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. How does the controller remember and use this important fact? Normally, the diversion of all inbound planes to the open runway produces an easily perceived reminder. In particular, the controller will detect only a single line of planes on approach to the airport, even though two lines --- one to each runway --- would normally be expected.
However, problems may arise in conditions of low workload. With few planes around, there is no visually distinct line of planes to either runway. Thus, the usual situation in which both runways are available is perceptually indistinguishable from the case of a single closed runway. The lack of perceptual support would then force the controller to rely on memory alone and thus increase the chance of error.
A controller's proneness to such errors (and many such are recorded) illustrates a general tendency in human behavior: to resort to habitual behaviors and assumptions despite available memories that indicate otherwise. That such errors will occur is entirely predictable. However, those who design procedures and equipment for air traffic controllers --- and operators in many other domains --- often fail to consider the memory load imposed by their designs and thus unnecessarily increase the likelihood of such errors.