What happened
On 15 April 2016, a pilot conducting a solo navigation training flight in a Guimbal Cabri G2, registered ZK-IIH, experienced a sudden mechanical failure shortly after departing Rotorua. While climbing, the pilot heard a loud bang and a continuous clattering noise, followed quickly by the smell and sight of smoke entering the cabin.
Despite the engine malfunction, the pilot maintained control and performed an immediate emergency landing in a nearby paddock. Although the pilot escaped the aircraft without injury, the helicopter was subsequently destroyed by fire. Witnesses observed smoke emanating from the engine area and around the mast, with the flames rapidly engulfing the airframe after the pilot had vacated the cockpit.
The investigation
Investigators from the Transport Accident Investigation Commission (TAIC) examined the wreckage and conducted engine tests to determine the origin of the fire. The examination of the engine's ignition system revealed that the ceramic insulator and center electrode from the number 4 cylinder's upper spark plug had been ejected.
Technical analysis and metallurgical testing showed that the failed DENSO W24EMR-C spark plug was a genuine, approved component. However, the investigation established that the plug had a manufacturing defect: the metal shell had not been properly swaged (crimped) around the ceramic insulator. This lack of compression allowed the insulator to become loose and eventually exit the plug body.
Engine test runs replicated the failure, demonstrating that the loss of the insulator allowed a jet of flame to emit from the spark plug body. This flame impinged on the engine'_s fiberglass cooling shroud, igniting the combustible material and leading to the fire.
Findings
- The in-flight fire was triggered by the ejection of the ceramic insulator from the number 4 cylinder upper spark plug.
- The engine remained operational after the failure, which allowed hot combustion gases to escape through the empty plug shell and ignite the engine's cooling shroud and carbon-fiber structure.
- The spark plug failed because it lacked the necessary swaging during its manufacturing process.
- The pilot's rapid response and the protection provided by the engine bay firewall were critical in preventing a more serious accident.
Safety action
Following the investigation, the Civil Aviation Authority of New Zealand issued a notice recommending that operators and maintenance providers inspect all DENSO W24EMR-C spark plugs for similar defects. Additionally, the helicopter manufacturer and the European Aviation Safety Agency (EASA) issued safety bulletins advising Cabri operators to check their installed spark plugs for serviceability. No further defective plugs were identified, indicating the manufacturing error was an extremely rare occurrence.