N904MS

On February 6, 2011, about 1115 central standard time, an amateur built Zenith STOL CH750, N904MS, experienced a separation of the propeller during flight near Cassville, Missouri. The pilot executed a forced landing to a snow covered field and the nose landing gear collapsed resulting in minor damage. The pilot and passenger were not injured. The airplane was owned and operated by a private pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed and no flight plan was filed. The local flight originated from the Cassville Municipal Airport, Cassville, Missouri, about 1105.

The airplane was powered by a Jabiru Model 3300 reciprocating engine rated to produce 125 horsepower. The propeller hub/extension was not integral to the engine crankshaft and was a separate piece bolted to the crankshaft using six high strength bolts. The separated propeller was recovered subsequent to the event. The propeller remained attached to a propeller hub/extension, which was separated from the crankshaft. The six hub retention bolts had failed. The recovered pieces included the hub/extension, and the failed bolts were subsequently retained for examination.

The National Transportation Safety Board Materials Laboratory examined the components and determined that the six bolts used to attach the propeller hub/extension to the crankshaft had failed and had fracture features consistent with fatigue. Further examination revealed fretting on the mating surfaces between the crankshaft and propeller hub extension. The washers between the retaining bolt heads and the forward surface of the propeller hub extension exhibited a dished shape. Three of the washers had the concave side of the washer installed facing the hub and a fourth washer was installed with the concave side facing the bolt head. The remaining two washers were not removed from the hub during the examination. The retaining bolt threads had a coating of a stiff green material consistent with thread locking compound. The aft faces of the retaining bolt washers had areas with the same material on them. A portion of the material was lifted from one of the washers and the thickness measured at 0.0016 inches.

The propeller was a multi-piece ground adjustable design with composite blades and an aluminum alloy hub. Disassembly of the propeller revealed that the bolts used to mount the aft half of the hub to the engine propeller flange were installed with the bolt shank facing forward. This installation is contrary to the written guidance provided by the propeller manufacturer and resulted in localized damage to the blade roots where the protruding bolt shank impinged on the blade root. The damage did not extend beyond the immediate area of the impingement. Interviews with the pilot indicate that the propeller operated normally with no excessive vibration until separation.

The engine manufacturer provided information relating to 6 previous events of hub/extension separation from the same or similar model engines as follows:

1 - The first event was a Jabiru 5100 series engine that experienced a hub/extension separation in the United Kingdom. The engine manufacturer attributed the separation to use of incorrect length hub/extension bolts.

2 – A Jabiru 2200 engine in Ireland experienced a hub/extension separation. The engine manufacturer attributed the separation to use of an unauthorized propeller.

3 – A Jabiru 2200 engine in the United Kingdom experienced a propeller hub/extension separation. The engine manufacturer attributed the separation to use of an unauthorized propeller.

4 – A Jabiru 2200 engine in Australia experienced a propeller hub/extension separation. The engine manufacturer attributed the separation to improper torque of the 6 attachment bolts.

5 – A Jabiru 2200 engine in the United States experienced a propeller hub/extension separation. The engine manufacturer attributed the separation to the prior use of an unapproved propeller type. At the time of the separation the engine was fitted with an approved propeller.

6 – A Jabiru 3300 engine in the United States experienced a propeller hub/extension separation. Details of the cause of this event were not provided.

The maintenance manual that was in effect at the time of the incident described applying thread locking compound to both the female threads in the crankshaft and to the male bolt threads. The engine manufacturer has instituted changes to the maintenance manual regarding the use of thread locking compound on the propeller hub extension. The revised manual contains detailed instructions and warnings to prevent contamination of the crankshaft, propeller hub extension, washers, and bolt faces with thread locking compound. In addition, the design of the hub attachment has been modified to incorporate three 8 millimeter dowel pins between the hub and crankshaft to prevent movement between the propeller extension and crankshaft.