Cessna 172P N52436 accident near Goodyear, Arizona

Probable cause

Bearing degradation that increased clearances between the bearing and crankshaft journal surfaces, resulting in fatigue cracks and the subsequent failure of the No. 3 connecting rod and a total loss of engine power. Contributing to the accident was the mechanics and owner/operator’s continued operation of an engine that exceeded the manufacturers’ recommended overhaul.

NTSB analysis

The flight instructor reported that, shortly after departing the airport, the engine oil pressure indication dropped to near zero. The instructor intended to turn back to the departure airport, but the engine exhibited an uncommanded reduction of power. The instructor then attempted to land at another airport but the engine lost all power. The instructor initiated a forced landing in a residential area, striking a tree and colliding with an unoccupied vehicle, which caused substantial damage to the airplane.

Postaccident examination of the engine revealed that the Nos. 3 and 4 connecting rods had separated from the crankshaft. The No. 3 connecting rod fractured at two locations on the strap due to multiple fatigue cracks that initiated at the outer surface next to the connecting rod bolt heads. The fracture of the No. 4 connecting rod strap and all four connecting rod bolts from the Nos. 3 and 4 connecting rods was secondary to the failure of the No. 3 connecting rod, which likely encountered elevated cyclic stress due to degradation of the No. 3 bearing. Heat tinting on the interior surfaces of the No. 3 connecting rod strap pieces and heat damage and circumferential scoring on the No. 3 connecting rod journal indicate bearing issues preceding the fatigue failure of the connecting rod.

As the No. 3 bearing degraded, higher stresses likely developed in the connecting rod strap and cap due to increased clearances between the bearing and the crankshaft journal surfaces, leading to fatigue crack initiation and growth in the connecting rod strap. Fracture surfaces on the No. 3 connecting rod cap also showed signs of high cyclic stress, likely associated with the No. 3 bearing issues. The No. 4 bearing also showed signs of excessive wear. The worn bearing condition was likely associated with excessive service time. Similarly, the No. 3 bearing failure was likely associated with the excessive engine operating time since new.

Maintenance records revealed that the engine total time since new was 3,745.6 hours since it was installed on the airplane about 44 years prior to the accident. No records of an engine overhaul were found in the supplied maintenance records. Lycoming Service Instruction No. 1009BE recommends that the accident engine model should be overhauled within 12 calendar years of the date it first entered service or the date of its last overhaul, or after 2,000 operating hours from the last overhaul.

Text from the NTSB. Open the NTSB docket for WPR24LA084 for the full record.