When can an aircraft be assigned to an altitude that another aircraft occupies without lateral separation applied?

An aircraft can be assigned to an altitude that another aircraft occupies without lateral separation only after it has been reported leaving that altitude. This means that once one aircraft has confirmed its descent or ascent through a specified altitude, air traffic control can then assign that same altitude to another aircraft.

This practice ensures safety by preventing two aircraft from occupying the same altitude at the same time, particularly in busy airspace. While visual sighting of the aircraft or clearance for a new altitude might seem like valid scenarios, they don’t guarantee that the first aircraft has left the altitude safely or that the necessary separation is maintained. Similarly, simply being on a different course does not provide the required assurance needed for safe vertical separation at the same altitude. Thus, reporting the departure from the altitude is a clear indicator that that specific altitude is available for assignment to another aircraft.