What happened
On August 9, 2013, a Socata TB-10 Tobago, registration EC-FPN, was conducting a solo flight training mission departing from Salamanca Airport. Approximately 20 minutes into the flight, while cruising at 3,600 feet near Vallesa de Guareña, the engine began to run irregularly. The pilot found that the engine would not respond to power adjustments, despite engine instruments showing normal oil pressure, temperature, and fuel levels.
Unable to maintain altitude, the pilot declared an emergency and executed a forced landing into a harvested cereal field. During the landing sequence, the aircraft struck a perimeter fence before coming to a halt against an embankment by a stream. The pilot, a 19-year-old student, was uninjured, but the aircraft sustained substantial damage to the landing gear, engine mount, wing, and the lower forward section of the engine compartment.
The investigation
The investigation focused on identifying the source of the engine power loss. Investigators examined the Teledyne Continental Motors D4LN-3000 dual magneto. Inspection of the magneto revealed that the breaker points' cam followers had worn down against the cam. This wear reduced the contact gap and delayed the timing of the breaker points, preventing the generation of an adequate spark at the spark plugs.
Laboratory tests were conducted to determine the cause of this wear. Testing showed that the cam had undergone a process of overheating, which led to a loss of its lubricating properties. Specifically, when the cam was heated to 90 °C, lubricant began to weep from its surface. Further tests confirmed that the heat generated by friction between the worn followers and the overheated cam was the primary driver of the component degradation.
Findings
- The engine power loss was caused by the magneto's inability to produce sufficient voltage for the spark plugs due to improper breaker point opening.
- This malfunction was a direct result of the overheating of the magneto cam, which caused it to lose its lubrication.
- The loss of lubrication led to accelerated wear of the breaker point cam followers.
- While the engine compartment typically maintains sufficient airflow during flight, the investigation noted that during ground operations or taxiing in high ambient summer temperatures, the magneto can reach temperatures high enough to trigger this loss of lubrication.