What happened
On 13 July 2002, a Renegable Spirit, registration G-MZIP, was conducting a private flight from Cumbernauld Airport toward Oban. The flight was part of a longer journey that had begun earlier that morning near Winchester, involving several refueling stops through Wales and the Lake District.
Following a period on the ground at Cumbernauld, the aircraft departed under favorable weather conditions, characterized by good visibility and a light north-westerly wind. While climbing through approximately 700 feet, the aircraft suffered a loss of power, making it impossible for the pilot to maintain altitude. After verifying that fuel selectors and switches were correctly positioned, the pilot determined that a forced landing was necessary. Due to the rising terrain ahead, the pilot was forced to land up-slope on a hillside. The aircraft struck the ground in a nose-down attitude.
The impact caused the fuel tanks to rupture. The pilot, who sustained minor bruising, managed to extract the passenger from the front cockpit. The passenger suffered two broken legs. Because a radio distress call had not been transmitted, the pilot used a mobile phone to alert the airport, which prompted a rapid response from a nearby Robinson helicopter.
The investigation
Investigators examined the engine to determine the cause of the power loss. A strip examination of the Rotax 532 engine showed that the unit had been well-maintained and was otherwise in good condition. However, inspectors discovered a buildup of carbon whiskers on the electrode of the spark plug in the number one cylinder. This buildup was significant enough to cause a short circuit, preventing the plug from sparking. Because this engine utilizes a single ignition system, the failure of this single spark plug resulted in the engine operating at only half its intended power.
Findings
- The primary cause of the loss of altitude was the engine operating at half power due to a fouled spark plug.
- Carbon whiskers had accumulated on the number one cylinder's spark plug electrode, causing it to short out.
- The single ignition configuration of the engine meant there was no redundancy to compensate for the failed plug.
- The aircraft was operating at a relatively high weight during the climb, which exacerbated the impact of the power loss.