ERA23LA013

The pilot reported that the glider was at 3,700 ft mean sea level when the canopy suddenly, without warning, separated from the glider. He subsequently landed uneventfully at his intended destination airport. Review of maintenance records revealed that about 2 years before the accident, a maintenance facility stop-drilled a hole to correct a canopy crack at the rear hinge along the glide frame. Metallurgical examination revealed that the drilled hole appeared to halt progression of the original crack along its initial propagation direction. However, both the new primary separation crack and a secondary crack propagated from the hole in diametrically opposite directions, which suggests they likely developed due to a stress state along the same plane. Additionally, both cracks propagated from internal surface corners of the hole, indicating the corners likely acted as stress risers that facilitated crack initiation. There was no approved method from the glider manufacturer to stop-drill a hole in plexiglass; however, manufacturer approval was not required as the glider was registered in the experimental category. The glider manufacturer stated that it was common practice for cracks in the plexiglass to be drilled-out to stop the crack progression as a temporary solution; however, for a proper repair, the canopy must be removed and re-glued. The presence of multiple cracks propagating out of the same stress riser of the internal hole corner indicated that the specific hole drilled did not prevent subsequent progression of further cracking and the eventual separation of the canopy.

On October 8, 2022, about 0950 eastern daylight time, an experimental Schempp-Hirth Discus CS glider, N520RJ, was substantially damaged when it was involved in an accident near Woodstock, Virginia. The private pilot was not injured. The glider was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that the glider was at 3,700 ft mean sea level when the canopy suddenly, without warning, separated from the glider. The pilot double-checked the canopy latch and jettison handle and confirmed that he did not jettison the canopy. He subsequently landed uneventfully at his intended destination airport. Review of maintenance records revealed a work order dated August 9, 2020, which stated “…Found canopy crack at rear hinge along frame and stop holed crack….” Separated sections of the glider canopy were recovered and forwarded to the National Transportation Safety Board Materials Laboratory, Washington, D.C. Metallurgical examination revealed that the drilled hole appeared to halt progression of the original crack along its initial propagation direction. However, both the primary separation crack and a secondary crack propagated out of the hole in diametrically opposite directions, consistent with occurrence due to a stress state along the same plane. Additionally, both cracks propagated from internal surface corners of the hole, consistent with the corners acting as stress risers that facilitated crack initiation (for more information, see Materials Laboratory Factual Report in the public docket for this accident). According to a representative from the glider manufacturer, the canopy was made from plexiglass and there are no manufacturer instructions for repairing cracks in the canopy. Additionally, it has been common practice for decades that cracks in the plexiglass are drilled out to stop the crack progression as a temporary solution; however, for a proper repair, the canopy must be removed and re-glued.