What happened
On 15 June 2013, a Zenair CH 601UL Zodiac, registration G-BZFV, departed from a private grass airstrip in Sibson, Leicestershire, for a private flight. Shortly after takeoff, the pilot experienced significant airframe vibrations, accompanied by a thud and a visible shift in the engine cowling. This was immediately followed by a loss of engine power.
In response to the power loss, the pilot executed a left turn and performed a forced landing into a nearby wheat field. During the impact, the aircraft's nose and left main landing gear collapsed. The aircraft sustained structural damage to the wings and the engine firewall. The pilot escaped the incident with no injuries.
The investigation
Following the accident, an inspection of the aircraft revealed that one blade of the two-bladed Woodcomp Varia composite propeller was missing. The investigation further determined that the engine's carburettors had also detached from the engine. A fragment of the missing propeller blade was recovered from the runway.
An engineering examination conducted by the LAA focused on the propeller's coarse pitch stop mechanism. Investigators discovered that a small, stepped washer, which is essential for maintaining spring tension in the pitch control system, was missing. This omission allowed the propeller blade to oscillate at a high frequency about its pitch axis while the engine was operating.
Findings
- The engine power loss was triggered by the structural failure and subsequent detachment of a composite propeller blade.
- The failure was caused by high cycle oscillation of the blade, which led to delamination and eventual tensile overload of the blade cuff.
- The oscillation was made possible because a vital washer was missing from the coarse pitch stop assembly.
- Maintenance records indicated that the coarse pitch stop had been disassembled to adjust the fine pitch stop; however, the washer was not reinstalled during reassembly, and the work was not verified by an inspector.