Você pode encontrar o texto completo da investigação do NTSB aqui e de acordo com esse relatório (em negrito para ênfase),
Examinations of the left propeller components indicated a propeller blade angle of about 3 degrees at impact. This position was based upon the position of the pitchlock acme screw. The left PCU ballscrew position indicated that the PCU had commanded a blade angle of 79.2 degrees. The discrepancy between the ballscrew position and the position of the pitchlock acme screw is a strong indication that a disconnect between these two components occurred prior to impact and that the left propeller had achieved an uncommanded blade angle below the normal flight range.
e mais tarde no relatório
Using measurements and the inspection procedures for the quill and transfer tube of the Hamilton Standard Alert Service Bulletin, it was determined that the left PCU quill spline was worn to the extent that its gear teeth did not engage the transfer tube spline. In addition, the test cell and flight tests showed that the propeller blade angle could not be controlled by the PCU with a disengaged transfer tube. In the test cell, the blade angle moved toward high pitch; however, the propeller was operating at zero airspeed and did not experience normal flight loads. In contrast, the flight tests showed that the blade angle would move toward low pitch with a disengaged transfer tube. The blade characteristics indicate that centrifugal and aerodynamic twisting moments tend to move the blades toward low pitch.
The Safety Board believes that the worn quill on the left engine PCU became disengaged from the transfer tube prior to the loss of control of the airplane during the approach to Brunswick. Moreover, the propeller blades moved to a low angle, resulting in an asymmetric lift and drag condition that exceeded the capability of the pilots to counteract with the airplane controls available.
Em termos simples, a unidade que controla o passo da hélice sofreu uma falha devido a componentes desgastados que levam a um caso em que a hélice não emborcou completamente. A parte específica parece ser a pena PCU (para responder diretamente à sua pergunta).
Ainda mais tarde no relatório, eles discutem por que a falha não foi realmente coberta pelos testes originalmente. O teste não foi necessariamente incorreto tanto quanto pode ter sido incompleto e não previsto esta questão com a pena.
The investigation found that wear of the quill was not considered during the certification of the propeller system because of the very light torque loading on the quill during flight. Service history of the PCU quill prior to the introduction of the titanium-nitrided transfer tube indicted that quill spline wear was not a problem. Additionally, the manufacturer provided an analysis during certification indicating that even in the event of a failure, the propeller would either drift into the feathered position or maintain the blade angle present when the failure occurred. However, the accident involving flight 2311 and the subsequent investigation have determined that these assumptions, though originally supported by numerous engineering evaluations and manufacturing experience, are invalid and that there are single failure modes that could result in an uncommanded propeller blade angles below flight idle.