Amateur-built IV N549CJ accident near Laramie, Wyoming

Probable cause

An environmental stress cracking failure of the acrylic windshield, which resulted in a rapid decompression during cruise flight.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for WPR19LA088 for the full record.