Stopover routes are routes where you sort of have two routes with one plane. To make a stopover route, first buy an airplane with a SO sign next to it. Then make a route from one city to another, then from the second city to a third city. Stopover routes make more profit than normal ones, due to the fact that it's actually two routes. If you make a stopover route but you have competition in one part of the route, you probably will win the competition because some of your passengers are flying from the first city to the third city. The first stopover plane is the IL-62M, introduced in 1969.05. To place a plane on a stopover route, the plane must have a range of 9800 kilometers or more, without upgrades. The minimum flight range fo a stopover route is 13000 kilometers, and the straight line from the first city to the second city must be over 10000 kilometers. If you are trying to make a stopover route, the route must fit these requirements.
We highly suggest stopover routes because you can make routes from one large city to another large city without competition if you do stopovers. For example, when you are making a route from Sydney to Phoenix, you can make a stopover route from Sydney to a low-rated city, and from that city to Phoenix. Even if there are airlines flying directly from Sydney to Phoenix, you can make the same route without having competition if you fly a stopover route. Another reason stopover routes are great is because you don't need to buy 2 planes to make routes from Male to London and Seoul, you can just buy one plane and make a route from London to Male to Seoul.
Some problems of stopover routes are that since it's technically two routes with one plane, the number of schedules from the second city to the first or third city will be decreased by half. For this reason, some routes shouldn't be stopover routes. For example, you might make a stopover route from Dubai to Madrid to Philadelphia, and you would only have 3 schedules from Madrid to Philadelphia or Dubai, whilst you can have 6 schedules from Dubai to Madrid if you're route isn't a stopover route.