What happened
On 21 February 2009, a pilot operating a home-built MW7 monoplane, registration G-BREE, performed an initial flight from Lower Upham Farm airfield. After an uneventful first leg, the pilot refuelled the aircraft to its maximum takeoff weight and departed again at 1115 hrs.
While cruising at roughly 1,000 ft agl, the engine experienced a sudden loss of power. Although the engine continued to run, it could no longer sustain the aircraft's altitude. The pilot initiated a descent at 50 kt to troubleshoot the issue but could not identify a cause while power was still available. He subsequently selected a field for a forced landing. As the aircraft reached an altitude of approximately 10 to 15 ft, the engine ceased running entirely. During the impact, the right main landing gear sheared off, causing the aircraft to roll onto its back. The pilot sustained minor injuries, and while fuel leaked from the tank vent, no fire occurred.
The investigation
The investigation focused on the sudden loss of engine performance and the subsequent mechanical failure. Investigators examined the engine and found that a spark plug had become unscrewed from the front cylinder. While the spark plug remained attached to the high-tension lead, the loss of power was directly linked to its displacement.
It was noted that the spark plugs and the front Cylinder Head Temperature (CHT) probe had been replaced more than six months prior. The specific spark plug involved was located beneath a newly fitted CHT probe. Although the ring from the CHT probe was missing following the accident, the pilot did not believe it had caused the plug to loosen. However, investigators considered the possibility that the threads of the spark plug may have been bound by the CHT probe during installation, resulting in an inaccurate torque reading and leaving the plug insufficiently tightened.