In the ASA Flight 2311 crash the failure of a quill resulted in the loss of propeller pitch control. The propellers ended up with a pitch angle of 3 degrees which caused reduced thrust and increased drag resulting in severe asymmetrical thrust situation. The airplane was uncontrollable in this configuration and crashed. The propeller manufacturer, Hamilton Standard, had tested the propellers in a laboratory and had shown that the propellers will feather with a loss of pitch control. However, a flight test orchestrated by the NTSB had shown that due to the "aerodynamic forces of flight" the propellers did not feather as designed.

My question is what exactly caused the propellers to not feather in flight when the propellers successfully feathered in the laboratory? I am looking for a more detailed answer than "aerodynamic forces of flight".

You can find the full text of the NTSB investigation here and according to that report (bolded for emphasis),

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.

And later in the report

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.

In simple terms the unit that controls the propellor pitch experienced a failure from worn out components that lead to a case where the propellor did not completely feather. The specific part seems to be the PCU quill (to answer your question directly).

Even later in the report they discuss why the failure was not really covered by the testing originally. The testing was not necessarily incorrect as much as it may have been incomplete and not foreseen this issue with the quill.

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.

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