What happened
On July 15, 2016, a single-seater SZD-51-1 “Junior” glider, registration PH-1006, was performing local flight maneuvers near Larserveld. After an hour of thermal soaring, the pilot attempted to lose altitude by performing several loops. The pilot increased the airspeed to 180 km/h and successfully completed one loop. However, during the final phase of a second loop, as the aircraft leveled out, the pilot heard a loud, sharp sound described as glass breaking. Despite the structural noise, the pilot maintained control of the aircraft and landed safely, though a subsequent inspection revealed significant damage to the left wing.
The investigation
The Dutch Safety Board (DSB) engaged the Netherlands Aerospace Centre (NLR) to perform a detailed failure analysis of the left wing spar. Using scanning electron microscopy, investigators examined the fracture surfaces of the glass-fibre reinforced plastic (GFRP) structure. The investigation focused on the interface between the aluminum insert and the composite spar cups.
Investigators discovered that the aluminum insert had lost its adhesion to the C-frames. This separation created a gap that allowed the insert to move freely within the structure. Because the insert was no longer bonded to the GFRP material, the structural loads were no longer distributed evenly; instead, they were concentrated into much smaller contact points at the end of the spar and the top of the insert. This concentration of force created extreme local stresses within the composite layers.
Findings
- The investigation established that a crack in the left wing spar existed prior to the second loop.
- The loss of adhesion between the aluminum insert and the GFRP material caused high local stresses in the spar's C-frames and wrapping.
- The structural failure occurred due to a high load at the end of the loop, which coincided with the moment the pilot heard the loud bang.
- While the crack had been growing for some time, the specific reason why the aluminum insert detached from the composite material could not be determined.
- Previous scheduled maintenance and 1000-hour inspections had failed to detect the internal detachment or the developing crack.