What happened
While operating in cruise flight at 11,000 feet MSL, the pilot of a twin-engine airplane heard a loud bang and felt the aircraft vibrate. In response to the event, the pilot shut down the right engine and feathered the propeller. The pilot then successfully landed the aircraft at an alternate airport; there were no injuries reported.
The investigation
An examination of the right propeller determined that one of the propeller blades had separated from the hub at a distance of approximately 8.5 inches. The blade was constructed from 7076 aluminum alloy, a material known as a 'hard alloy.'
Upon inspecting the fracture surface of the propeller blade, investigators found that the failure was caused by fatigue cracking. This cracking originated from an area affected by intergranular corrosion. Records indicated that the propeller had been overhauled 222 hours before the accident. The overhaul facility utilized Alodining for corrosion control, which is an approved method. However, the propeller manufacturer noted that while anodizing is a more expensive and time-consuming process, it is the preferred method because its properties allow for better corrosion detection.
Findings
Following the investigation, the propeller manufacturer issued a service bulletin stating that 'hard alloy' blades should be removed from service due to their susceptibility to intergranular corrosion.