What happened
On 6 April 2007, a Kolb Twinstor Mk 3 Extra, registration G-CDFA, was conducting a private flight from Rufforth to Rossall Field, Cockerham. While cruising at 2,500 ft, the pilot and passenger heard a rattling sound, followed shortly by a mechanical noise and the sudden stoppage of the engine. Believing the engine had seized, the pilot focused on locating a suitable field for an emergency landing. The pilot successfully performed a forced landing in the Trough of Bowland, near Clitheroe, without causing damage to the airframe. There were no injuries to the two occupants.
Following the landing, the aircraft remained in a field for 11 days before being recovered. Upon inspection, the crew discovered that the two-bladed wooden propeller was missing.
The investigation
The AAIB conducted a metallurgical examination of the engine components. The aircraft utilizes a Jabiru 2200A engine in a 'pusher' configuration, where the propeller is mounted on an extension fitting bolted to the crankshaft with six bolts. The investigation established that the propeller had detached because the bolts securing the extension fitting to the crankshaft failed. One bolt was entirely missing, while the remaining five had fractured.
Technical analysis revealed that the bolts used were threaded along their entire length. This design caused the bolts to cut into the holes of the fitting, which exacerbated fretting. Furthermore, the investigation noted surface corrosion on the crankshaft fitting. The investigation also found that the wire locking, which should secure the bolts in pairs, had failed due to high-cycle fatigue.
Findings
- The propeller detached because the bolts holding the crankshaft extension fitting to the crankshaft failed.
- The failure was caused by long-term high-cycle bending fatigue in the bolts.
- This fatigue was driven by progressive slackening of the bolts resulting from fretting within the fitting holes.
- The use of full-length threaded bolts contributed to the degradation of the fitting holes.
- The 'pusher' configuration of the engine, which places the propeller in the disturbed air behind the wing, subjects the propeller and its attachments to higher-than-normal stresses compared to a standard tractor layout.