The same task can be easy or difficult depending on how we do it. Research indicates that people are able to perform two tasks concurrently only in limited circumstances, even if they are skilful in performing each task separately
During periods of head-down activity, such as programming the FMS, the pilot‟s eyes are diverted from other tasks. Some FMC entries involving one or two keystrokes can be performed quickly and may be interleaved with other cockpit tasks. However, attempting to perform longer programming tasks, such as adding waypoints or inserting approaches during busy segments of flight, increases the risk of error in one task or the other.