What happened
On April 28, 2011, a Cessna 152, registration PR-NEA, departed from the Carlos Prates Aerodrome in Minas Gerais, Brazil, for a flight training mission involving touch-and-go maneuvers. The flight was operated by Aeroclube do Estado de Minas Gerais. Approximately 12 minutes into the flight, the crew noticed smoke emanating from the right side of the aircraft accompanied by engine vibrations.
In response to the developing issue, the crew attempted to return to the departure airfield. During the return leg, the engine failed completely. The instructor performed an emergency landing in a decommissioned landfill located roughly 4 km northwest of the airport. Both occupants of the aircraft escaped without injury, though the aircraft sustained light damage.
The investigation
CENIPA investigators conducted a detailed examination of the engine following the occurrence. Preliminary inspections revealed abnormalities within the cylinders and connecting rods, including connecting rod bolts from the third rod found inside the engine block.
Upon disassembly at a certified workshop, investigators discovered that the torque applied to the bolts on the number 2 and number 4 connecting rod bearings did not meet the manufacturer's specifications, which required 480 in.lbs. Laboratory analysis of the bolts from the number 3 bearing by the Department of Aerospace Science and Technology (DCTA) revealed elongation, evidence of insufficient lubrication, and overheating. The bolts showed signs of failure due to overload.
Findings
- The engine failure was caused by the improper torque applied to the connecting rod bolts during a previous maintenance service.
- The inadequate torque led to the loosening of the number 3 bearing bolts, resulting in lubrication loss, overheating of the crankshaft journal, and subsequent bolt elongation and rupture.
- The aircraft had flown approximately 23 hours between the maintenance event and the accident.
- Maintenance practices and management oversight were identified as contributing factors.
Safety action
CENIPA issued a recommendation to the National Civil Aviation Agency (ANAC) to work with the operator to improve administrative and operational mechanisms for receiving and verifying maintenance services performed by contracted mechanics or organizations.