What happened
On the evening of 9 January 2004, a Piper PA23-250E Aztec, registration ZK-DGS, was completing a scheduled freight flight from Napier to Paraparaumu. The flight, operated by Sunair Aviation Limited, carried two pilots and approximately 90 kg of bank documents. After a routine landing on runway 34, the crew taxied toward the fuel pumps to offload cargo and refuel for the final leg to Palmerston North.
As the aircraft was coming to a stop at the fuel pumps, the right main landing gear suddenly collapsed rearwards. The impact caused the right wing, engine, and propeller to strike the ground. Both pilots were able to exit the aircraft without injury, and no fire was reported.
The investigation
Investigators from the Transport Accident Investigation Commission (TAIC) examined the wreckage and the aircraft's maintenance history. The inspection of the landing gear revealed that the forward attachment bolt of the right main landing gear drag brace had failed.
Metallurgical analysis of the failed bolt showed evidence of reverse bending fatigue. The investigation also noted that the bushings within the drag brace were slightly elongated, which allowed for movement that contributed to the fatigue process.
Historical records showed that ZK-DGS had been involved in a serious accident in 1995, which resulted in the aircraft being written off and subsequently rebuilt. There was no evidence that the specific drag brace or the failed bolt had been replaced during that 1995 reconstruction, suggesting these components may have been reused.
Findings
- The primary cause of the collapse was the failure of the drag brace forward attachment bolt due to reverse bending fatigue.
- The elongation of the drag brace bushings likely facilitated the bending motion that led to the fatigue.
- It is possible that the components were subjected to excessive stress during the aircraft's 1995 forced landing, creating the initial stress points that eventually led to the bolt's failure.
- The reuse of parts from accident-damaged aircraft may have prevented the detection of existing damage during the 1996 rebuild.
Safety action
Following the investigation, the Commission recommended that the Director of Civil Aviation establish guidelines for operators and maintenance facilities regarding the appropriate inspection and reuse of components salvaged from accident-damaged aircraft.