What happened
On July 23, 2001, a Beechcraft F-33A, registration EC-COX, departed from Salamanca Airport for a dual instruction flight. The crew, consisting of an instructor and a student pilot, was performing aerial exercises involving a nearby radio navigation aid. During the flight, the crew experienced a violent vibration in the engine, followed by a significant loss of engine power and oil pressure. The crew declared an emergency and successfully performed an emergency landing on runway 21 of the airport. There were no injuries to the crew and no damage to the aircraft structure beyond the engine.
The investigation
Following the incident, an external inspection of the engine revealed a hole in the area where the two engine crankcase halves meet. This opening exposed the connecting rod cap of cylinder number 4. A subsequent teardown of the Continental IO-520-BA engine revealed that the connecting rod caps for cylinders 3 and 4 had detached. This caused the rods to move uncontrollably, striking internal engine components. The investigation found that the crankshaft counterweights had sustained impacts in the area aligned with the middle cylinders, and the camshaft was slightly bent. Debris found within the engine included fragments of the detached rod caps, connecting rod bolts (some of which were sheared), and parts of the crankshaft counterweights.
Maintenance records showed the engine had undergone two major overhauls since 1979, including a repair in May 2000 to address vibrations. Since that repair, the engine had flown 181 hours without reported mechanical issues. An oil analysis performed in May 2001, 131 flight hours prior to the accident, had shown normal metal content levels.
Findings
- The engine failure was characterized by a sudden vibration, loss of power, and oil loss.
- The primary cause of the internal damage was the detachment of the connecting rod caps for cylinders 3 and 4.
- The investigation identified the failure of a connecting rod bolt or a lack of lubrication at the rod cap bearings as the likely initiating factors, though the exact origin could not be definitively determined.