Introduction
On 24 July 2001 at 1435 hours UTC, a Cessna 421C registered N6315X sustained damage during a landing at Oxford Airport. The aircraft was on a private flight with one pilot and one passenger. No injuries were reported. The aircraft had previously been involved in a 1998 overrun incident at Denham Aerodrome and had undergone repairs at Kidlington Airport, including replacement of the right wing and a strip examination of the right main landing gear (MLG) hydraulic actuator/sidestay unit.
History of the Flight
After a normal takeoff, the landing gear was retracted. As the aircraft climbed through 600 feet above ground level, the airspeed indication decreased to zero, the vertical speed indicator showed no reading, and the altitude indication remained at 600 feet despite continued climb. The pilot made a right turn and requested a priority landing. During downwind checks, the landing gear was lowered, but only the nose and left main landing gear indicated green; the right main landing gear showed a red light. Emergency procedures, including yawing, applying positive g, and operating the landing gear blow-down system, were attempted but did not resolve the red light. The aircraft was flown past the control tower for visual inspection, and the controller reported that all three landing gears appeared fully extended. The pilot landed on Runway 20 with the Airport Fire Service in attendance. The aircraft touched down gently on both main wheels, and the pilot shut off the fuel, closed down the engines, and feathered both propellers. During the rollout, the right main landing gear leg slowly retracted, causing the right propeller and right wing to contact the runway, damaging the propeller, engine, wing lower skin, and flap. The aircraft veered off the right side of the runway onto the grass. No fire occurred, and the pilot and observer evacuated the cabin.
Investigation
After the accident, the aircraft was placed on jacks for landing gear system testing with AAIB participation. During multiple operating cycles, all three landing gears consistently locked down with three green lights illuminated. However, when pulled into position by hand without hydraulic power, the right MLG actuator consistently failed to lock while the left MLG actuator locked positively. The right MLG actuator was removed and functioned on a hydraulic test rig, where it appeared to lock in the extended position under hydraulic pressure but not when pulled by hand.
Actuator Examination
The actuator was stripped and examined. No failed or defective components were apparent, but wear was observed on the non-chamfered edges of the locking segments and on the chamfered face of the groove in the ram. A 20 mm sliver of the backing ring was found free within the locking mechanism area, but it did not restrict sleeve movement. Microscopic examination revealed metal transfer from the bronze locking segments onto the chamfered face of the locking groove and locally on the ram surface, along with evidence of wear. A similar actuator from service with no landing gear problems showed little wear and no metal transfer. During reassembly, it was noted that a locking segment could be easily inserted in the wrong orientation due to difficulty detecting the chamfer when covered in oil. The maintenance manual instructed that the chamfer of each segment must face away from the retaining washer. Maintenance personnel involved in the previous reassembly after the 1998 accident believed they had installed all segments correctly. In further tests, the actuator was reassembled with one segment deliberately reversed. On the test rig, a lock in the extended position could still be achieved, but the locking sleeve only moved partially towards the locked position. It was considered that any restriction to sleeve movement would prevent full engagement of the segments, and a positive lock would not be assured.
Conclusion
The source did not state a definitive probable cause, only that the observed conditions could lead to incomplete locking.