Incident Overview
On 9 April 2017, an ATR72-212A aeroplane, registration ZK-MCY, operated by Mount Cook Airline, was on approach to land at Nelson with 66 passengers and five crew on board. When the crew selected the landing gear down, they received an alert that the right-hand main landing gear was not locked in the down position. The red light remained illuminated for the right main gear.
Crew Actions
The crew initiated a go-around and notified air traffic control. They held in visual conditions north of Rabbit Island while completing the landing gear unsafe indication and gravity extension checklists, but the unsafe indication persisted. The crew then recycled the landing gear by retracting and lowering it again, which is not in accordance with documented procedures. The problem remained. After discussion, they decided to divert to Palmerston North Aerodrome, citing its longer runway and favorable weather. A 'Pan Pan' call was made, and emergency services were requested.
During the diversion, the crew retracted the landing gear and climbed to 15,000 feet. The cabin attendants briefed passengers and prepared the cabin for an emergency landing. The operator's duty pilot offered assistance. After holding and fuel burn, the crew lowered the gear a third time and performed steep turns to attempt to lock the gear, but without success.
Landing
At about 1635, the captain positioned the aeroplane for a visual approach to runway 07 at Palmerston North. The captain landed on the left main gear first, then lowered the right gear and nose gear onto the runway. Both engines were shut down, and the aeroplane was brought to a halt. Passengers disembarked via the rear door, and no injuries occurred. The right outer main tyre was found deflated, with a skid mark indicating a locked wheel that skidded about 150 metres before bursting.
Investigation Findings
The Transport Accident Investigation Commission determined that the landing gear problem was caused by the failure of the two locking springs designed to hold the right main landing gear in the down and locked position. The springs failed due to corrosion-induced cracking. Salt from the saline operating environment was considered the likely initiator of the corrosion.
The investigation also identified a safety issue: the maintenance inspection program for the locking springs was unlikely to detect corrosion cracking before failure, and no required preventive maintenance limited corrosion damage.
The crew's decision to divert to Palmerston North was appropriate, and preparation for the emergency landing was well conducted. However, recycling the landing gear several times was outside documented procedures and had the potential to exacerbate the condition.
Safety Actions
Due to safety actions taken by the operator and aeroplane manufacturer, no new recommendations were made.