Beech E90 N60BA incident near Eau Claire, Wisconsin

Probable cause

The failure of the copilot’s windshield inner layer due to a peel chip that developed as a result of mechanical and thermal expansion stresses.

NTSB analysis

The operator reported that, while in cruise flight at flight level 190, the copilot's inner layer windshield cracked. Windshield heat was not being used at the time, and pressurization was not compromised. The pilot conducted a precautionary landing without incident. Examinations of the windshield revealed that the inner glass ply had fractured due to a peel adhesion chip at the windshield's lower forward corner. The peel chip developed as a result of the differences in coefficient of expansion between the inboard glass ply and vinyl interlayer and had propagated due to mechanical and thermal expansion stresses until the center tension of the thermally tempered glass was penetrated, which caused a spontaneous fracture. The windshield was redesigned in 2001 to reduce the thermal expansion stresses.

Text from the NTSB. Open the NTSB docket for CEN14IA085 for the full record.