But when the two tasks have high conscious requirements, such as programming an FMS, it is hard to combine them without risking error. In-between cases are unclear. Flying is a mix of conscious and automatic processes. It is easy to become preoccupied, lose focus on other tasks and forget to switch back to them. It is the ability to remember to do something that must be deferred, and has not been studied until the past few years.
Most memory studies involve memorizing something and then being tested on it, which acts as a cue.
And in many tasks, sequential steps cue each other. But with prospective memory, no one tells you it is time to remember. An example of the failure of prospective memory is checking the standby power during pre-start activities. The crew often defers this task because it interferes with refueling, but they may leave the switch guard up as a reminder. But nothing makes them look at the switch guard, and the standby power remains unchecked.