WPR19LA088

The pilot reported that, during cruise flight at 15,500 feet mean sea level in the pressurized airplane, he heard a loud “bang” as the windscreen fractured. The pilot diverted to a nearby airport, where he landed the airplane uneventfully. An examination of the windshield pieces revealed that the fracture origin area displayed features consistent with a preexisting crack that developed due to environmental stress cracking (ESC). ESC is a fracture mechanism in polymer materials that occurs when a susceptible polymer material is exposed to a combination of tensile stress and exposure to a substance that degrades the integrity of the material. The ESC was likely a result of contact with a degrading substance, the origin of which could not be determined. Although a stress analysis of the windscreen structure was not completed, the location of the fracture origin was likely an area of high stress on the windscreen due to the curvature at that location. Sustained high stress in acrylic can promote crack development, particularly when the acrylic is also exposed to certain chemical agents. The overall appearance of the windscreen edge showed limited visual evidence of adhesive on the surface, suggesting a relatively low bond strength between the windscreen at the frame. Evidence of voids in the epoxy at the interior bond surface were observed; if portions of the windscreen became disbonded, this could have affected the stress state of the windscreen and contributed to increased stress at the fracture origin area.

On February 10, 2019, about 1600 mountain standard time, a Lancair IV-P airplane, N549CJ, sustained substantial damage when it was involved in an accident near Laramie, Wyoming. The pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 personal flight. The pilot reported that, following departure, he configured the airplane to climb to 15,500 feet mean sea level (msl). The airplane reached that altitude and was cruising about 235 kts when the pilot heard a loud “bang,” as the windscreen suddenly blew out of the airframe. The pilot reduced the airplane’s speed and lowered the landing gear in an effort to stabilize the airplane. He diverted to Laramie and made an uneventful landing. The windshield was manufactured from cast acrylic and the frame was from a carbon fiber reinforced polymer (CFRP) with layers of carbon-fiber fabric and epoxy. According to the window installation instructions, the exterior surface of the windshield is bonded in position with Hysol paste epoxy structural adhesive. Recovered portions of the windshield pieces were sent to the National Transportation Safety Board (NTSB) for examination. (see Figure 1.) The outer edge of the windshield displayed a white appearance consistent with a roughening process used during the windshield installation process before bonding to the windshield frame. Figure 1: Overall view of the submitted windshield pieces. Unlabeled arrows indicate fracture propagation directions. Most of the windshield fractures exhibited fracture planes that varied across the thickness with curving hackles turning toward the interior or exterior surface, features consistent with fracture under a combination of tension and bending loads. One fracture (shown as “A”) displayed smooth, mirror-like fracture features along most of the fracture surface in a flat plane perpendicular to the interior and exterior surfaces consistent with fracture under primarily tension loads. A portion of that fracture surface had curving rib marks and with a somewhat hazy surface, consistent with preexisting progressive crack growth. The features were consistent with the crack’s origin positioned at the forward left side of the windshield above the edge of the frame. The surface adjacent to the fracture appeared disturbed consistent with a surface scratch. The origin area fracture displayed features consistent with a preexisting crack that developed due to environmental stress cracking (ESC). ESC is a fracture mechanism in polymer materials that occurs when a susceptible polymer material is exposed to a combination of tensile stress and exposure to a substance that degrades the integrity of the material. At high magnification, the crack surface had fracture features that were significantly different than features observed in laboratory-created impact fracture surfaces. The examination further revealed liquid oozing from secondary cracks. The liquid was identified as aplasticizer, a chemical that could cause ESC in acrylic. The source of the plasticizer was unknown, but potential sources include flexible polymers such as vinyl tape potentially used in the construction of the windshield or possible contact with a vinyl material if that material had been used in the airplane interior. Plasticizers can exude from the adjacent material due to changes in temperature or humidity, mechanical stress, or weathering. The windshield edge showed limited visual evidence of adhesive on the bond surface, with evidence of voids in the epoxy at the interior bond surface.