The acronym means Extended-range Twin-engined Operations Performance Standards. The International Civil Aviation Organisation (ICAO) used the term for any twin-engined aircraft operating over one hour from its diversion airport, assuming one engine inoperative. Aircraft may be approved to fly more than that one hour based on stringent technical and operational criteria. Modern aircraft may now be approved for ETOPS operations up to 330 minutes from their chosen diversion airport. The rule allows passenger operations using twin-engined aircraft on routes where aviation rules previously required four-engined aircraft.