Synopsis: This page describes a theory of Cockpit Task Management (CTM) and studies we conducted to understand its nature and significance. CTM was a precursor to Agenda Management and is superceded by it.

Keywords: system, behavior, goal, task, cockpit task management, workload, aircraft accident reports, aircraft incident reports.

Last update: 24 Jun 97

Based on what we learned in developing the Task Support System, we developed a preliminary, normative theory of Cockpit Task Management (Funk, 1991). The first part of this page is based on that paper.

Formally speaking, a theory is a collection of statements about some domain. These statements contain terms that are used to denote things and relationships that are considered to be important elements of the domain. For the theory to be sound and, of equal or greater importance, for it to be useful in analysis and design, these terms must be clearly defined. Terms essential to a theory of CTM are defined next.

A dynamic system is an entity that may be described in terms of input, output, and state. Input is matter, energy, or information flowing into the system. Output is the flow of matter, energy, or information out of the system. State is the set of system attributes at a given time. In addition, state is a compact representation of the history of the system that, with input given, makes possible the prediction of future outputs and states (Padulo & Arbib, 1974).

Two systems that are connected by inputs and outputs form a more complex system called a supersystem. If a system is formed from simpler systems through input output connections, the simpler systems are called subsystems. For example, an aircraft system can be partly defined as a collection of pilot, autopilot, airframe, and engine subsystems.

Note that this is "relative" terminology because whether something is called a system, a subsystem, or a supersystem depends on the analyst's perspective. For example, if the aircraft is considered a system, then the autopilot is a subsystem. On the other hand, if the analyst is primarily concerned with the autopilot, then the autopilot is a system, the aircraft is a supersystem, and the altitude hold circuitry in the autopilot is a subsystem. When using this terminology, the analyst must be careful to identify his or her purpose and frame of reference.

A system behavior is a discrete sequence or a continuous series of system input, state, and output values over a time interval. For example, given a system composed of airframe and engine subsystems, a behavior could be defined as time series of throttle setting (input), altitude (state), and sound pressure level (output) values. A system exhibits a behavior if observed values of input, state, and output values match those of the behavior.

An event is a set of system behaviors in which some state component changes in a significant way at the end of the time interval. For example, the event reach 10,000 ft consists of a set of aircraft behaviors, each ending with an altitude value of 10,000 ft.

A goal for a system is defined by a set of desired behaviors. If one of the behaviors is exhibited by the system, the goal is achieved, otherwise the goal is not achieved.

A goal has an initial event that defines the conditions under which the goal becomes relevant. For example, a typical flight path consists of a series of waypoints, which are geographical points along the route serving as intermediate destinations. So a goal to be at Waypoint 8 is relevant only after the initial event arrive at Waypoint 7 has occurred.

A subgoal of a goal is a set of behaviors consistent with those of the goal, but restricted in time and/or in scope. For example, a single goal to approach the destination airport and arrive at landing position (prior to final approach) could be decomposed into several subgoals: cleared to approach waypoint, at approach waypoint, approach flaps, approach power, and approach speed.

A goal and all of its subgoals form a hierarchy with the goal at the apex. The topmost goal for a flight mission will be referred to as the mission goal.

Goal priority reflects an ordering of a set of goals according to the relative importance or urgency assigned to them by the flight crew. More important or urgent goals have higher priorities. For example, a goal to remain clear of terrain and other aircraft established to maintain the safety of the aircraft and its passengers is clearly more important than a goal to avoid sudden maneuvers established for passenger comfort. The first goal should have a higher priority than the second.

This description of goal priority is a preliminary and highly simplified one. It does not accurately reflect the complex and dynamic nature of priority assignment which must take into account compliance with air traffic control directives, Federal Aviation Administration regulations, and company policies, to name just a few considerations. It reflects a minimal definition of goal priority which must be expanded as the theory is developed.

A task is a process that is completed to cause a system to achieve a goal. A task involves the behaviors of one or more secondary systems or subsystems necessary in order to produce inputs to the primary system to achieve the goal. For example, for the goal to arrive at Waypoint 7, there must be a fly to Waypoint 7 task. The pilot, the primary flight controls, the cockpit displays, the hydraulic system, and the engines are just a few of the secondary systems required to complete the fly to Waypoint 7 task to achieve the goal for the primary system (the aircraft) to arrive at Waypoint 7. These secondary systems are called resources. Stated another way, tasks require resources to achieve goals.

A task has state. Initially, a task is latent. When the initial event of its goal is imminent, the task becomes pending. When the initial event occurs, the task becomes active. A task becomes active in progress when resources are allocated to it to achieve the goal (i.e., while it is being executed). If the task has been in this state but resources are deallocated from it and execution ceases, the task returns to the active state. A task may be terminated if its goal is achieved, if the goal is unachievable, or if the goal becomes irrelevant. In the case of an unsuccessful termination, the task may be considered to be aborted. Further state decomposition is possible and perhaps desirable, but the set of states just described is satisfactory for the preliminary theory to be presented later in this document.

