History of the Flight
On Wednesday, 25 July 2007, at 14:45 local time, aircraft EC-FTJ, a Socata Tobago TB-10, departed Ocaña Aerodrome (Toledo) on a training flight for a private pilot license. The crew consisted of one instructor and one student. The planned route was Ocaña–Requena–Casas de los Pinos–Ocaña. Approximately three hours into the flight, while at 3,500 ft on the return leg, the engine experienced repeated power losses, causing altitude loss. The instructor took control and diverted toward Lillo, the nearest aerodrome, but due to the power loss was forced to land off-field approximately 2 NM north of Lillo Aerodrome and 14 NM southeast of Ocaña. The landing was performed with full flaps; the aircraft sustained minor damage to the fuselage. Both occupants exited under their own power and were not harmed.
Aircraft Information
The aircraft, serial number 1573, was built in 1993 and registered in Spain that same year. It had been used for instruction. It was powered by a Lycoming O-360-A1AD engine (serial L33583-36A). At the incident, the airframe had 6,530 flight hours and the engine 2,528 hours. The last 100-hour inspection occurred on 20 July 2007 at 6,525 airframe hours; the 500-hour inspection on 2 July 2007 at 6,500 hours. The engine was last overhauled on 3 October 2005 at 6,000 airframe hours.
The dual magneto, a Teledyne Continental Motors D4LN-3000 (P/N 10-682555-11, S/N J079232G), was installed on 21 March 2006 with 6,074 airframe hours and had 0 hours since overhaul. It had previously been on another TB-10 and was overhauled in 2005 after a propeller ground strike, during which the distributor block (P/N 10-682054) was replaced. At the incident, the magneto had 456 operating hours since overhaul.
Magneto Failure and Examination
Post-incident inspection revealed that the distributor block (P/N 10-682054) had fractured in two through the center where the magneto shaft sits. The fracture exposed the metallic bearing liner between the roller bearing and the plastic block. The roller bearing (part number 30) showed severe overheating, friction, deformation, and loss of rollers. The bearing liner and roller bearing had fused to the shaft and rotated with it. Two diametrically opposed friction marks were found on the bearing's contact face with the liner. The bushing (part 53) rotating inside the bearing had severe friction marks. The magneto shaft spun off-center at the distributor end, and the area showed overheating. One platinum point did not open; the other opened incorrectly.
The engine manufacturer (TCM) reported no prior similar failures. The maintenance center inspected other magnetos from the same production lot and found no anomalies. TCM stated it would continue evaluating processes but had not identified any anomalies.
Analysis
The power loss resulted from uncoordinated ignition due to magneto failure. The dual magneto design means any failure of shared components affects both magnetos. The roller bearing and liner overheated and fused to the bushing and shaft, causing the bearing assembly to rotate with the shaft. This transmitted torsion to the distributor block, fracturing it and misaligning the cam and platinum points, preventing proper opening and likely contributing to power loss. The origin was the roller bearing; the distributor breakage was secondary. Maintenance records showed required actions were performed; the bearing was supplied pre-lubricated from the manufacturer, and maintenance prohibits additional lubrication. The diametrically opposed friction marks suggest eccentricity between the bearing and its liner, leading to uneven contact and overheating.
Conclusion
The investigation identified that the loss of power originated from a roller bearing failure in the dual magneto, which led to the distributor block fracture and ignition disruption. No causal factors beyond the bearing anomaly were identified.