This database search and evaluation was performed to ascertain what types of situations pilots consider surprising or unexpected. The purpose of the investigation was to determine if natural categories of events emerge from the existing accident, incident, and event reporting data commensurate with the current operational and research focus concerned with unexpected events and aviation safety. The underlying question was whether the involvement of surprise or unexpectedness in events may be a precursor to a loss of attention, increased workload, or other interruptions of ongoing processes, which may then contribute to an unwanted outcome of a maneuver, or an entire flight as revealed in these database reports. The study was also conducted to facilitate the development of a conceptual framework for the study of unexpected events in aviation.

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In the earliest days, air transports were operated by a single pilot because the airplanes in the 1920s were small and rudimentary. As the size and the sophistication of equipment increased, a copilot was added to help the captain in controlling the airplane and monitoring the systems, thereby improving safety. The 1930s brought about four-engine transports, the artificial horizon, and the first autopilot prototypes. As flights became longer and cockpits acquired more equipment, a flight engineer was added to the crew. Soon thereafter, multiple engineers, navigators, radio operators, and stewards were also necessary to accomplish all of the tasks facing the team on a typical flight or mission.By the 1950s, the size of the human team began to shrink as navigators and radio operators were replaced with new, more reliable ground-based communication and navigation equipment. At first, the main thrust for the rapid development and implementation of automation revolved around human performance limitations as aircraft flew higher, faster, and for longer periods of time. Early research studies on pilot stress and fatigue (e.g., McFarland, 1953) were often presented as evidence for the advantages of 243

Abstract. Emergencies and other threatening situations challenge the cognitive capabilities of even the most skilled performers. While the effects of acute stress on cognition and performance have been examined in diverse laboratory studies, few studies have focused on skilled performers. We analyzed 12 airline accidents to determine the types of errors arising in situations that are highly challenging and probably stressful. We identified 212 flight crew errors from accident investigation reports; these errors were grouped into eight higher-level error categories. Cognitive factors contributing to vulnerability to these errors were identified and related to theoretical models of stress. Finally, we suggest specific ways to guard against stress-related errors by enhancing training, operating procedures, and cockpit interfaces.