On September 8, 2002, at 1136 Pacific daylight time, a Lockamy Sonex experimental airplane, N164JL, collided with the terrain during a forced landing at the Oxnard Airport, Oxnard, California. The pilot/owner was operating the airplane under the provisions of 14 CFR Part 91. The private pilot, the sole occupant, sustained serious injuries; the airplane sustained substantial damage. The personal local flight was originating from Oxnard. Visual meteorological conditions prevailed, and a flight plan had not been filed.

During a telephone conversation with a National Transportation Safety Board investigator, the pilot stated that while in the initial climb, about 300 to 400 feet above ground level, the engine surged and the airplane could not sustain lift. The engine was still producing power, but failed to distribute that power to the propeller. With a lemon grove directly ahead of him, the pilot opted to turn back to the runway to land. As he began a left turn toward the runway, the left wing impacted the dirt and the airplane "cartwheeled." He felt a violent reaction in the cockpit as he was thrown from the airplane.

The pilot further stated that, while doing an engine teardown, he discovered that the propeller hub had separated from the crankshaft. The non-aircraft grade bolt that connected the hub to the crankshaft had "backed out" leaving the propeller spinning freely. He also noted that 2 weeks prior to the accident he had a similar propeller hub to crankshaft separation problem, at which time he removed the propeller hub and reattached it. He thought that the bolt's threads were loaded improperly and the bolt was not sufficient in transferring the load.

The Volkswagen Type 4 engine crankshaft end has a conically tapered end with a shoulder reduction from the crankshaft diameter that in an automotive application contains a press fit pulley. To adapt the engine to aircraft application, a conically tapered metal sleeve (the kit manufacturer refers to this as a "tapered spacer"), which is seated on the shoulder to bring the diameter of the shaft end to a constant taper for the propeller hub to fit over.

Great Plains Aircraft, a distributor of the Volkswagen Type 4 engine, released a Service Bulletin after investigating this and a similar accident. Their examinations revealed that the bolt did not fail; rather, the bolt loosening and backing out is a result of the tapered spacer spinning on the shaft. They further stated that normally about 51 percent of torque is transmitted through the tapered spacer to the tapered crankshaft. They noted that if the tapered spacer breaks loose, the entire load is distributed to the rest of the crankshaft. The length of the tapered spacer is about 1.250 inches, with the remaining length of the crankshaft/propeller hub taper about 1.200 inches. When the separation of the spacer occurs, the instantaneous torque load to the crankshaft results in the propeller hub separating from the remaining taper, and the bolt unscrewing from the crankshaft.

The pilot failed to submit a pilot/operator report, NTSB Form 6120.1/2.

According to Federal Aviation Administration Advisory Circular AC 20-27F, Certification and Operation of Amateur-Built Aircraft, "Amateur builders are free to develop their own designs or build from existing designs. We do not approve these designs and it would be impractical to develop design standards for the wide variety of design configurations, created by designers, kit manufacturers, and amateur builders."

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