Grob G115E Tutor, G-BYXJ, 29 June 2004
Grob G115E Tutor, G-BYXJ
Summary:
The aircraft was completing an aerobatic manoeuvre when one of the propeller blades separated from the hub. Despite severe vibration, the pilot was able to shut down the engine quickly and perform a successful forced landing in a field. There were no injuries to either crew member. The investigation determined that the No 1 propeller blade had detached due to a high-cycle fatigue failure of the blade socket in the aluminium alloy hub. The pattern of cracking suggested that the failure may have been vibration related. It was also established that the propeller blade-retaining nut preload decreases rapidly in the first few hours of propeller operation, raising concerns that the reduction in blade retention stiffness could increase the blade’s propensity to vibrate, thereby increasing the stresses in the hub. A safety recommendation concerning the need for further vibration testing to be carried out in order to fully understand the mechanism of the failure was made on 1 December 2004. Two further safety recommendations have also been made concerned with the continued airworthiness of the propeller and focusing on propeller blade retaining nut maintenance procedures and the non-destructive testing of propeller blade sockets to detect fatigue cracks.
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