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, the pilot applied full power for takeoff. At this moment, a loud bang was heard, and a propeller blade detached from the engine. The resulting imbalance caused the engine to shake violently from its mountings, partially separating the engine from the aircraft structure. The pilot managed to maintain level flight despite the asymmetric forces and brought the aircraft to a halt on the runway.
The investigation
Investigators examined the wreckage, including the damaged engine, the propeller, and maintenance records. They found that the propeller was missing one blade, which was recovered near the touchdown point. Debris from the propeller had struck the paved surface with enough force to create depressions. The engine's crankshaft had fractured near its forward end, and a portion of the crankcase casting had also broken.
Laboratory analysis of the propeller hub revealed that while one part of the hub failed due to simple overload, the forward section exhibited fatigue. Crucially, the pitch-change bearing for the separated blade showed a significant lack of lubrication, whereas the remaining blade's bearing was fully greased.
Findings
- The propeller blade detached because a part of the hub carrying a pitch change bearing failed due to fatigue.
- The loss of lubrication in the pitch change bearing contributed to the failure.
- The engine separation and crankshaft fracture were caused by the extreme bending loads and imbalance following the blade loss.
- The aircraft's maintenance records showed that a manufacturer's Service Bulletin regarding potential hub fillet cracks had not been implemented on this propeller.
- The lack of a mandatory Airworthiness Directive meant that the specific inspections required to identify at-risk propellers were not performed during standard maintenance cycles.
Safety action
- The AAIB recommended that the CAA alert UK maintenance organisations to the importance of the Hartzell Service Bulletin.
- The AAIB recommended that the FAA, EASA, and other regulatory bodies take steps to make the inspection requirements of the Hartzell Service Bulletin mandatory.