What happened
While operating in cruise flight at 3,500 feet near the Potomac River, the aircraft experienced a partial loss of engine power. The pilot attempted to troubleshoot the issue by checking the magnetos and engine controls; adjusting the mixture provided a brief restoration of power. However, within three minutes, the engine suffered a total loss of power. To avoid the river, the pilot maneuvered the aircraft away and executed a forced landing into a farm pasture. There were no injuries reported.
The investigation
Post-accident examination of the engine at the scene identified several mechanical failures centered on the #3 cylinder. Investigators found that the #3 cylinder was loose on the crankcase, and the #3 intake tube had not been installed, instead resting at the bottom of the engine compartment. Additionally, the #3 exhaust riser had separated at the flange. This flange was loose because the two nuts were not tight, and one nut lacked both a washer and a lock washer.
Further inspection revealed that two case studs and two thru studs along the bottom of the #3 cylinder had broken off. Upon removal and visual examination of the cylinder, investigators noted chafing and scratching on the cylinder skirt, with matching scratches found on the cylinder case pad material. Analysis of the fractured cylinder studs showed fracture surfaces consistent with metal fatigue.
Records indicated the engine had been overhauled 14 years and 917 flight hours prior to the accident, with no cylinders removed since that time. The engine manufacturer's recommendation for overhaul was every 2,000 hours or 12 years, whichever occurred first.