What happened
On 14 December 2000, a Piper PA23-250D Aztec, registration ZK-DIR, landed at Gisborne Aerodrome following a charter flight from Hamilton. The flight, operated by Sunair Aviation Limited, included the pilot and four passengers. The approach was conducted normally, with the pilot confirming that all three green undercarriage position lights were illuminated, indicating the gear was down and locked.
Upon touchdown, the aircraft landed more firmly than usual. Approximately 10 seconds after the aircraft had slowed on the runway, the nose undercarriage leg suddenly collapsed toward its retracted position. The nose of the aircraft dropped to the runway, causing the aircraft to veer approximately 30 degrees to the right. The impact caused the propellers to strike the runway, destroying the blades, and damaged the lower nose cone structure. Despite the substantial damage to the aircraft, there were no injuries to the pilot or the four passengers.
The investigation
Investigators examined the aircraft's mechanical systems and maintenance history. The aircraft had been maintained according to the required schedule, with the most recent 100-hour inspection completed shortly before the incident. The investigation focused on why the nose gear failed to remain locked.
Testing at a repair facility in Hamilton explored three potential scenarios. First, investigators considered if the gear had failed to lock during the approach; however, the pilot's confirmation of the green lights and the lack of an unsafe warning horn made this unlikely. Second, they investigated whether the undercarriage selector lever had been accidentally moved to the 'up' position; this was deemed improbable due to the specific design and safety lock of the lever.
Findings
While the exact cause of the collapse could not be definitively established, the investigation identified a primary area of interest. The most likely cause was that play in the nose undercarriage drag strut bushes allowed enough movement during landing forces to cause the strut to move out of its over-center position, thereby unlocking the leg. Although this specific failure could not be replicated during subsequent testing, the presence of play in the bushes was confirmed. Additionally, it was noted that an anti-retraction valve on the left main gear was defective, meaning it would not have prevented gear retraction if the selector had been moved, though this was not the primary cause of the event.