What happened
On July 1, 1999, a Socata TB 10, registration F-GGNL, was conducting a local instructional flight near Muret, France. While cruising between 2,000 and 3,000 feet, the instructor noticed a dull thud followed by a loss of engine power. Although the crew attempted to restore power by activating the electric pump and switching fuel tanks, the engine power decreased again during a turn to return to the airfield.
Unable to maintain level flight, the pilot was forced to perform an emergency landing in a field near Mondavezan. The landing site was cluttered with straw bales. During the landing roll, the aircraft's left wing struck a bale, causing the wing to detach. The aircraft subsequently overturned. All three occupants—the instructor and two students—escaped without injury.
The investigation
The investigation focused on the cause of the engine power loss. While the fuel system showed no defects, investigators examined the Bendix D3000 dual magneto. The inspection revealed that the magneto was failing to deliver energy to any of the eight spark plugs.
Technicians found that the nylon cam rollers had partially melted, reducing the gap between the breaker points. This damage was traced to electrical discontinuities. Specifically, the investigation found that the torque on the breaker point fixing screws was below the manufacturer's specifications. Furthermore, the investigation identified a discrepancy in the magneto's traceability, as the part number on the aircraft did not match the documentation.
Findings
- The primary cause of the accident was a loss of engine power resulting from a malfunction in the dual magneto.
- The failure was caused by the melting of the nylon cam rollers, which led to an insufficient gap between the breaker points and prevented proper ignition.
- This melting was triggered by electrical arcing caused by a break in electrical continuity, likely occurring during a recent maintenance visit.
- The severity of the aircraft damage was exacerbated by the low altitude at the time of the failure and the presence of obstacles (straw bales) in the landing field.
- The operator had not implemented the recommendations of Teledistance Continental Service Bulletin No. 651, which addresses similar issues regarding capacitor mounting and electrical continuity.