Event Description
A twin-engine airplane was in cruise flight at 11,000 feet mean sea level when the pilot heard a loud bang and felt vibration. The pilot shut down the right engine, feathered its propeller, and landed at an alternate airport without further incident.
Examination Findings
Post-flight examination of the right propeller revealed that one blade had separated approximately 8.5 inches from the hub. The blade was manufactured from 7076 aluminum alloy, described as a 'hard alloy.' Examination of the fracture surface indicated that the blade failed due to fatigue cracking, which originated from an area of intergranular corrosion.
Propeller Corrosion Control
The propeller had been overhauled 222 hours prior to the event. The overhaul facility reported using an Alodining process for corrosion control, an approved method. However, the propeller manufacturer stated that anodizing is the preferred method because it offers corrosion detection properties, though it is more expensive and time-consuming.
Service Bulletin
In response to this event, the propeller manufacturer issued a service bulletin directing that 'hard alloy' blades be removed from service due to their susceptibility to intergranular corrosion.