What happened
On 3 September 2004, a Piper PA-2/8-161 Warrior II, registration G-BTSJ, was conducting a training flight from Plymouth City Airport when the aircraft's left main landing gear partially detached during takeoff from Runway 31. As the aircraft became airborne, the pilot heard a thud, and shortly after, air traffic control notified the pilot that debris and the inner cylinder of the landing gear assembly, including the wheel, had been left on the runway.
The pilot maintained the climb to 300 feet and performed a circuit pattern to inspect the aircraft's condition. To mitigate further damage upon landing, the pilot shut down the engine and isolated the electrical systems, attempting to land on the right landing gear while supporting the left wing. The aircraft sustained damage to the left wing, left flap, and left landing gear. Both the pilot and the passenger were unhurt during the evacuation.
The investigation
Investigators examined the left landing gear and discovered that the lower torque link pivot bolt had fractured. This failure allowed the inner cylinder to separate from the torsion link and eventually fall away from the outer cylinder. Although the aircraft was not currently fitted with wheel spats, the bolt used was a specific design intended for use with spats, suggesting they had been removed at some point in the past.
Metallurgical analysis of the bolt revealed that it had failed due to a rotational reverse bending fatigue mechanism with multiple points of crack initiation. The investigation also found evidence of corrosion and surface polishing on the bolt shank, which indicated that the bolt had become loose within its sleeve. While the aircraft had received maintenance only seven hours prior to the event, there was no specific requirement to inspect these bolts, only to lubricate the fitting every 100 hours.
Findings
- The lower torsion link attachment bolt suffered a fatigue failure.
- The bolt had become loose in its fitting, leading to rotation and subsequent surface polishing and corrosion.
- The failure was identified as a fatigue-related fracture caused by rotational reverse bending.
- A similar failure had recently occurred on another Piper PA-28-161, G-BTBC.