N60BABeech E-90

On December 11, 2013, at 1008 central standard time, the pilot of a Beech E90, N60BA, operating as flight 25, made a precautionary landing at Chippewa Valley Regional Airport (KEAU), Eau Claire, Wisconsin, after the copilot's windshield cracked in flight. The pilot and one passenger were not injured. The airplane sustained minor damage. The airplane was registered to and operated by Bemidji Aviation Services, Inc. (BEMA), Bemidji, Minnesota, under the provisions of 14 Code of Federal Regulations Part 135 as a non-scheduled domestic passenger flight. Visual meteorological conditions prevailed at the time of the incident, and an instrument flight rules flight plan had been filed. The flight originated from Park Rapids, Minnesota, and was en route to Wheeling Illinois.

According to the operator, the airplane was at FL (flight level) 190 when the copilot's inner layer windshield cracked. Windshield heat was not being used and pressurization was not compromised. The pilot put on his oxygen mask, and descended to 10,000 feet. A precautionary landing was made at KEAU without incident. The airplane was later ferried to its base in Bemidji, Minnesota.

The windshield was removed and shipped to PPG Aerospace in Huntsville, Alabama, where, on May 6, 2014, it was examined under the auspices of the National Transportation Safety Board. The windshield, part number 50-420069-32 CHGA, serial # 95061H3863, was manufactured in March 1995. The inner glass ply fractured due to a peel adhesion chip at the lower forward corner of the windshield. The peel chip developed as a result of the differences in coefficient of expansion between the inboard glass ply and vinyl interlayer. The peel adhesion chip propagated, becoming wider and deeper due to mechanical and thermal stress encountered in service, until the center tension of the thermally tempered glass was penetrated causing spontaneous fracture.

In 2001, PPG Aerospace redesigned the windshield adding a urethane interlayer next to the outboard surface of the inboard glass ply. This reduced the thermal expansion stresses on the glass surface. The new windshield now bears part number 50-420069-37/-38.