What is the standard longitudinal separation between DME-equipped aircraft?

The standard longitudinal separation between DME-equipped aircraft is 20 miles. This separation is established to ensure safe distances between aircraft flying on the same or converging flight paths, particularly in terminal airspace where vectors or procedures are in place that utilize Distance Measuring Equipment (DME). The 20-mile separation allows pilots and air traffic controllers to maintain adequate spacing, reducing the risk of conflicts and ensuring safe operations, especially when aircraft are using the same navigation aid for guidance.

Other options reflect larger or smaller separations that may not align with the operational standards set for DME usage in controlled airspace. For instance, a separation of 10 miles may be insufficient in certain scenarios, while 30 or 50 miles may be considered excessive or unnecessary for the effectiveness of traffic management in common air traffic settings.