The engine was rated for a maximum operating time of 9,000 hours (more than one year if operated continuously) and had a special mechanism that could keep it clean for the entire New York-to-Paris flight. It was also, for its day, very fuel-efficient, enabling longer flights carrying less fuel weight for given distances.[Note 1] Another key feature of the Whirlwind radial engine was that it was rated to self-lubricate the engine's valves for 40 hours continuously. Lubricating, or "greasing," the moving external engine parts was a necessity most aeronautical engines of the day required, to be done manually by the pilot or ground crew prior to every flight and would have been otherwise required somehow to be done during the long flight.[8]