What happened
On 15 October 2010, a Rans S6-ES Coyote II, registration G-BYRS, was performing a check flight from Sandy Airfield, Bedford to verify engine performance. The flight proceeded normally through the initial climb. However, as the pilot increased the throttle to begin a timed climb, the engine RPM rose rapidly, and the propeller ceased to receive drive from the engine.
After attempting to restore power without success, the pilot transitioned into a glide. In an effort to return to the airfield, the pilot initiated a left turn; however, the aircraft subsequently performed a forced landing in a nearby field. During the landing, the nose landing gear collapsed, causing the aircraft to overturn. There were no injuries to the two occupants on board.
The investigation
The investigation focused on the mechanical failure of the Rotax 582-48 engine's gearbox. A strip inspection of the unit revealed that the secondary gear angular ring's two retaining half rings had fractured into several pieces. This failure allowed the gear to move axially, which caused the overload dog-clutch to disengage and subsequently severed the drive to the propeller.
The investigation also examined the propeller's specifications. While the propeller was approved by the LAA, its exact mass moment of inertia was unknown. It was noted that similar propellers from the same manufacturer have been known to exceed the maximum limits specified by Rotax for B-type gearboxes. The engine manufacturer's service documentation explicitly warns that using propellers exceeding the maximum allowable mass moment of inertia can lead to reduced gearbox life or mechanical damage.
Findings
- The failure of the retaining half rings within the gearbox allowed for axial movement of the gear.
- This movement caused the overload dog-clutch to disengage, resulting in the loss of propeller drive.
- The propeller's mass moment of inertia may have exceeded the maximum allowable limit for the B-type gearbox, potentially contributing to the component failure.