A faulty spark plug caused a fire that forced a helicopter to make an emergency landing in a Rotorua field last year.
The pilot was on a solo navigation training flight in the central North Island in a Helicopteres Guimbal Cabri G2 craft on April 15. Shortly after take-off he heard a loud bang and a continuous clattering noise before smoke began to enter the cabin.
The uninjured pilot landed immediately in a paddock on Hawthornden Drive in Tikitere and sprinted from the helicopter before flames destroyed it.
An investigation by the Transport Accident Investigation Commission found that the fire was started by a ceramic insulator and centre electrode that ejected from one of the engine spark plugs. The loss of the spark plug allowed its combustion flame to ignite the engine cooling shroud.
The commission also found that the failed spark plug was a genuine part that had been approved by the relevant airworthiness authority for use in the helicopter's ignition system, but it failed because it had not been "swaged" during the manufacturing process.
Swaging bent the cold metal into place to hold in the insulator.
The type of failure that caused the helicopter fire is so rare that it does not have significant implications for aviation safety.
The investigation praised the pilot's actions, describing them as appropriate and commendable. The immediate landing, combined with the protection provided by the engine bay firewall, prevented a more serious outcome.
The commission said the key lesson arising from the inquiry was that maintenance personnel need to be vigilant for product anomalies when putting components onto aircraft, even when the components come from approved suppliers. It made no recommendations.