The reasons behind an emergency landing involving an Air New Zealand flight at Palmerston North Airport in 2017 has been revealed.
A report by the The Transport Accident Investigation Commission said an issue with the landing gear was to blame.
"The problem was caused by the failure of the two landing gear locking springs, designed to hold the right-hand landing gear in a down and locked position.
"The springs failed because of corrosion-induced cracking. Salt, as a result of the saline environment in which the aeroplane was operated, was considered as the likely initiator of the corrosion."
The alert was raised when the flight crew received a warning their right-hand main landing gear was not locked down, as the aircraft approached Nelson.
Attempts to resolve the issue while circling Nelson were unsuccessful so the decision was made to diver to Palmerston North.
The aircraft landed safely and the landing gear did not collapse, there were no injuries and damage was limited to one tyre.
The commission also said aircraft engineering teams would have been unable to detect the corrosion or cracking, so preventative maintenance wouldn't have been possible.
Despite agreeing with choice to divert the aircraft to Palmerston North, the TAIC said the decision by the pilots to raise and lower the landing gear several times could have exacerbated the condition and was outside documented procedures for the situation.
Air New Zealand told Newshub it fully supports the TAIC's report into the landing gear failure of the Mt Cook Airlines operated ATR aircraft.
A spokesperson said following the incident the airline has completed a fleet-wide inspection of the locking springs installed on its aircraft.
The inspection found no failed or fractured springs.
The airline also undertook a programme to replace those locking springs that had accumulated the number of cycles recommended in the report.
A flight cycle refers to the number of times an engine is switched on, flown and switched off over time.