The arc then escaped through the glass fracture, allowing it to come into contact with the humpseal sealant. This contact resulted in melting of the humpseal. However, there were no indications of any burning or fire events on the inboard surface of the window, including the area around the power terminal block. The entire event was contained to the outboard glass ply only.
Inspection Findings
The power terminal block was removed for inspection, and no damage was noted to the wiring or solder connections. Additionally, evidence of moisture ingress and degradation of the interlayer was observed in some areas around the periphery of the window. The examination also noted humpseal degradation and improper humpseal repair techniques.