What happened
Shortly after departing from a private grass airfield, the pilot of a Zenair CH 601UL Zodiac, registration G-BZFV, experienced significant vibrations throughout the airframe. This was immediately followed by an audible thud and a noticeable shift in the engine cowling, alongside a sudden reduction in engine power.
Unable to maintain flight, the pilot performed a forced landing into a nearby wheat field. During the landing sequence, the aircraft's nose and left landing gear collapsed, resulting in damage to the wings and the firewall. There were no reported fatalities or injuries.
The investigation
The investigation focused on the mechanical sequence that led to the loss of power. It was determined that the engine power loss was triggered by the failure and subsequent detachment of a single blade from the composite propeller. The loss of this blade created the intense vibrations felt by the pilot, which were severe enough to cause the carburettors to separate from the engine.
Findings
- The primary cause of the incident was the structural failure of a composite propeller blade.
- The failure was driven by high cycle oscillation occurring about the blade pitch axis.
- This oscillation was made possible by a missing component within the coarse pitch stop assembly, which allowed for the unstable movement that led to the blade's detachment.