Incident Details
On December 11, 2013, at 1008 central standard time, a Beech E90, registration N60BA, operating as flight 25, performed a precautionary landing at Chippewa Valley Regional Airport (KEAU) in Eau Claire, Wisconsin. The flight was a non-scheduled domestic passenger operation under 14 Code of Federal Regulations Part 135, operated by Bemidji Aviation Services, Inc. (BEMA). The aircraft had departed Park Rapids, Minnesota, en route to Wheeling, Illinois, with an instrument flight rules flight plan filed. Visual meteorological conditions prevailed at the time.
According to the operator, while at flight level 190, the copilot's inner layer windshield cracked. Windshield heat was not in use, and the aircraft's pressurization remained uncompromised. The pilot donned an oxygen mask and descended to 10,000 feet, subsequently making a precautionary landing at KEAU without further incident. The pilot and one passenger were not injured. The airplane sustained minor damage. The aircraft was later ferried to its base in Bemidji, Minnesota.
Examination and Findings
The windshield was removed and shipped to PPG Aerospace in Huntsville, Alabama, for examination. On May 6, 2014, under the auspices of the National Transportation Safety Board, the windshield—part number 50-420069-32 CHGA, serial number 95061H3863, manufactured in March 1995—was inspected. The inner glass ply had fractured due to a peel adhesion chip at the lower forward corner of the windshield. This peel chip developed from differences in the coefficient of expansion between the inboard glass ply and the vinyl interlayer. The 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.
Windshield Design History
In 2001, PPG Aerospace introduced a redesigned windshield that incorporated a urethane interlayer adjacent to the outboard surface of the inboard glass ply. This change reduced thermal expansion stresses on the glass surface. The updated windshield now carries part number 50-420069-37/-38.