What happened
On 21 December 2006, a Piper PA23-250-E Aztec, registration ZK-PIW, was conducting a multi-engine instrument flight training mission when a series of landing gear malfunctions occurred. The flight, operated by Mountain Air, began with a departure from Auckland International Airport toward Great Barrier Island.
After an initial landing at Great Barrier Aerodrome, the crew noticed the nose gear position light had extinguished. Following advice from a maintenance engineer, the pilots lubricated the nose gear door linkages and micro-switches, which restored the green indication. Per the engineer's suggestion, the pilots kept the landing gear in the down position for the subsequent flight to Ardmore Aerodrom.
During the flight to Ardmore, the right main landing gear position light also failed. Despite the loss of the indicator, the pilots believed the gear remained mechanically locked, as the control lever had not been moved. After a low flyover at Ardmore was visually cleared by the UNICOM operator, the aircraft attempted a landing on runway 21. While the aircraft initially touched down normally, the right main gear collapsed during the landing roll. The right wing contacted the runway, causing minor damage to the aircraft, but the pilots managed to keep the aircraft on the runway and sustained no injuries.
The investigation
TAIC investigators examined the mechanical state of the landing gear and the maintenance history of the aircraft. The investigation focused on why the gear failed despite the pilots' belief that it was securely locked. Investigators reviewed the aircraft's maintenance logs, noting that while the aircraft was maintained according to schedule, it was a 33-year-old airframe that had required increasing amounts of unplanned maintenance for hydraulic and landing gear components.
Findings
- The collapse was likely caused by a combination of worn landing gear components and hydraulic leaks.
- An internal leak in the actuator allowed hydraulic pressure to bleed off during the flight, potentially undermining the mechanical down lock.
- The failure of the nose gear and right main gear lights may have been symptoms of the overall worn condition of the landing gear assembly.
- The decision to keep the gear down for the duration of the flight to Ardmore may have contributed to the collapse by allowing pressure to equalize within the actuator.
Safety action
Following the investigation, the Commission recommended that the Director of Civil Aviation define "ageing aircraft" and collaborate with the industry and manufacturers to develop specific guidance and regulatory documentation to ensure the safe operation of older aircraft fleets.