What happened
On 6 September 2015, a Schemlipp-Hirth Duo Discus T glider, registration G-SAXT, was conducting a private flight from RAF Halton to Bembridge. While flying over the region of Lee-on-the-Solent, the pilot deployed the aircraft's self-sustaining engine to assist with a water crossing. After approximately five minutes of engine operation, a loud bang and intense vibration occurred as one of the five propeller blades detached from the hub.
Following the incident, the pilot and passenger noted that the propeller was missing a blade. The detachment caused a detached blade to strike the aircraft, embedding itself in the left wing root and causing a restriction in aileron movement. The pilot identified a suitable stubble field approximately 10-12 km away and performed a successful forced landing. There were no fatalities and no injuries to the two occupants.
The investigation
AAIB investigators examined the aircraft and found significant damage to the propeller and fuselage. The detached blade had impacted the left wing root, damaging the aileron control circuit, while an adjacent blade had been struck by the departing piece. The severe imbalance caused by the loss of the blade also caused the engine to move on its flexible mounts.
A metallurgical analysis of the propeller hub revealed that the failure originated from metal fatigue cracking within the aluminium alloy hub blocks that secure the blades. The investigation established that cracks had propagated through the metal, eventually causing a section of the block to break away and release the blade. Similar fatigue cracks were also identified in the blocks for two other blades.
Findings
- The propeller blade detached due to the failure of the metal hub blocks.
- The primary cause of the failure was pre-existing fatigue cracking in the hub blocks.
- The cracks were consistent with high-frequency oscillations rather than material defects.
- The manufacturer had previously issued technical notes regarding the need for visual inspections and hub revisions for this propeller model due to known cracking issues.