Aircraft and Flight Details
The aircraft involved was a Piper PA-38-112, registration G-BYMC, manufactured in 1982 and powered by a single Lycoming O-235-L2C piston engine. It was being used for an instructional flight on 23 November 2001, departing for an area suitable for practice forced landings (PFLs) after a normal takeoff and climb out of Manchester Airport controlled airspace.
Incident Sequence
The crew, consisting of an instructor and a student pilot, successfully completed two PFLs into a field near Alsager, Staffordshire. While climbing away to move to a new area for further PFLs, they heard a loud 'pop' followed by a reduction in engine power. The engine was observed running very roughly and vibrating badly.
After performing all relevant checks without success, the instructor decided to carry out a full forced landing. The landing was executed without injury to either occupant. However, during the landing roll, the left wing tip struck a hedge at the end of the field, causing superficial damage to the left wing tip and leading edge.
Investigation Findings
Inspection by the aircraft's maintenance organisation revealed that the lower spark plug of the front right (No 1) cylinder had come out of its threaded hole and was retained only by its lead. Examination of the threads on both the plug and the cylinder head showed that only the last two threads on each exhibited any evidence of engagement and wear, indicating that the plug had not been installed correctly and tightened during the last 50-hour maintenance check, which had been performed 16 flying hours prior to the incident.
Maintenance Action
The maintenance organisation has taken action to prevent a recurrence of this problem, though the specific measures are not detailed in the report.