Longitudinal separation is the separation of aircraft following the same course at the same altitude. In simpler terms, it is the separation required between two aircraft flying one in front of the other. There are a few different ways that longitudinal separation is accomplished in the air. The first way is by air-traffic control. It is a requirement by the Federal Aviation Administration to keep longitudinal and lateral separation between aircraft at all times. An air-traffic controller could require the pilots of two aircraft to make position and true airspeed reports. This would help the controller gauge where the aircraft is and will be in a specified amount of time. However, the most common method of longitudinal separation is RADAR. Using the help of a transponder, the RADAR can accurately identify aircraft flying within its cover zone and display them on a screen for a controller to see. This allows the controller to determine the best course of action if a separation error seems likely.
The Federal Aviation Administration also requires that pilots flying by visual flight reference, or VFR, keep separation from other aircraft and report when that separation is violated. This, as well as the possibility of a crash, deters the pilot from having any kind of longitudinal separation violation and helps maintain the safety of the flight. Although longitudinal separation is important to maintain at all points of the flight, there are times when restrictions and regulations pertaining to it get stricter. During trans-oceanic flight and instrument approaches and departures, the minimum separation requirements become larger for both pilots and air traffic control.