What happened
On 21 January 2004, a Piper PA-3-200 Seneca, registration G-TEST, was performing a training flight at Stapleford Aerodrome, Essex. Following the completion of an instrument training session, the pilot executed a visual landing on the asphalt section of Runway 22 using full flaps. The touchdown was described as normal, and the pilot applied slight to moderate braking after the aircraft had travelled approximately 70% of the runway length. Suddenly, the right main landing gear collapsed, causing the aircraft to veer 90 degrees to the right. The aircraft eventually came to a halt near the edge of the runway. There were no injuries to the three crew members on board, though the aircraft sustained damage to the propeller, landing gear, and right wing.
The investigation
Investigators examined the right main landing gear oleo strut to determine the cause of the collapse. They discovered that the cylindrical tube had fractured near its lower end within the assembly that holds the wheel axle. This fracture caused the wheel and half fork assembly to detach, leaving the remaining part of the tube to make contact with the runway surface.
Metallurgical analysis of the components revealed that the failure was caused by a fatigue mechanism. This fatigue originated at the forward bolt hole of the strut. While the upper fracture surface was heavily abraded by the runway, the lower portion of the component remained protected. The investigation also looked into the history of the aircraft, noting that while it was manufactured in 1974, it was registered in the UK in 1989. The investigators noted that the components involved in the failure might have been the original parts from the time of manufacture.
Findings
- The primary cause of the failure was fatigue cracking that initiated at the forward bolt hole of the oleo strut.
- The fatigue crack spread around approximately 25% of the strut's circumference before a rearward bending overload caused the final separation.
- The fatigue resistance of the component was significantly compromised by abusive machining during the drilling of the bolt hole, which left a very poor surface finish.
- This poor surface finish was likely caused by improper drilling techniques, such as using a blunt drill, insufficient lubricant, or excessive feed speeds.
- The failure was driven by standard in-service stresses from landing, braking, and taxiing acting upon a weakened component.