What happened
On September 17, 2020, a Tecnam P20L6T, registration SP-ADF, was conducting a training flight within a training zone. The crew, consisting of an instructor and a student pilot, had been in flight for approximately 18 minutes at an altitude of 3,200 ft when the right engine experienced a drop in RPM. Within approximately 30 seconds, the engine ceased operation. The crew performed a single-engine landing at the departure aerodrome, EPWT, without further incident.
Post-flight inspections of the engine revealed a partially unscrewed valve cover bolt on cylinder number 1 of the right engine. Investigators found metal shavings on the magnetic plug, as well as trace amounts of debris in the oil filter and the oil itself. While coolant levels were normal, an oil leak had occurred, leaving only 1 liter of oil remaining in the engine.
Upon disassembly of the engine for further inspection, investigators discovered that the connecting rod bearing of cylinder number 1 had seized against the crankshaft. This seizure resulted in significant damage to the connecting rod, cylinder, and piston, with fragments of the cylinder and piston found within the oil sump and reduction gear.
The investigation
The PKBWL examined the engine components and the maintenance history of the Rotax 912 UL engine. The engine had been installed on the airframe by a previous owner in Italy in 2016, and the current owner had been operating the aircraft since March 2020.
Investigators reviewed the engine's line and base maintenance instructions, noting that while bolt tightening occurs during engine installation, there is no specific requirement to check these bolts during routine operations. The investigation also considered that while visual inspections or oil leak detection should theoretically identify loose bolts, the rubber gasket around the cover perimeter often remains stuck in place, which can mask the loosening of the bolt during inspections.
Findings
- The primary cause of the engine failure was the unintentional loosening of the valve cover bolt on cylinder number 1 of the right engine.
- This loosening led to an oil leak, which caused the engine to seize.
- The investigation could not determine the specific reason why the bolt became unscrewed.