Stage 3 / Stage 4 noise
Stage 3 and Stage 4 (ICAO Chapter 3 and 4) are certification noise standards for jet aircraft, each roughly 10 decibels quieter than its predecessor; the newest aircraft meet Stage 5 (Chapter 14). Nearly all business jets built since the 1990s are Stage 3 or better, while older Learjets, Gulfstream IIs and early Citations that were only Stage 2 have been banned from most of the US, Europe and many other countries. Some airports go further, restricting or surcharging Stage 3 aircraft at night or banning them outright. For an aircraft buyer, an older jet's noise category can determine where and when it can operate and its future resale market.