What happened
On June 24, 2002, at approximately 12:20 local time, a Beechcraft B55 Baron, registration EC-COA, was conducting a dual-instruction flight near Salamanca Airport, Spain. The flight, involving an instructor and a student, was performing instrument flight rules (IFR) maneuvers.
While the crew was adjusting engine RPM to 2,500, they experienced abnormal vibrations. During this process, the left engine's RPM surged to 3,000. When the pilot attempted to reduce the RPM to stabilize the engine, the power dropped abruptly, and the engine began to fail. The crew observed lubricating oil spraying from the engine cowling and noted that oil pressure had dropped to zero. The pilot promptly shut down the affected engine, secured the right engine, and successfully returned the aircraft to Salamanca Airport without further incident. No injuries were reported.
The investigation
Following the incident, a detailed inspection of the Teledyne Continental IO-470-L engine was conducted with the participation of a manufacturer technician. The investigation revealed that the engine's two lateral semi-crankcases had been punctured by internal components. Upon opening the engine, investigators found significant debris in the oil sump, including fragments of the number 5 connecting rod, piston wall pieces, bearings, and bolts. The inspection also showed that the camshaft had been severed and the interior of cylinder number 6 had been damaged by impacts from moving parts.
Investigators focused on the number 5 connecting rod, which they were able to fully reconstruct. The examination of the rod revealed that the connecting rod cap-to-body bolt had failed. While the propeller governor and pressure transfer lines were inspected and found to be functioning correctly, the maintenance history of the engine was scrutinized. The engine had 3,274 total hours since new, with 278 hours since its last overhaul, which had occurred in 1990. Maintenance manuals require that these specific bolts be replaced during every overhaul; however, documentation from the 1990 overhaul could not confirm if this replacement had taken place.
Findings
- The primary cause of the left engine failure was the fatigue failure of a bolt connecting the connecting rod cap to the rod body on cylinder number 5.
- This bolt failure caused the connecting rod cap to detach from the crankshaft, triggering a chain reaction of internal component failures and the subsequent puncturing of the engine crankcase.
- The investigation could not definitively determine the origin of the fatigue, though three possibilities were identified: a maintenance error involving the failure to replace the bolt during the last overhaul, an error in the application of the required torque during assembly, or the use of a bolt that did not meet design specifications.