Piper PA 31T accident near Missoula, Montana, December 5, 2016

The NTSB's narrative final · quoted from the NTSB record

The private pilot reported that, during cruise flight, the left windshield suddenly departed the airplane. The airplane was below its published service ceiling of 29,000 ft and the windshield was not subjected to internal pressure above its limitations at the time of the failure. The pilot entered an emergency descent and landed the airplane at an airport without further incident. The lack of windshield debris in the cockpit and the glass embedded in the left propeller indicated that the left windshield blew outward. Examination of the right windshield, which remained on the airplane, revealed evidence of moisture ingression into the laminate, to include cloudiness in the vinyl layer, interlayer cracking, delamination around the edges of the windshield, and corrosion of the aluminum retainer. Remnants of the left windshield were insufficient to establish if the laminate exhibited the same evidence of moisture ingression. Examination of the right windshield showed that the discrepancies were consistent with delamination as defined in the manufacturer's service manual. The discrepant areas had only progressed up to ½ inch into the daylight opening. The cloudiness, interlayer cracking, and delamination noted along the upper and lower edges of the right windshield were visible to the pilot before the accident flight. However, the delamination of the right windshield would not have necessitated immediate replacement based on the published service information, and although it is impossible to determine the condition of the left windshield before the flight, it was likely similar to that of the right windshield. The left windshield aluminum retainer exhibited corrosion around the periphery that was more extensive than that on the right windshield. The corrosion of the aluminum retainer is impossible to see with the windshields installed in the airplane, since the fuselage windshield frame covers the entire flange area. Windshields were designed in a manner to transfer their internal stresses equally around the windshield into the airframe structure. The corrosion of the aluminum retainer eliminated the load path for the internal windshield stresses to be transferred to the airframe. For each of the corroded areas on the aluminum retainer, there would have to be a redistribution of the internal stresses toward the intact areas, which would cause significant stress concentrations in the windshield laminate. These stress concentrations could be high enough to exceed the capabilities of the laminate in local areas. If coupled with a fracture of one glass layer, the stress concentration would be intensified even more adjacent to the corroded retainer areas. The corroded retainer on the left windshield led to areas of stress concentration within the windshield laminate. The precise initiation of the left windshield failure could not be conclusively determined. The little amount of glass remaining suggested that the fracture of the glass layers initiated in the aft portion of the windshield near the center. It is possible there was a peel chip that began the event, or simply that the stress concentrations adjacent to the corroded retainer were high enough to cause failure of the windshield laminate.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.