 The investigation revealed that as the flight crew attempted to reduce thrust on the left engine during the final approach, they were unable to remove the 'cruise pitch lock' mechanism that is used to maintain a cruise thrust setting.When they subsequently attempted to perform an emergency feathering and shutdown procedure on that engine, they were able to shut down the engine but unable to achieve a feathering of the propeller.This eventually resulted in the left propeller creating a high amount of asymmetric drag while windmilling; so much so, that the other engine operating at full power was not able to arrest the resulting uncontrollable descent.