An empty leg is when an aircraft is flying empty on its way to pick up passengers or after dropping off passengers. These flights are trips that must go, whether empty or with clients either because another client has booked the aircraft or because the aircraft has to return to its operating base. Supposedly a client or a company has a booked flight on the route Athens to Mykonos. After delivering to the point of destination in Mykonos, the aircraft should return to the base in Athens, what means empty flight without passengers from Mykonos to Athens. An empty leg gives the opportunity to make a flight much cheaper than in comparison with its nominal cost.

Nonetheless using empty legs requires some flexibility. For example, if the client whose flight created the empty leg opportunity cancels his flight, the empty leg no longer exists. In other words, the empty leg is contingent upon the plan of others.