What happened
On 23 November 2001, a Piper PA-38-112, registration G-BYMC, was conducting a training flight near Alsager, Staffordshire. The crew was performing practice forced landings (PFLs) after departing from Manchester Airport controlled airspace. Following the successful completion of two landings in a single field, the aircraft was climbing to reposition for further exercises when the crew heard a loud popping sound. This was immediately followed by a loss of engine power and intense vibrations throughout the airframe.
After performing the necessary emergency checks, the instructor determined that a full forced landing was required. The aircraft landed in a field, and while there were no injuries to the two crew members, the aircraft sustained damage to the engine, the left wing tip, and the leading edge after the wingtip made contact with a hedge at the edge of the field.
The investigation
The investigation focused on the mechanical failure of the Lycoming O-2CL piston engine. A subsequent inspection by the aircraft's maintenance organisation identified that the lower plug of the front right (No 1) cylinder had dislodged from its threaded hole. Although the plug was still held in place by its lead, it had failed to remain seated.
Upon examining the threads of both the plug and the cylinder head, investigators found that only the final two threads of each component showed signs of engagement or wear. This indicated that the plug had not been properly installed or tightened during the most recent 50-hour maintenance check, which had been performed 16 flying hours prior to the incident.