What happened
On 14 June 2008, a PZL-104 Wilga 80, registration G-EPZL, was performing a private flight in Wortham, Suffolk. During the landing phase, the aircraft suddenly pitched forward and became inverted. Following the impact, the rear seat passenger managed to exit through the right door with help from the airfield owner, while the front seat passenger also escaped via the same door after unbuckling his seat belt. The pilot was initially pinned by the aircraft's internal structure but was able to crawl out once the aircraft was lifted by bystanders. The accident resulted in minor injuries to one crew member and two passengers, though the aircraft sustained damage beyond economic repair.
The investigation
Investigators examined the landing gear assembly to determine why the aircraft had overturned. The aircraft is equipped with a tailwheel configuration, where each main gear consists of a fixed upper leg and a pivoting trailing arm. The investigation established that the trailing arm had detached from the upper leg during the landing. Specifically, the upper portion of the gear leg dug into the ground after the lower part of the right leg had separated.
Technical analysis focused on the spindle that secures the trailing arm to the upper leg. The investigation found that the retaining bolt and two welds had failed, allowing the spindle to move outward. While the internal surfaces of the cylinder and spindle showed some corrosion, this was not considered a significant factor. The examination of the sheared bolt was complicated by smearing on the fracture surfaces, which prevented investigators from identifying any pre-existing defects.
Findings
- The trailing arm assembly detached from the upper leg because the spindle migrated outboard following the failure of the retaining bolt and welds.
- The failure of the attachment components was likely the result of cumulative damage occurring over a series of landings.
- Although the non-perpendicular axis of the shock absorber could theoretically generate axial loads capable of causing such a failure during a heavy landing, the investigators considered cumulative fatigue more probable.