What happened
On 15 June 2000, during a crew training flight at Gisborne Aerodrome, a Fairchild SA22 7-AC Metro III, registration ZK-RCA, experienced a significant mechanical failure upon landing. During a training circuit, the aircraft experienced a firm touchdown, which caused the left undercarriage drag braces to fail. This failure caused the left landing gear to collapse rearwards, leading the aircraft to veer off the runway. The crew immediately initiated a go-around.
Following the incident, the crew noted that the left undercarriage indicator light remained red, signaling that the gear was not properly locked. After attempting to force the gear into position with vertical acceleration, the crew determined the gear was stuck in a trailing position. To ensure safety, the pilots elected to fly the aircraft to Hamilton Aerodrome, which offered superior emergency services and a longer runway.
Upon arrival at Hamilton, the crew performed several low approaches to inspect the gear. Following manufacturer recommendations, the captain decided to perform a controlled wheels-up landing. Before touchdown, the crew shut off the fuel and feathered the propellers to mitigate fire risks. The aircraft touched down on runway 18, sliding and eventually coming to rest on the grass. No injuries were sustained by the two crew members on board.
The investigation
Investigators examined the failed components and discovered that a fatigue crack had developed in the outboard lower drag brace of the left undercarriage. The crack originated in a machined recess designed for a grease fitting. This specific geometry created a stress concentrator that facilitated the crack's growth. When the aircraft landed firmly at Gisborne, the crack had already reached a critical length, causing the outboard brace to fail and subsequently overloading the inboard brace, which also failed.
Findings
- The primary cause of the undercarriage collapse was a design deficiency in the drag brace, where square-edged recesses acted as stress concentrators.
- A fatigue crack had grown to a critical size in the left undercarriage outboard lower drag brace.
- The failure was triggered by a firm landing at Gisborne Aerodrome.
- Standard visual maintenance inspections were insufficient to detect the cracks, as they were located within the machined recesses.
- The failure of the outboard brace caused the immediate failure of the inboard brace due to overload.
Safety action
Following the investigation, the aircraft manufacturer issued Service Bulletins requiring routine inspections of the drag brace links using dye penetrant testing. The New Zealand CAA subsequently issued an Airworthiness Directive to mandate these inspections for all New Zealand-registered aircraft of this type. Additionally, the manufacturer announced a design change to use a tougher aluminum alloy and a redesigned surface geometry to eliminate the stress-concentrating square edges.