History of the Flight
The pilot approached Guernsey from the north and was cleared for a right-hand base leg join for Runway 27. On base leg, he selected the first stage of flap and lowered the landing gear, observing normal indications. He confirmed three green 'down and locked' indications and visually checked the gear via the landing gear viewing mirror. On final approach, he selected landing flap and received clearance to land. The approach was stable, with no crosswind. The aircraft touched down on the runway just beyond the numbers. As the nose wheel was lowered, the nose continued to drop, and it quickly became apparent from the nose-down attitude and damage to the now stationary propellers that the nose gear had collapsed. The aircraft travelled in a straight line with its nose scraping on the runway surface, coming to rest about 300 metres along the runway. The pilot, who was the sole occupant, was uninjured and exited normally via the rear door.
Aircraft Information
The Cessna T303 Crusader is an all-metal, low-wing, six-seat, twin-engined aircraft with a tricycle landing gear. G-CYLS, serial number T303-00005, was built in 1982 and held a current EASA Certificate of Airworthiness valid until 18 May 2008.
Nose Landing Gear Description
The nose gear retracts forwards and consists of a shock strut mounted in a trunnion assembly, a nosewheel, shimmy damper, and a double-acting hydraulic actuator for extension and retraction. The upper end of the actuator is fixed to the aircraft structure, and the end of the actuator ram is attached to the rear of the nose gear. During gear extension, hydraulic pressure retracts the actuator ram, causing the nose gear to extend rearward. When fully down, spring-loaded locking hooks on the end of the ram engage onto downlock pins on the body of the actuator. The gear is thus mechanically locked in the down position by the actuator body acting as a rigid drag strut. The downlock pins are located in locking lugs on the actuator body and retained by roll-pins.
Nose Gear Actuator Examination
The actuator fitted to G-CYLS, part number 1280514-11 and serial number 133, was manufactured in 1981. Enquiries with the aircraft manufacturer revealed no overhaul or special inspection requirements for the nose gear actuator, indicating the failed actuator may have been on the aircraft since build. The failed actuator underwent detailed metallurgical examination. Both locking lugs had fractured, liberating a segment of each lug and its downlock pin, although the roll-pins remained in the lugs. Of the two fractures on each lug, one was solely due to ductile overload and secondary to the initial failure; the other exhibited much less deformation. One primary fracture was very flat with no obvious signs of ductile overload and considerably more discoloured by corrosion products than the other. The other primary fracture had a lesser degree of discolouration and a clearly visible region of ductile overload. Fractographic examination using optical and scanning electron microscopy found the primary fractures to be generally intergranular, with no evidence of fatigue propagation. It was not possible to state with certainty what mechanism caused the primary fractures. Metallographic examination revealed an unfavourable grain orientation around the ends of the lugs, such that applied loads acted along grain boundaries, as evidenced by the primary fractures being parallel to the grain boundaries.
Discussion
The discolouration of the primary fractures indicated they had been present for some time, in contrast to the fresh appearance of the final ductile overload failures that resulted in the nose gear collapse. The cracks had occurred over a period, but the mechanism causing the primary fractures was not determined. It is conceivable that a discrete occurrence such as a shock load from a heavy landing or mishandling during towing may have been the initiating event. The unfavourable orientation of the grain boundaries at the ends of the lugs likely had a significant influence on local material strength, providing planes of weakness for crack initiation and propagation. The aircraft manufacturer reported awareness of occurrences of lugs/pins failures on this actuator and indicated that rough ground handling creates more damage than normal operations, suggesting the failure was likely induced by earlier rough ground handling.
Safety Action
In the original design, a groove was cut into the downlock pin and the roll-pin engaged with this groove to retain the pin. In a new design, the groove was eliminated, as it acted as a stress concentration, and replaced by a hole through the pin into which the roll-pin is inserted.