An Air New Zealand aircraft that suffered engine failure after being hit by a bird had been released to fly.
The Air New Zealand Airbus A320 was landing at Wellington Airport in June 2012 when a black-backed gull struck its right engine, a Transport Accident Investigation Commission (TAIC) report said.
As the bird strike had only involved one engine and no damage had been observed, the plane was allowed to continue for one more take-off and landing, before having a more thorough inspection.
It was released back into service later that day for a flight to Auckland under this "continued operating allowance", with 165 passengers and five crew members onboard.
But as it came into land at Auckland International Airport, the same engine failed.
The captain had to reduced the engine thrust to idle to continue with the landing.
"Although damaged internally, the engine continued to run and was used during the landing," the TAIC report said.
A later inspection of the failed engine revealed damage caused by the bird being ingested into the core of the engine.
This led to cracking a compressor blade in the third-stage compressor, which grew under stress as the engine was used in a damaged state.
It fractured completely and caused significant damage to other components as it passed through other compressor stages in the jet engine.
This incident was the first reported worldwide where a V2500 engine had failed while operating under the continued operating allowance following a bird strike, TAIC said.
After reviewing the requirements around this continued operating allowance, the TAIC found the resulting risk to aviation safety was reasonable and made no recommendations.
It said that systems would have generated automatic reports to the operator's maintenance operations control during the flight, which could have alerted it that the damage from the bird strike was worse than initially thought.
However, these did not reach the control centre as intended, but the reasons for this had now been rectified, it said.
The TAIC also reviewed Wellington International Airport's measures to control bird life around the facilities and found they met industry best practice.
Although safety was not compromised by releasing the aircraft, the TAIC said operators needed to balance the cost of having inspection services available at airports with the cost of an engine failure at this scale.
It also said that if mandatory minimum checks had been made to an engine after bird strike, engineers should increase their vigilance around engine performance until full checks could be completed.