What happened
On 16 February 2006, a Socata TB10, registration G-BNRA, was performing a training flight at Nottingham Airport (Tollerton) when a significant mechanical failure occurred during a touch-and-go landing. As the pilot applied full power for the subsequent takeoff, a loud bang was heard, and a propeller blade detached from the engine. The resulting imbalance caused the engine to shake violently and partially separate from its mounting structure, shifting at angles between 30 and 40 degrees. Despite the severe imbalance and the engine's movement, the pilot managed to maintain level flight and brought the aircraft to a halt on the runway. There were no injuries to the two crew members on board.
The investigation
Investigators examined the wreckage, including the damaged engine, the remaining propeller, and the detached blade. The investigation revealed that the engine's crankcase had suffered damage due to an overload, and the propeller and drive flange had separated from the engine. Analysis of the propeller hub, a Hartzell HCC2YK-1BF, showed that the pitch change bearing for the separated blade lacked lubrication, whereas the remaining blade's bearing was fully charged with grease.
Metallurgical examination of the hub's forward casing identified a complex failure involving fatigue. Specifically, a crack had propagated from several locations on the inside surface of the hub. The investigation also found that the engine crankshaft had fractured near its forward end. Furthermore, records indicated that a manufacturer's Service Bulletin (HC-SB-61-269), which recommended inspections for this type of hub, had not been implemented on G-BNRA during the 105 flight hours following the bulletin's release.
Findings
- The engine partially detached from the airframe due to the imbalance caused by the loss of a propeller blade.
- The primary cause of the blade separation was a fatigue failure within the propeller hub.
- The fatigue cracking was likely influenced by non-optimal temperature conditions during the forging of the hub material.
- The failure to implement the manufacturer's Service Bulletin meant the necessary inspections were not performed.
- A lack of lubrication in the pitch change bearing of the failed blade was noted.
Safety action
Following the accident, several regulatory bodies took action to mandate inspections:
- The UK CAA issued a letter to operators recommending that aircraft affected by the Hartzell Service Bulletin undergo eddy current inspections.
- The FAA issued Airworthiness Directive 2006-18-15 to make the inspection requirements mandatory.
- The EASA issued Airworthiness Directive No 2006-0092, also mandating the inspection or replacement of the hub parts.