An ultra-safe, high ratio planetary transmission, for application as a helicopter main rotor final drive, has been designed, fabricated and tested. The transmission improvements are reduced weight, reduced noise and improved fail-safety and efficiency. This paper discusses the fabrication, assembly and testing of the planetary transmission. An existing planetary transmission utilized a two-stage conventional spur gear design with fixed internal ring gears. The new double helical planetary (DHP) system design uses a compound planetary arrangement with staggered planets and high contact ratio gearing in a unique configuration. Double helical gears in the planet to ring meshes balance axial tooth forces without axial planet bearing reactions. The spur gear sun to planet meshes are staggered to achieve a compact arrangement. The sun gear is fully floating.