A helicopter crash in Rotorua last year was caused by an event so rare it has no implications for aircraft safety, the Transport Accident Investigation Commission has found.
The crash happened soon after the French-made Guimbal Cabri G2 helicopter had taken off from Rotorua airport in April 2016.
The pilot heard a loud bang, and saw smoke pouring into the cabin.
He immediately landed the craft and got out without injury.
Shortly afterwards, the helicopter erupted in flames and was destroyed.
In a report into the accident, the TAIC found the fire was caused by the ejection of a ceramic insulator and centre electrode from one of the engine spark plugs.
That allowed flame from combustion to ignite the engine cooling shroud.
The commission found the spark plug insulator was a genuine part that had been formally approved for use, but it failed because it had not been swaged during the manufacturing process.
Swaging is a process that bends a piece of metal around another object to hold it tight.
In this case, the spark plug was held in place by a tight fit and an adhesive compound, not by swaging.
But after 121 hours of service, the adhesive compound could not hold against the pressure within the cylinder and the insulator was ejected. This allowed a jet of flame approximately 870°C to pour out of a hole in the engine.
Despite this, the commission found the crash did not have long term safety implications.
It said a relatively small number of spark plugs were made each year and the assembly process and quality control were intensively manual, leaving open the opportunity for human error.
No other problems were found in any other spark plugs, so it was likely human error during assembly was to blame, it said.
The commission made no safety recommendations in its report.