Beechcraft A36TC N36SG

The pilot reported that, shortly after he began a letdown of the airplane from 9,500 ft mean sea level for landing at the airport, the airplane experienced a "very sudden" onset, high-frequency vibration. The pilot unsuccessfully attempted to diagnose and correct the problem. Although the pilot initially believed that the airplane could reach the airport, it was unable to maintain altitude. When the airplane was about 2 miles from the runway and over a lake, the pilot began a right 180-degree turn to land on an unobstructed portion of the shoreline. During the turn, the right wing struck the lake, and the airplane then came to rest inverted in about 2 ft of water. Postaccident examination of the airframe and engine did not reveal any preimpact mechanical problems that would have precluded normal operation and continued flight. However, detailed examination of the three-bladed, controllable-pitch propeller revealed that one steel actuating pin, located inside the propeller hub and which controlled the pitch of one blade, had failed in fatigue. The pin's failure caused the pins to be torn from the other two propeller blades, which allowed the blades to rotate freely in the propeller hub and resulted in a loss of propeller thrust and the vibration that was felt by the pilot. About 7 years before the airplane was manufactured, the propeller manufacturer changed the design of the component hole (the "propeller ferrule") from a through hole to a blind hole that only penetrated one surface of the ferrule. The ferrule accepted the threaded end of the pin, and the corresponding pin thread length was reduced so that the pin would not contact the bottom of the hole when installed. The postaccident examination indicated that the failed pin was likely the longer pin associated with the previous design as evidenced by the contact and deformation of the bottom of the ferrule hole, which resulted in visible ferrule damage. The manufacturer's guidance explicitly addressed and prohibited installation of the longer pins in the blind ferrule hole. Bottoming of the pin in ferrule hole and the hole's subsequent deformation during installation prevented the proper seating of the pin and permitted operational bending loads to be transmitted to the threaded portion of the pin, which was not designed for such bending loads. The bending loads on the threaded portion resulted in crack initiation in the pin thread valley and the eventual fatigue failure of the pin. Review of the maintenance records indicated that multiple people had owned the airplane and that it had been continuously maintained in an airworthy condition through annual inspections and scheduled and unscheduled maintenance since at least 1998. However, the propeller was last overhauled more than 15 years before the accident, which exceeded the propeller manufacturer's recommended overhaul interval by a factor of three. The investigation could not determine why the owners or maintenance personnel did not comply with the propeller manufacturer's recommended overhaul interval. Further, the maintenance records did not provide sufficient detail to determine why the incorrect part was used or why its incorrect use and resulting damage were not detected during the overhaul. If the propeller had been overhauled in accordance with the propeller manufacturer's guidance, the improper pin installation likely would have been detected and the damaged, nonairworthy propeller would have been replaced with an airworthy unit.

Quoted verbatim from the NTSB record.