ETOPS is an acronym for Extended-Range Twin-Engine Operations. Without an ETOPS rating, an aircraft with only two engines must be able to get to an airport where it can safely land within 60 minutes if an engine fails in-flight. ETOPS extends this "rule time" to 90 minutes or more, up to a maximum of 180 minutes. Obtaining an ETOPS rating requires certification of the reliability of an airframe/engine combination as well as an airline's flight operations and maintenance. Usually extra equipment is required as well, such as additional backup systems for electrical power. ETOPS does not require over-water equipment (e.g., life rafts) or additional fuel tanks, though these are usually required for the typical missions of ETOPS-rated aircraft.