Dnl65 From United States of America, joined Jun 2001, 79 posts, RR: 0 Reply 1, posted (10 years 2 months 1 week 6 days 4 hours ago) and read 1432 times:
There are currently three stages or chapters (as they are known in Europe) established by ICAO with Stage/Chapter 4 in final stage of development. In the US all aircraft weighing more than 75,000 pounds GTOW were required to meet stage 3 noise limits by January 2000 in accordance with the Airport noise and capacity Act. This could be done by meeting the rule at certification, modification of the engine (hush-kit) or re-engineing. Most of the modified airplanes went to the desert in the industry downturn following 9/11 although a few are still operating.