What happened
On July 25, 2007, a Socata TB-10 aircraft, registration EC-FTJ, was conducting a dual flight training mission near Ocaña, Spain. The flight, which included an instructor and a student, was part of a planned route between Ocaña and Casas de los Pinos. Approximately three hours into the flight, while cruising at 3,500 feet, the aircraft experienced several instances of engine power loss.
In response to the loss of altitude and power, the instructor took control of the aircraft and attempted to divert to the nearest aerodrome in Lillo. However, the engine power continued to fail, forcing the crew to perform an off-field landing approximately 2 nautical miles north of Lillo Aerodrome. The aircraft sustained minor damage to the fuselage, but the crew remained uninjured and were able to exit the aircraft under their own power.
The investigation
Following the incident, investigators performed several field checks to rule out issues with the fuel supply, induction system, spark plugs, and cylinder intake or exhaust. The investigation then focused on the engine's ignition system.
Upon disassembly of the D4LN-3000 dual magneto, investigators discovered a fracture in the distributor block. The inspection revealed that the roller bearing and its metallic liner had overheated and fused to the bushing that rotates with the magneto shaft. This fusion caused the bearing liner to rotate with the shaft, creating a torsion movement that eventually snapped the plastic distributor block in two. This mechanical failure disrupted the movement of the platinum points, leading to the uncoordinated electrical supply and the subsequent loss of engine power.
Findings
- The primary cause of the power loss was the malfunction of the dual magneto's roller bearing.
- The bearing had been installed and pre-lubricated by the manufacturer as part of the distributor block assembly.
- The most likely cause of the bearing failure was an off-center installation of the bearing inside its liner during the manufacturing process, which created uneven contact and excessive friction.
- The fracture of the distributor block was a secondary effect of the bearing's overheating and subsequent fusion to the shaft assembly.
- All required maintenance intervals for the aircraft and engine had been strictly followed, and no maintenance errors were identified.