What happened
On July 27, 2006, a PIPER PA-34-200T, registration EC-FDE, was conducting a dual-control instructional flight from Cuatro Vientos Airport to Valladolid Airport. During the initial landing attempt at Valladolid, the crew observed the aircraft veering to the right and noted an intermittent signal from the nose gear position light. The crew elected to perform a go-around to verify the gear's operation.
After cycling the landing gear in flight and receiving a normal indication, the crew attempted a second landing. Approximately 300 meters down the runway, the aircraft began tilting to the right at low speed. The right wing and propeller subsequently contacted the ground, causing the aircraft to come to a stop on its right wing. The occupants of the aircraft, an instructor and a student pilot, sustained no injuries.
The investigation
The investigation focused on the structural integrity of the landing gear following the discovery that the aircraft had lost its right wheel, wheel fork, and axle. Post-accident inspection of the PIPER PA-34-200T revealed that the shock absorber piston had completely fractured. Laboratory analysis of the fracture surface identified an area of progressive fatigue crack growth, followed by a brittle failure that occurred when the wheel separated from the fork during the landing impact.
The investigation examined the maintenance history of the aircraft, noting that the operator had voluntarily implemented a 500-hour inspection of the piston using a camera, following a similar incident in 2005. The most recent inspection, conducted only 48 flight hours prior to the accident, had failed to detect any defects.
Findings
- The primary cause of the incident was the fracture of the shock absorber piston on the right main landing gear leg due to metal fatigue.
- The fatigue crack initiated on the surface of the lock screw for the piston plug, where the rough surface finish and high stress concentrations facilitated crack growth.
- The instructional nature of the flight may have contributed to the failure, as training maneuvers and landings can subject the landing gear to a higher frequency of load cycles and harsher impacts than standard operations.
- The existing inspection method used by the operator was unable to detect the crack, either because the method was ineffective or because the crack progressed from an undetectable size to a critical state very rapidly.