Pilots have multiple and complex tasks to perform; these are normally shared between the pilot flying (PF) and the pilot monitoring (PM). Flight crew workload varies, even during routine flights, from low to high and will rise in the event of abnormal weather conditions or aircraft malfunctions. During high workload, flight crew are especially vulnerable to error if their strategies for effective multi-tasking break down. This is the aspect of workload considered here. However, during periods of low workload in the cruise, a different sort of error vulnerability may arise which stems from the low Level of Arousal which typifies this flight phase and sometimes from Complacency as well.