During the early 1980s the Federal Aviation Administration (FAA) began rethinking its policy on the minimum number of engines an aircraft needed to be permitted to fly long distances non-stop over water. The FAA's rethink had been prompted by the increasing reliability of jet engines powering new widebodied airliners as well as other technological advances. Mid-air engine shutdowns that necessitated an emergency landing at the nearest diversion airfield and which could harm the structure and/or occupants had become a rare phenomenon. The result was ETOPS, a set of rules permitting twin-engined aircraft to fly long over water routes, provided that there were usable diversion airfields within 120 minutes' non-stop flying distance plus a 15% margin from the aircraft's route.