History of the Flight
The aircraft, a Piper PA-34-200T Seneca II (registration G-ROUS), departed from Oxford Kidlington Airport for a 30-minute training flight with an instructor in the right-hand seat and student pilots in the left and rear seats. The departure and initial climb into the Runway 19 circuit were flown without incident. After the aircraft turned downwind, the student attempted to lower the landing gear. As the main gear deployed, there was a loud bang. The green 'gear down and locked' indication lights illuminated for the right main and nose gear but not the left main gear. The red 'gear unsafe' warning light remained on.
The instructor told the student pilot to continue flying the circuit normally. Once the aircraft turned onto the base leg, the instructor recycled the gear up and down, but the gear indication lights returned to the same state. The aircraft continued onto final approach, and the instructor informed the tower controller of the problem, requesting a visual check of the gear as the aircraft passed over. Both the tower controller and the pilot of an aircraft at the holding point advised that the gear appeared to be down.
The instructor initiated a go-around and selected the gear up, but the 'gear unsafe' warning light remained illuminated. Again, the instructor told the student pilot to continue flying a normal circuit, and on the downwind leg he advised ATC of his intention to land. During the downwind leg, the instructor confirmed the indication by interchanging the bulbs from the left and right main gear green lights. At the end of the downwind leg, the instructor used the emergency gear lowering switch, but this made no difference to the cockpit indication.
The instructor then became the handling pilot for the final approach and landed on the right main gear, attempting to hold off the left main gear as long as possible. When he felt the left wing drop, he feathered both propellers and retarded both mixture levers. The left wing then contacted the runway, and the aircraft veered left, departing onto the grass at relatively low speed before coming to rest with the tail over the tarmac. The aircraft was rapidly shut down, and all three occupants departed through the rear door.
Landing Gear System
The aircraft is equipped with a retractable tricycle landing gear, hydraulically extended and retracted by an electrically powered reversible pump. When the gear is 'down and locked', three green lights above the gear selector switch indicate that condition. Activation of all three gear down limit microswitches shuts the hydraulic pump off and energizes the green lights. A red light illuminates when the gear is 'unsafe' (neither limit switch contacted). As engine manifold pressure drops below approximately 14 inches of mercury, and if the landing gear has not been extended, a throttle switch actuates a warning horn.
The landing gear is normally extended and retracted by means of the gear selector switch. In the event of hydraulic or electrical system failure, the gear can be extended by pulling the free-fall valve, which bypasses hydraulic fluid and permits the gear to fall under gravity. Once down, a spring maintains the side-brace truss assembly in the locked position until released by hydraulic pressure. A downlock hook prevents the truss assembly from moving until the gear is hydraulically retracted.
The hydraulic actuator ram for the main gear leg is attached to a retraction fitting, located at the top of the truss assembly. This fitting is a key component in the extension and retraction system. As the hydraulic actuator ram extends, the fitting rotates forward, pushing the truss assembly against spring pressure until the gear leg is down and the over-centre position of the truss assembly is reached. The downlock hook is also pushed forward until it latches onto the lower truss link. Retraction works in the opposite sense.
Engineering Inspection
Inspection of the left landing gear revealed that the retraction fitting had failed along three fracture lines. The top sections of the lugs, where the hydraulic actuator eye end and the spring are attached, had separated from the lower half of the fitting, which remained bolted to the truss assembly. A section of the retraction fitting, forward of the attachment point for the downlock hook assembly, was missing completely. The downlock hook assembly had detached from the retraction fitting and was lodged between the down limit microswitch plate and the lower truss link assembly.
Detailed inspection identified various impact and wear marks around the bolt holes and on the body of the fitting. A wear mark on the rear under-surface correlated with a witness mark on the truss assembly. The retraction fitting had been installed as new three years (1,508 flight hours) previously. The fitting is an aluminium silicon casting, and the fracture surfaces had a characteristic granular appearance. The fracture surfaces for failures B and C were significantly darker in colour than for failure A and showed evidence of polishing of the raised sections.
Analysis
The discolouration and polishing on the fracture surfaces of failures B and C suggest that these failures occurred prior to failure A. All fracture surfaces were consistent with ductile overload. Cast aluminium has low fracture toughness and is susceptible to shock overload type failures. Failures B and C probably occurred during a retraction cycle in a previous flight, when the load generated by the downlock hook release was applied to the fitting through the downlock hook assembly for reasons that are not evident. Continued operation of the gear meant that the fitting and downlock assembly would not be working symmetrically, causing the bolt holes to wear. This may eventually have resulted in the downlock hook dropping to a lower position than normal during the first extension cycle of the accident flight and, rather than hooking around the 'stop' on the lower truss link, it butted against it. As the hydraulic ram tried to extend the gear fully, the load transferred up the downlock assembly and into the remaining intact side of the retraction fitting, causing it to fail in overload and the downlock assembly to drop down. The gear would therefore have been 'down' as observed by the tower controller and holding pilot. However, the lack of hydraulic actuator connection or downlock hook securing the side-brace truss meant that it was not over-centre and not locked, leading to the cockpit indication. When the aircraft landed, the unsupported gear leg collapsed under the weight of the aircraft, trapping the downlock hook assembly.
Conclusion
The operator has commenced a fleet-wide inspection programme of the retraction fitting and truss assembly at the next 100-hour maintenance check, with a repeat inspection each annual maintenance check. At the time of writing, six aircraft have been inspected with no adverse findings. The operator also comments that this is the first failure of a retraction fitting in their extensive experience of operating this aircraft type.