On September 6, 2015, about 1540 eastern daylight time, a DeHavilland DHC-6-200, N181CS, operated by Rampart Aviation, LLC, sustained substantial damage during landing at North Raleigh Airport (00NC), Louisburg, North Carolina. The certificated airline transport pilot flying received minor injuries, the airline transport pilot not flying was seriously injured, and the passenger was not injured. Visual meteorological conditions prevailed and no flight plan was filed for the aerial observation flight that departed Washington County Airport (AFJ), Washington, Pennsylvania around 0915. The flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91.According to the pilot, the airplane was on the final leg of the traffic pattern when he reduced the power levers for landing, and noticed that the right engine sounded like "the [propeller] was heading towards beta." The pilot increased the engine power and the sound "went away." Closer to the runway, he decreased engine power and the noise returned, the airplane yawed to the right, and he applied left aileron and rudder inputs in order to remain aligned with the runway centerline. While over the runway, the pilot reduced the engine power to idle and the airplane "pushed hard to the right." Then, the pilot applied full power in an attempt to perform a go-around maneuver; however, the airplane yawed about 30 degrees off the runway centerline, touched down in the grass, and impacted trees prior to coming to rest. The airplane came to rest about 80 feet from the right side of the runway. The right wing, right engine, and right propeller assembly were impact separated. In addition, the right side of the cockpit exhibited crush damage. The right engine propeller came to rest approximately 50 feet forward of the main wreckage and was observed in the feather position. Examination of the bolt holes where the bolts secured the propeller to the right engine exhibited elongation and smearing. According to Federal Aviation Administration records, the airplane was manufactured in 1968. It was equipped with two Pratt and Whitney Canada PT6A-27, 620 horsepower engines that utilized 4-bladed McCauley controllable pitch propeller assemblies, which were installed per Supplemental Type Certificate (STC) SA1385GL. Postaccident examination of maintenance records indicated that the most recent continuous airworthiness inspection was performed on September 3, 2015. In addition, an overhauled propeller was installed on the right engine and a test flight was performed on September 4, 2015, which was 8.7 flight hours prior to the accident. The maintenance logbook entry for the right propeller stated: "Reinstalled propeller [in accordance with] McCauley Owner Operators Manual…[torqued] nuts to 57 ft lbs with [torque] wrench. No defects noted." At the time of the accident the airplane had accumulated 6,915.4 total hours. According to the McCauley Propeller Owner/Operator Manual, the propeller was designed to operate in two modes of operation, the beta mode and the governor mode. The beta mode could be selected "for ground reversing or taxi operation by means of the aircraft engine mechanical linkage. The linkage repositions the propeller reversing lever and beta valve to provide access for high pressure oil to reach the propeller piston and move the blades toward reverse pitch." In addition, "Propellers are a single acting unit in which hydraulic pressure opposes the forces of springs and counterweights to obtain the correct pitch for engine load. Hydraulic pressure urges blades toward low pitch (increasing RPM), while springs and counterweights urge blades toward high pitch (decreasing RPM)." According to STC SA1385GL, the McCauley propeller installation on the accident airplane required the propeller retaining nuts to be torqued to 68 to 72 foot pounds. The right propeller governor was sent to the manufacturer for examination and testing. During the examination, the governor functioned without anomaly. Nicks and "chatter marks" were observed around the mounting hole of the base closest to the drain port. The speed setting lever was bent outward, the max stop screw had been adjusted out an "excess number" of threads, and the control lever return spring was not engaged to the speed setting lever. The maximum speed was set about 170 rpm below the factory specification, and the pneumatic control valve settings for overspeed and underspeed were reset to compensate for the incorrect maximum speed setting. In addition, the beta valve travel from null, or the neutral position, was out of tolerance; however, during testing, the governor operated without anomaly. [Additional information about the governor examination can be found in the public docket for this case.] The right propeller governor control rigging was unable to be examined and tested due to the damage to the right wing that incurred during the accident sequence. However, according to manufacturer installation guidelines "make sure of proper rigging of engine controls. Refer to aircraft maintenance manual or STC maintenance manual supplement." In addition, it stated that "feather, reverse, and low blade angles are set during assembly or overhaul. These angles are NOT adjustable in the field." Also, the propeller manufacturer preflight checklist indicated, "the control system (governor) should be checked to determine whether the system is operating properly and is not leaking."