Lessons Learned

Technical Related Lessons

Manufacturing processes for critical propeller control components, and changes to those manufacturing processes, must be assessed by robust process validation procedures before being implemented. With proper process validation, it is expected that no propeller failures due to manufacturing-induced defects will occur in the life of the propeller fleet. (Threat Category: Uncommanded Thrust Reversal)

  • The transfer tube is approximately 36 inches long, and designed to very close tolerances over this length. The close tolerances on the transfer tube were difficult to maintain after application of the nitrate coating. The hot bath method used to coat the spline of the transfer tube with nitrate produced a finish scaling and a straightening problem. To produce a more consistent transfer tube surface finish, a minor design change was made to a titanium nitrate coating method. This changed process employed a vapor deposit technique to apply the titanium nitrate at a much lower temperature. This design change solved the manufacturing issue but introduced a more coarse and harder surface on the transfer tube spline.

The coating change on the transfer tube had the unintended effect of causing a wear rate on the quill that was three or four times greater than the wear rate for the tube. However, this higher wear rate was not considered critical due to the relatively small design loads which were believed to exist between the quill and the transfer tube. This proved to be false, as vibration between the two parts caused the splines to be "filed" off of the quill. The quill splines became so worn that they were no longer able to engage the transfer tube, and the ability to change the propeller pitch was eventually lost. Consequently, due to the loss of engagement between the quill and transfer tube, the propeller blades on the left engine went to an uncommanded low pitch during approach, leading to very high asymmetric airplane drag, and a loss of airplane control.

Common Theme Related Lessons

It is essential that critical manufacturing processes, including changes to these processes, are subjected to robust validation. It is expected that when implemented, these validation processes will achieve a level of safety which results in no loss of propeller control events occurring in service during the life of a propeller fleet. (Common Theme: Flawed Assumptions)

  • The titanium nitrate coating on the transfer tube produced a range of finishes from glossy, to matted or dull. The surface finish specifications allowed this range in finishes. The matted finish was much rougher, and caused a more rapid wear of the quill. During validation testing, the titanium nitrate transfer tube used had a "bright gold" or "shiny" finish. Testing was not conducted using a transfer tube with a "matted gold" or dull finish; i.e. tests were not conducted to evaluate the effects of the range of finishes that could result from the coating process. Also, tests were conducted using a General Electric turbine engine, not the Pratt & Whitney PW120 engine as installed on the EMB-120. The tests did not take into account the different aerodynamic loads on the propeller and the dynamic effects of engine and airplane vibration.

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