What is a DC airplane? A DC-10? Speeds of all transport category aircraft are roughly the same, but the additional engines serve two primary purposes. One is to provide adequate design thrust (especially for takeoff) and the second is to be able to fly further away from land on over water flights. Twin engine aircraft, such as the Boeing 757/767 can fly over water but must stay within a maximum flying time on one engine of a suitable airport of 180 minutes or 240 minutes depending on the approvals the carrier has tested for...