Increasing passenger loads created greater demand on the world's most popular air lines, especially across the Atlantic. Traditionally, regulators had insisted that all passenger aircraft be powered by at least three engines, but the development of more sophisticated airplanes eventually made twin-engine, long-distance travel feasible. A new standard was introduced, known as Extended Range Twin-engine Operation, or ETOP. It allowed twin-engine aircraft to fly long distance routes if they complied with certain techinical specifications and sufficient diversion options in case of emergency.