What happened
On the morning of 25 August 2021, a Jabiru J450 with registration ZU-DFR was conducting a ferry flight from Wonderboom Aerodrome (FAWB) to Rhino Park Airfield. The flight followed a recent annual inspection completed on 19 August 2021.
While climbing after departing Runway 29 at approximately 5800 feet, the pilot noticed a significant drop in engine performance. The engine revolutions per minute (rpm) decreased from 2800 to roughly 2100 rpm. In response to the partial power loss, the pilot issued a Mayday call to the tower and was directed to return to Wonderboom. The pilot successfully repositioned the aircraft for landing on Runway 06, completing the return without further incident. There were no injuries and no damage to the aircraft.
The investigation
Following the landing, the aircraft was moved to a hangar where a mechanical inspection was performed. Upon removing the engine cowlings, investigators discovered that the rear spark plug in the number 6 cylinder had been forced out of its position.
An examination of the hardware indicated that the threads within the cylinder head had been compromised. The investigation determined that the damage likely occurred because the spark plug had been cross-threaded and over-torqued during its recent installation. To rectify the issue, the number 6 cylinder was sent to an engineering facility where a new thread shell was inserted into the spark plug hole. Following the repair, the spark plug was reinstalled using a correctly calibrated torque wrench according to manufacturer specifications, and subsequent engine tests confirmed normal operation.
Findings
- The primary cause of the power loss was the ejection of the rear spark plug from the number 6 cylinder.
- The cylinder threads were damaged due to cross-threading and over-torqueing during the installation process during the aircraft's recent annual inspection.