What happened
On July 1, 2000, a DHC-3T Otter floatplane, registration N9758N, was conducting a flight from Lyon Bron to Saint-Brieuc Armor. While cruising at 4,500 feet near Vichy Charmeil, the pilot experienced a sudden surge in the turbine engine accompanied by a loss of thrust. The propeller had moved into the feathered position, preventing the engine from providing necessary traction.
Unable to maintain altitude, the pilot attempted to reach the Vichy airfield but was forced to perform an emergency landing in a field used for equestrian training. During the landing, the left float struck an obstacle, causing the aircraft to break apart. Shortly after the aircraft came to a halt, the dry grass ignited, and the wreckage was destroyed by fire.
The investigation
The investigation focused on the engine, propeller, and the mechanical components of the propeller governor. Investigators examined the turbine, the governor, and the propeller assembly, finding that the propeller was indeed in the feathered position at the time of impact. While the fuel control unit (FCU) and propeller governor connections showed no signs of failure, significant damage was found on the beta plate.
A 90-degree section of the beta plate's forward guiding ring had ruptured. The investigation revealed that the guide pad, which slides within the beta plate groove, had fragmented. Physical evidence, including corrosion on the pad, indicated that the component had been failing prior to the accident. Furthermore, investigators identified a deformation on the external support of the pad, which appeared to have been caused by a heavy impact from a tool.
Findings
- The primary cause of the accident was a mechanical failure of the propeller regulation mechanism.
- This failure originated from a heavy impact, likely caused by a tool during a previous maintenance procedure, which damaged the guide pad's external support.
- The resulting fatigue and subsequent fragmentation of the guide pad created axial forces that led to the rupture of the beta plate.
- This rupture caused the propeller governor to stop delivering oil, triggering the automatic transition of the propeller to the feathered position.
- The loss of thrust during cruise flight necessitated the forced landing in the field.