Crash of a De Havilland DHC-6 Twin Otter 100 in Clayton: 2 killed

The airplane had not been flown for about 5 months and the purpose of the accident flight was a maintenance test flight after both engines had been replaced with higher horsepower models. Witnesses observed the airplane depart and complete two uneventful touch-and-go landings. The airplane was then observed to be struggling to gain altitude and airspeed while maneuvering in the traffic pattern. One witness, who was an aircraft mechanic, reported that he observed the airplane yawing to the left and heard noises associated with propeller pitch changes, which he believed were consistent with the "Beta" range. The airplane stalled and impacted trees in a wooded marsh area, about 1 mile from the airport. It came to rest about 80-degrees vertically. Examination of the wreckage did not reveal any preimpact malfunctions; however, the lack of flight recorders and the condition of the wreckage precluded the gathering of additional relevant information. Damage observed to both engines and both propellers revealed they were likely operating at symmetrical power settings and blade angles at the time of the impact, with any differences in scoring signatures likely the result of impact damage. The reason for the yawing and the noise associated with propeller pitch changes that were reported prior to the stall could not be determined.