What happened
On 24 December 2014, a Piper PA 32, registration G-OCTI, was conducting a private flight from the Isle of Wight to Galway. The aircraft was carrying one pilot, three passengers, and a pet dog. While flying over the south midlands of Ireland, the pilot noticed engine vibrations accompanied by a drop in power. Shortly after, oil smoke began filling the cabin, prompting the pilot to open the storm window for ventilation.
During the flight, the pilot heard loud bangs and observed what appeared to be an object exiting the top of the engine cowling. Following further power loss, the pilot elected to shut down the engine to mitigate risk. The pilot then executed a forced landing into an agricultural field in Lorrha, County Tipperary. The aircraft sustained minor damage to the cowling, but there were no injuries to the occupants and no fire occurred.
The investigation
The AAIU examined the engine and the wreckage to determine the cause of the power loss. The investigation focused on the Lycoming engine and the condition of its components. Investigators found that the number five cylinder had completely separated from the crankcase. The piston was broken into three pieces, and the connecting rod had detached from the crankshaft.
Technical analysis of the engine revealed that the cylinder retention studs had suffered fatigue failure. While some red rust was present on the threads, metallurgical testing confirmed that the failures did not originate from corrosion pitting. Furthermore, the investigation reviewed maintenance records, noting that while the engine had undergone an annual inspection recently, such inspections do not typically include checking the torque of cylinder retention nuts.
Findings
- The loss of engine power was caused by the detachment of the number five engine cylinder.
- The detachment resulted from the fatigue failure of several cylinder retention studs.
- Metallurgical analysis suggests the fatigue was caused by either insufficient torque during assembly or the nuts loosening during operation.
- During engine disassembly, investigators found that several nuts on the same side of the engine were below the required torque specification.
- The engine had not undergone a full overhaul since 1987, though it was within its hourly operating limits.