What happened
On April 2, 1965, a Jodel U2 aircraft, registration HB-SVN, departed from Biel airfield for a series of practice circuits. Shortly after takeoff, while the pilot was attempting to increase the propeller pitch for the climb, the engine experienced a sudden overspeed. As the pilot attempted to manage the power, the aircraft's airspeed dropped significantly from 110 km/h to approximately 70 km/h, and the aircraft began to descend rapidly.
In an attempt to avoid obstacles, the pilot turned toward open terrain, but the flight controls became unresponsive. The aircraft subsequently struck treetops and crashed into a wooded area near Pieterlen. The two occupants of the aircraft sustained two serious injuries.
The investigation
The investigation focused on the mechanical state of the engine and propeller assembly. Investigators examined the propeller, which consisted of a steel drive shaft, a steel hub, and wooden blades. The inspection of the engine itself revealed no pre-existing damage or malfunctions.
However, the examination of the propeller assembly uncovered a critical defect. A crack was discovered on the outer surface of the propeller hub, originating from the press fit area between the hub and the drive shaft. Furthermore, the hub had shifted forward by 3.5 mm along the shaft. The investigation also noted that the gear mechanism lacked the standard stops used to limit the range of blade angle adjustment.
Findings
- The primary cause of the accident was an engine failure during the initial climb.
- A defect in the propeller hub, specifically a crack and a subsequent forward shift of the hub on the drive shaft, led to the malfunction.
- This displacement altered the blade angles, reducing the effective pitch and causing the propeller to act as a brake rather than providing thrust.
- The engine overspeed occurred precisely when the pilot attempted to adjust the propeller pitch, likely triggering the mechanical instability in the compromised hub.