What happened
On 4 March 2006, a Scheibe SF25B motorglider, registration G-BLZA, was performing a private flight near RAF Halton in Buckinghamshire. While flying at 1,000 ft on the downwind leg of the circuit for Runway 02, the pilot heard a clattering noise from the engine. This was immediately followed by intense airframe vibrations that lasted approximately five seconds before the engine stopped abruptly.
Upon realizing that the propeller had detached from the engine, the pilot executed a successful emergency landing on an alternate runway, Runway 06. There were no injuries to the pilot during the incident.
The investigation
The AAIB investigation focused on the mechanical failure of the propeller assembly. Following the incident, the propeller, spinner, and back-plate were recovered. Examination of the engine revealed that the starter ring gear and generator pulley had fallen into the lower engine cowling.
Investigators analyzed the bolts used to secure the back-plate to the crankshaft. They found that four of the bolts had failed due to fatigue, while the remaining two bolts had failed due to overload. The investigation also identified that the engine had been fitted with spark plugs of a shorter reach than specified by the manufacturer, which could lead to torque fluctuations. Furthermore, the bores of the holes in the back-plate showed evidence of damage caused by the bolt threads, suggesting movement between the assembly and the crankshaft.
Findings
- The loss of the propeller was caused by the fatigue failure of the bolts securing the propeller back-plate to the crankshaft.
- The failure was likely driven by insufficient torque loading or the loosening of bolts due to thermal contraction from previous flight operations in low temperatures.
- The installation of incorrect-reach spark plugs may have contributed to torque fluctuations.
- The bolts used were M 8.8 type, and the investigation noted that the threads were making contact with the back-plate bores, reducing the effective clamping force.