What happened
On April 2, 1998, a private Cessna FR 172F, registration OH-CAH, departed from Helsinki-Malmi airport for a flight to Tallinn. Shortly after takeoff, while flying over the Viiki area, the pilot noticed a slight drop in engine RPM. Although the pilot increased power to compensate, fluctuations in the fuel flow and manifold pressure meters soon followed, accompanied by a change in engine sound and a decrease in RPM.
As the engine performance continued to degrade, smoke began to emerge from the engine cowling. The pilot decided to perform an emergency landing in a nearby frozen, snowless field in Viiki. The landing was successful, and the aircraft sustained no damage. However, the engine had suffered catastrophic internal failure.
The investigation
The investigation focused on the engine's internal condition and the oil system. Upon disassembly of the Teledyne Continental Motors IO-360-D engine, investigators found that the crankshaft bearings had seized. Specifically, the connecting rod bearings for cylinders 1, 4, and 5 had melted, and the connecting rod for cylinder 3 had snapped. The failure was so severe that the connecting rod struck the cylinder wall and piston, and the engine crankcase sustained a hole through which oil smoke had escaped during flight.
Investigators examined the oil filter housing and found that the bypass valve flap was missing. A laboratory analysis of debris found in the oil pan revealed a small piece of piston wood, suggesting that fragments from the failed connecting rod had entered the oil pan and subsequently blocked the oil pump suction inlet. This blockage prevented sufficient oil from reaching the bearings, leading to the total loss of lubrication.
Findings
- The primary cause of the engine failure was a complete loss of oil lubrication resulting from the seizure of the crankshaft bearings.
- The failure sequence likely began with the failure of the bypass valve flap in the oil filter adapter, which allowed debris into the oil circulation system.
- The investigation identified that a mandatory Service Bulletin (SEB93-1) regarding the inspection and modification of the oil filter adapter had not been implemented on this aircraft.
- While the oil pump was functional, the accumulation of metal fragments and engine debris in the oil pan blocked the suction of the oil pump, cutting off the oil supply to the bearings.
Safety action
It was recommended that the aviation authority issue an Airworthiness Directive (AD) regarding the Cessna SEB93-1R1 service bulletin. This would ensure that mandatory inspections and modifications of the oil filter adapter and its bypass valve are strictly enforced across all affected aircraft.