ENG21LA023

Factual narrative

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 located under the outboard ply reached a high enough temperature during the arcing event that it melted and fractured the outboard glass ply of the window. The arc escaped via the glass fracture, allowing the arc to come into contact with and melt the humpseal sealant. There were no indications of any burning or fire events on the inboard surface of the window (including the power terminal block). The event was contained to the outboard glass ply. The power terminal block was removed, and no damage was noted to the wiring and solder connections. There was evidence of moisture ingress and degradation of the interlayer of window was noted in some areas around the periphery of window. Humpseal degradation and improper humpseal repair techniques were noted. The outboard glass ply failed due to a fracture along the bottom aft edge of the window caused by an electrical arcing event. The arcing event was caused by moisture ingression that came into contact with the window heating terminal block braid wires. Source: NTSB Aviation Accident Database Retrieved: 2026-02-12

NTSB Findings

FAA avdata. C = Cause, F = Factor.

• — Aircraft-Aircraft systems-Ice/rain protection system-Ice/rain prot sys wiring-Malfunction
• — Aircraft-Aircraft structures-Windows-windshield system-Flight compartment windows-Incorrect service/maintenance
• — Personnel issues-Task performance-Maintenance-Scheduled/routine maintenance-Maintenance personnel