What happened
On 15 May 2019, a private flight involving an Ace Aviation As-tec 13, registration G-CKUL, was nearing completion at Shotteswell Airfield in Oxfordshire. While cruising at an altitude of 2,100 ft, the pilot noticed a sudden increase in engine speed. During this period, the aircraft's pusher propeller became detached from the airframe. In response to the failure, the pilot deactivated the engine and initiated a glide descent toward the airfield, which was located roughly 1 nm to the north. The pilot successfully performed a power-off landing without any injuries to the crew.
The investigation
Investigators examined the single-cylinder Simonini Mini 3 piston engine and its drive system. The engine utilizes a reduction drive belt to rotate the pusher propeller, with tension managed via an eccentrically-mounted bearing on the propeller driveshaft. Upon inspection of the engine, it was determined that the propeller and the eccentric bearing assembly had separated from the support bracket. The investigation established that the separation was caused by a fatigue failure of the bearing support. The propeller itself was not recovered following the incident.
Safety action
Following the findings of this investigation, the engine manufacturer, Simonini Racing SRL, issued a security campaign to all owners and operators of the Mini 3 engine. This safety notice mandated the replacement of the eccentric bearing support prior to any subsequent flight operations.