What happened
On 26 April 2025, a DG-1000T powered sailplane, registration G-CKLY, was conducting a private sightseeing flight near Weston-on-the-Green Airfield. The flight was part of a series of local sorties, and the engine had been used several times during the day's operations.
While flying approximately 12 km south of the airfield, the pilot engaged the engine to gain altitude for a return to base. During this maneuver, a loud bang was heard, followed by an engine overspeed warning. The pilot immediately realized the propeller had detached from the engine. In response, the pilot closed the throttle, switched off the ignition, and retracted the engine. After verifying that the aircraft's control surfaces remained functional, the pilot performed a straight-in approach and landed safely at Weston-on-the-Green Airfield. There were no injuries to the pilot or the passenger, though the aircraft sustained damage to the fuselage, solar panel, and left wing caused by the departing propeller.
The investigation
The AAIB recovered the propeller from a wooded area and examined the engine's coupling mechanism. The investigation focused on the two-part rubber coupling, which is designed to connect the propeller to the engine's drive via a vulcanized Nitrile Butadiene Rubber (NBR) element.
Laboratory analysis, including Fourier Transform Infrared spectroscopy, confirmed the material composition of the rubber. Examination of the metal components revealed that the failure occurred in two stages: an adhesive failure between the rubber and the drive-side aluminium component, and a subsequent cohesive failure of the rubber itself. Detailed inspection of the drive-side coupling showed that the surface finish was inconsistent; while the top of the groove had been appropriately roughened via media blasting, the deeper sections of the groove retained visible machining marks and lacked the necessary roughness for a secure bond.
Findings
- The primary cause of the propeller separation was an adhesive bond failure between the rubber element and the drive-side coupling component.
- The effective bonded area was only approximately 33% of the total surface, which was insufficient to withstand the engine's torque loads.
- The lack of adhesion was caused by insufficient surface roughness within the machined groove of the aluminium component.
- The manufacturing process made it difficult to achieve a consistent blasted finish within the narrow groove.
- There were no specific production acceptance criteria or inspection standards to verify the surface finish after the media blasting process.