The North Atlantic Tracks are the most heavily used oceanic routes in the world. Most North Atlantic Tracks are covered by ETOPS 120-minute rules, removing the necessity of using 180-minute rules. However, because North Atlantic diversion airports are subject to adverse weather conditions, affecting their availability for use, the JAA and FAA have given 15% extension to the 120-minute rules to deal with such contingencies, giving the ETOPS-138 (i.e. 138 minutes), thereby allowing ETOPS flights with such airports closed. By the mid-2010s, virtually all North Atlantic plane routes are dominated by twin-engined aircraft.