One controller will often be assigned to the task of guiding planes through a region of airspace called an approach sector. This task involves taking planes from various sector entry points and getting them lined up at a safe distance from one another on landing approach to a particular airport.
Some airports have two parallel runways.
In such cases, the controller will form planes up into two lines. Occasionally, a controller will be told that one of the two runways is closed and that all planes on approach to land must be directed to the remaining open runway. A controller's ability to direct planes exclusively to the open runway depends on remembering that the other runway is closed and then using this knowledge in deciding how to direct each plane. Normally, the diversion of all inbound planes to the open runway produces an easily perceived reminder. However, problems may arise in conditions of low workload.