What happened
On 3 February 2011, a Tipsy Nipper T.66 Series 2, registration G-ATBW, was conducting a private local flight from Stapleford. While flying south of Flemings Farm, near South Hanningfield, the pilot experienced a sudden lateral vibration and a thud from the front of the aircraft. Upon reducing engine power, the pilot discovered that the propeller had detached from the engine.
Following the detachment, the pilot transitioned to a glide. Although a MAYDAY call was initially unsuccessful due to a radio connection failure caused by the propeller striking the aerial, the pilot managed to relay the emergency via another aircraft. The pilot subsequently performed a safe landing in a small grass field and sustained no injuries.
The investigation
The AAIB examined the propeller assembly and the engine mounting hardware. Investigators found that the bolts securing the wooden propeller to the engine crankshaft had failed. Specifically, four of the six bolts had worked loose, leaving the remaining two to bear the entire load.
Analysis of the bolt surfaces revealed evidence of fatigue cracking and fretting, indicated by a fine black dust in the bolt holes. The investigation also identified several maintenance discrepancies: two pairs of bolts had not been wire locked, one pair had been incorrectly routed, and the wire used was over-twisted, making it brittle. Furthermore, while the aircraft's maintenance logs indicated that torque checks were performed within the required hourly intervals, the aircraft's low usage meant the bolts had not been physically checked for nearly two years.
Findings
- Four of the six bolts securing the propeller to the engine had worked loose.
- The inadequate wire locking failed to prevent the bolts from moving.
- Fatigue cracking and fretting occurred due to the vibration caused by the loose bolts.
- The aircraft's low flight frequency meant the required torque checks had not been performed for approximately two years, despite being within the 25-hour interval requirement.