Schempp-Hirth DISCUS CS N520RJ accident near Woodstock, Virginia

Probable cause

Improper repair of a crack by maintenance personnel, which resulted in a subsequent failure of the glider canopy.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for ERA23LA013 for the full record.