What happened
On 22 April 2009, a Speedtwin ST2 prototype, registration G-STDL, was conducting a private test flight near Woodbridge Airfield, Suffolk. The aircraft, operated by a CAA test pilot, was accelerating toward its maximum level speed of 184 mph IAS at 4,000 ft. During this phase, a loud bang was heard, accompanied by engine vibration, smoke, and the smell of hot oil.
The pilot attempted to shut down the right engine and feather the propeller, but the propeller failed to feather and continued to rotate. Due to the severe vibration, the pilot performed an immediate forced landing at the disused Woodbridge Airfield. During the descent, the pilot managed to maintain control using the left engine, though the intense noise in the cockpit prevented a response to the initial MAYDAY call.
Upon landing, it was discovered that the right engine had stopped rotating and one propeller blade was missing. The departing blade had struck the aircraft nose and the left propeller assembly, causing damage to the front fuselage and the carbon fibre skin of the left propeller.
The investigation
Investigators examined the propeller assemblies, which featured wooden blades encased in carbon fibre. The investigation focused on the right propeller hub, where a portion of the failed blade' and the hub threads were analyzed. Metallurgical examination of the failed root section revealed intergranular cracking and delamination, features characteristic of stress corrosion cracking (SCC).
Further analysis showed that the threads in the hub had been machined rather than rolled. This machining process exposed the end grains of the metal, making the material more susceptible to corrosion. Additionally, the investigation found corrosion products on the threads and noted that no jointing compound had been used to seal the assembly against moisture.
Findings
- The failure of the right propeller blade was caused by stress corrosion cracking within the root section threads.
- This cracking resulted from a combination of residual stresses left over from the thread machining process and a corrosive environment caused by moisture ingress.
- The use of machined threads instead of a thread-rolling process left the metal end grains exposed and vulnerable.
- The lack of a jointing compound or sealing mechanism allowed moisture to reach the threaded components.