What happened
During a short local flight departing from and returning to Linköping/Saab Airport (ESS/ESSL), an aircraft carrying a pilot and one passenger experienced an excursion from the runway. The approach to runway 11 followed a standard glide path, with the aircraft touching down approximately 300 meters along the runway. Following a brief period of rolling on the runway, the aircraft began to yaw toward the left and roll toward the right. This uncontrolled yaw, a ground loop, caused the aircraft to exit the runway, leading to significant damage to the right wing and the landing gear.
The investigation
Investigators examined the structural failure of the right landing gear, which had broken at its attachment points to the airframe and folded underneath the fuselage. The inspection revealed that the bolt for the inboard attachment was bent and the threads were damaged. Notably, the nut for this bolt was missing and could not be located. The evidence suggests that the threads on the nut sheared due to an instantaneous longitudinal overload on the bolt. The investigation also considered whether the integrity of the bolted joint had been previously compromised by insufficient tightening torque or worn threads on the nut.
Findings
The primary cause of the accident was the pilot's decision to continue the landing attempt despite having experienced several bounces prior to touchdown.
Several contributing factors were identified, including the pilot's limited experience with this specific aircraft type and the fact that the touchdown occurred on the main landing gear rather than the intended three-point landing. The resulting substantial lateral forces exerted on the wheel caused the inboard bolted joint of the right landing gear to overload, leading to the gear failure and subsequent contact between the right wing and the ground.