The 727 is one of the noisiest commercial jetliners, categorized as Stage 2 by the U.S. Noise Control Act of 1972, which mandated the gradual introduction of quieter Stage 3 aircraft. The 727's JT8D jet engines use older low-bypass turbofan technology, whereas Stage 3 aircraft use the more efficient and quieter high-bypass turbofan design. When the Stage 3 requirement was being proposed, Boeing engineers analyzed the possibility of incorporating quieter engines on the 727. They determined that the JT8D-200 engine could be used on the two side-mounted pylons. The JT8D-200 engines are much quieter than the original JT8D-1 through -17 variant engines that power the 727, as well as more fuel efficient due to the higher bypass ratio, but the structural changes to fit the larger-diameter engine ( fan diameter in the JT8D-200 compared to in the JT8D-1 through -17) into the fuselage at the number two engine location were prohibitive.