In early combat operations, the Zero gained a reputation as an excellent dogfighter, achieving a kill ratio of 12 to 1.[80] However, Japan was unable to keep improving the aircraft through the war, primarily limited by lagging engine technology, and by mid-1942 a combination of new tactics and the introduction of better aircraft enabled the Allied pilots to engage the Zero on equal or superior terms.[81] For instance, the larger and heavier Grumman F6F Hellcat had superior performance to the Zero in all aspects other than manoeuvrability. Combined with the US Navy's superior training standards, units equipped with the type achieved a large victory-to-loss ratio against the Zero and other Japanese aircraft.[82]