What happened
On 29 June 2004, a Grob G115E Tutor, registration G-BYXJ, was performing a series of aerobatic manoeuvres as part of a navigation training exercise. The flight was operating in good weather conditions with clear visibility. After completing a series of manoeuvres, including a loop and a half-Cuban, the aircraft was flying at approximately 120 kt when a loud bang and intense vibration occurred.
The instructor observed debris passing the canopy and noted that the canopy itself had detached from the airframe, despite the latching system remaining intact. The extreme vibration caused by the propeller imbalance made it impossible to read the cockpit instruments. Upon inspecting the propeller after the engine was shut down, it was discovered that one blade was missing and the remaining blade was severely damaged.
Following the engine failure, the instructor performed forced landing procedures and identified a suitable field. The aircraft touched down in a standing crop at roughly 60 kt. During the ground roll, the left landing gear collapsed, causing the aircraft to yaw and slide. The two crew members sustained no injuries and were able to exit the aircraft safely.
The investigation
An investigation by the AAIB focused on the mechanical failure of the propeller assembly. Examination of the wreckage revealed that the number one propeller blade had separated from the hub. The detached blade and a portion of the fractured hub were located approximately 3.5 km from the crash site.
Analysis of the fracture surfaces indicated that the aluminium hub had succumbed to high cycle fatigue cracking, which originated at the front and back of the blade arm. The loss of the first blade caused it to strike the second blade, resulting in significant damage to the remaining blade. The resulting imbalance created massive centrifugal forces that caused the engine mountings to distort, the engine mounting lug to sever from the crankcase, and the canopy to detach from the aircraft.