What happened
On 8 April 2006, a Piper PA-18-180 Super Cub, registration G-BEUA, was performing a private glider towing operation at Dunstable Airfield. Following the completion of its second glider tow of the morning, the aircraft touched down with very low forward speed. During the landing roll, the right under-fuselage ‘A’ frame failed, causing the right landing gear to collapse. This failure resulted in the right wing dropping and the wing tip making contact with the ground. There were no injuries to the pilot or the passenger, and the engine was shut down safely with the propeller remaining clear of the ground.
The investigation
Investigators examined the structural integrity of the landing gear assembly. The aircraft had been operating from Dunstable since 1979 and had undergone a complete refurbishment in 1992. At the time of the accident, the airframe had recorded 11,750 flying hours and roughly 58,000 landings.
Technical analysis focused on the attachment lugs for the left side of the inverted ‘A’ frame. The engineer involved in the investigation noted that the geometry of the joint between the ‘A’ frame and the attachment lugs made cracks difficult to detect during routine daily or 50-hour inspections, especially given the accumulation of oil, dust, and dirt in the area. The examination of the fracture surfaces revealed that the cracks originated near the run-out of the weld beads used to secure the washers. The inspection showed that the aft lug had failed completely, while the forward lug had failed due to an overload following the initial crack.
Findings
- The primary cause of the landing gear collapse was a fatigue mechanism within the attachment lug for the left side of the ‘A’ frame.
- The cracks originated near the weld beads of the washers, an area subject to significant stress concentrations.
- The aircraft had previously experienced a similar failure of the left-side attachment lugs in 1994, which had been repaired by replacing the lugs.
- Since that 1994 repair, the aircraft had completed approximately 21,300 landings.
- The uneven surface of the Dunstable Airfield likely contributed to higher-than-normal loads being placed on the landing gear compared to operations on paved runways.