What happened
On 12 November 2008, a Piper PA-34-200T Seneca II, registration G-ROUS, was conducting a routine training flight at Oxford Kidlington Airport. The flight involved an instructor and two student pilots. During the circuit, a loud noise was heard as the landing gear was extended, followed by a 'gear unsafe' warning light. While the tower controller and another aircraft observed the gear to be down, the cockpit indication remained red.
In an attempt to rectify the indication, the instructor cycled the gear up and down, but the warning persisted. The instructor then utilized the emergency gear lowering switch, which failed to change the cockpit status. During the final approach, the instructor took control of the aircraft. As the aircraft touched down, the left main gear leg collapsed, causing the aircraft to veer off the runway onto the grass. There were no injuries to the three occupants, though the aircraft sustained damage to the left landing gear mechanism, wing tip, aileron, and flap.
The investigation
Investigators examined the left landing gear and discovered that the retraction fitting had failed along three distinct fracture lines. This failure caused the top sections of the fitting to separate from the lower half, which remained attached to the truss assembly. A portion of the fitting and the downlock hook assembly were found lodged near the microswitch plate.
Detailed analysis of the aluminium silicon casting revealed wear marks around the bolt holes and on the underside of the fitting. The fracture surfaces showed evidence of ductile overload. Furthermore, the investigation determined that the discolouration and polishing on certain fracture surfaces indicated that two of the failures had actually occurred during a previous flight cycle.
Findings
- The primary cause of the gear collapse was the failure of the retraction fitting due to ductile overload.
- The failure was initiated during a previous flight when the downlock hook assembly improperly engaged with the fitting, causing the hook to drop to an abnormal position.
- The subsequent hydraulic extension of the gear transferred excessive loads through the weakened fitting, leading to the structural break.
- The gear appeared visually extended to external observers, but because the side-brace truss was not over-centre and locked, the cockpit 'unsafe' warning remained active.