What happened
On 9 April 2017, an ATR72-212A, registration ZK-MCY, was performing a scheduled flight from Auckland to Nelson with 71 people on board. During the approach to Nelson, the flight crew activated the landing gear lever, only to receive an alert indicating that the right-hand main landing gear had failed to lock in the down position.
Following a go-around, the crew attempted to resolve the issue by performing various checklists and even recycling the gear by retracting and re-extending it. The crew also performed steep turns to use centrifugal force to attempt to lock the gear. Despite these efforts, the unsafe indication persisted. The crew decided to divert to Palmerston North Aerodrome, a location with a longer runway and favorable weather.
Upon arrival at Palmerston North, the aircraft landed, with the captain intentionally touching down on the left main gear first before lowering the nose and right gear. During the landing roll, the right outer main tyre deflated and eventually burst after skidding. There were no injuries to the 66 passengers or five crew members.
The investigation
Investigators examined the aircraft's landing gear assembly and the fractured components. The investigation focused on the two locking springs responsible for holding the right-hand main landing gear in the down and locked position.
Metallurgical analysis of the springs revealed that the failure originated from intergranular corrosion and stress corrosion cracking. Chemical analysis (EDAX) identified the presence of chlorine and sodium at the crack initiation sites, suggesting that the saline environment of the aircraft's operating area played a role. The cracks progressed through fatigue and corrosion fatigue until the remaining metal could no longer support the load, leading to an overload failure.
Findings
- The primary cause of the landing gear failure was the failure of the two landing gear locking springs due to corrosion-induced cracking.
- The corrosion was likely initiated by salt exposure from the aircraft's saline operating environment.
- The crew's decision to divert to Palmerston North was appropriate and well-managed, though the decision to recycle the landing gear was outside of documented procedures and had the potential to worsen the mechanical condition.
- The existing maintenance inspection programme was unlikely to have detected the corrosion cracking before the failure occurred, as there was no specific requirement for preventive maintenance of these springs.
Safety action
- The investigation highlighted the importance of pre-flight visual inspections for critical components like landing gear.
- It was noted that while recycling the gear did not cause further damage in this instance, such actions can exacerbate mechanical failures and should be avoided unless permitted by the flight crew operating manual.