The AH-64 helicopter tail rotor gearbox is a grease lubricated right-angle transmission which frequently leaks through both its input and output seals. An experiment was designed to create a worst-case scenario for a leaking output seal on three different high-life gearboxes, which were to be run for 500 hours in a seeded fault condition. The test objective was to demonstrate that aircraft with leaking output seals could continue to operate until a scheduled phase maintenance which occurs every 250 hours. Although previously considered impossible, during the study it became evident that grease freely moves from the main gear compartment into the static mast As a result, the output seal leaks caused lubricant starvation on the gear mesh surfaces, ultimately leading to catastrophic failures of the input gear teeth. The three gearboxes tested survived 490, 487, and 573 hours after fault seeding, and numerous vibration and thermal observations were recorded as the gearboxes approached failure.