This paper details PT6 reduction gearbox reliability problems as revealed by development testing and regular service. Initial bench tests showed that despite attempts to design within the state-of-the-art, the application was such as to require an extension of contemporary knowledge of gear design. Cavitation erosion of the planet gear bushings gave rise to some anxiety and excessive wear of coupling splines necessitated a redesign. These and other minor problems were largely resolved before certification and delivery of the first production units.Except for an incident with units operating in Arctic conditions, no significant reliability problems have occurred in regular service. Some wear problems have been corrected with subsequent benefit to overhaul costs but most complaints have been generated by weepage from the propeller shaft oil seal. This was not an operational hazard. Models in current production are little different from the initial production standard, although some have been certified for maximum powers nearly 50% greater than the first units and the minimum approved time between overhauls, for operation in the United States, is now 3000 hr.