What happened
On August 23, 2023, a Beechcraft S35 Bonanza, registration CX-BCF, departed from the “Ángel S. Adami” International Airport (SUAA) in Montevideo, Uruguay, for a repositioning flight to Cerro Largo (SUMO). Shortly after reaching an altitude of 4,500 feet, the pilot noticed a drop in engine power and requested a return to the departure airport.
During the return flight, the engine performance continued to degrade, accompanied by significant vibrations and a marked drop in oil pressure. The pilot reported the presence of smoke and the smell of oil inside the cockpit, prompting the crew to close all air inlets to prevent inhalation. Approximately 4.5 miles from the airport, at an altitude of 6,500 feet, the engine suffered an abrupt and total shutdown. The pilot successfully executed a forced landing on runway 19, utilizing remaining momentum to clear the runway and coming to a stop at the taxiway C holding point. There were no injuries to the pilot.
The investigation
The CIAIA investigation focused on the mechanical state of the Continental IO-520-BB engine and the presence of oil residue on the aircraft. Investigators found lubricant sprayed across the engine cowling, the windshield, and the underside of the aircraft, trailing toward the tail. The investigation specifically examined the oil filler/inspection cap and the internal components of the engine.
Upon disassembly of the engine at a maintenance facility, investigators discovered that the crankshaft had seized due to extreme temperatures. The connecting rods showed discoloration consistent with overheating, and metallic debris was found within the oil pan. Additionally, a connecting rod bolt was found to have pierced the engine block, though no oil leakage was observed from this specific puncture.
Findings
- The primary cause of the engine failure was the loss of lubricating oil through the oil filler/inspection cap.
- It is suspected that the oil cap may not have been properly secured during the pre-flight inspection.
- Evidence suggests that damage to piston 2, consistent with detonation or pre-ignition, may have created internal overpressure, contributing to the expulsion of oil.
- The engine continued to operate under severe oil starvation, leading to cumulative internal damage and the eventual seizure of the crankshaft.
- The pilot's skill and timely decision-making allowed for a safe return and landing despite the engine failure.