The goal to approach the airport and arrive at landing position was decomposed into cleared to approach waypoint and at approach waypoint subgoals. Similarly, an approach task could be decomposed into get approach clearance and fly to approach waypoint subtasks.

An agenda is a hierarchy of tasks to be completed during a mission. Each task is defined to achieve a specific goal and should become active when the goal's initial event occurs.

When an initial event occurs, the corresponding task becomes active. Two tasks that are simultaneously active are called concurrent tasks.

Executing a task involves the coordinated behaviors of one or more systems or subsystems called resources. Certain resources are required to complete each task, and if the resources are not available, the task cannot be completed satisfactorily and the goal cannot be achieved.

A variety of resources are required for cockpit tasks. Equipment resources include autopilots, radios, displays, and controls. Human resources include the captain, first officer, and flight engineer. Because resources are systems, they can be decomposed into simpler subsystems. Human resources can be decomposed into personal sensory, motor, and cognitive resources. Cognitive resources can be further decomposed into the verbal and spatial resources identified and studied by Wickens and his colleagues at the University of Illinois (Wickens, 1984; Wickens & Liu, 1988).

Because two concurrent tasks may require the same resources, this poses a potential problem. Behaviors of necessary resources that are compatible with achieving one goal may be incompatible with achieving another goal, and the performance of one or more of the tasks may suffer. That is, task performance is limited by resource availability. With resources like displays or hands and feet, this is obvious. But it is also true for cognitive resources (Navon & Gopher, 1979; WickensWickens, 1984). A situation in which task resource requirements exceed resource availability is called a task conflict.

For example, in the situation described above, if air traffic control clearance to an approach waypoint is obtained the set and maintain approach power task would become active. Assume that this task requires a multifunction display resource on which an engine display format must be shown. Suppose that now a primary electrical system failure event occurs and a subtask to diagnose/correct the electrical system becomes active. Assume that this subtask requires an electrical system display format on the same display resource. If the two formats cannot be displayed simultaneously, a resource shortage resulting in a task conflict exists.

Even if two displays are available to complete both of these tasks simultaneously, there still might be a task conflict due to cognitive resource limitations. Assuming for the purpose of this illustration that no other crew member is available to assist the pilot in completing these two tasks, he or she may lack sufficient cognitive resources to attend to both of them simultaneously. This might result in errors in completing one or both of the tasks.

The process by which the flight crew manages an agenda of cockpit tasks is called Cockpit Task Management (CTM). CTM is described as a procedure that is executed by the flight crew as follows:

• Create initial agenda.
• Until mission goal is achieved or determined to be unachievable:
• Assess current situation.
• Activate tasks whose initial events have occurred.
• Assess status of active tasks.
• Terminate tasks with achieved or unachievable goals.
• Assess task resource requirements.
• Prioritize active tasks.
• Allocate resources to tasks in order of priority:
• Initiate newly activated high priority tasks.
• Interrupt low priority tasks (if necessary).
• Resume interrupted tasks (when possible).
• Update agenda.

This procedure and the following explanation comprise a preliminary normative theory of CTM that seeks to identify the task management functions which should be performed by the flight crew. Together they represent an initial formalization of the functions the flight crew should use to manage their activities successfully.

Given a hierarchy of goals to accomplish in a mission, the first CTM step for the flight crew is to create the initial agenda. This agenda consists of a task to achieve each goal. An initial event must be defined for each goal/task pair.

Once the agenda has been created, a process of agenda management begins and continues until the mission goal is achieved or unachievable. In the latter case, the process should end only after the aircraft and its subsystems reach some safe state.

The flight crew must assess the current situation. The states of all relevant aircraft systems and subsystems must be considered to determine if significant events have occurred.

When initial events occur, the flight crew must activate tasks that are contingent upon those events. This means that these tasks enter the active and should become active in progress as soon as resources are available.

The flight crew must assess the status of active tasks to determine if satisfactory progress is being made toward achieving the tasks' goals. Not only must the current status of each task be assessed, but if the task's goal is not yet achieved, the status of the task must be projected into the future to determine the likelihood that the goal will be achieved. A task's status may be declared satisfactory if its goal is achieved or is likely to be achieved, marginal if achievement of its goal is uncertain, or unsatisfactory if the goal is violated or is unlikely to be achieved without corrective action.

Based on this assessment, the flight crew should terminate tasks with achieved or unachievable goals. Tasks whose goals become irrelevant due to changing circumstances should also be terminated. Termination removes tasks from competition for resources.

For the remaining active tasks, the flight crew should assess task resource requirements to determine what resources are required to complete them. A newly activated task might be started with minimal resources, but a task of marginal or unsatisfactory status might require additional resources to achieve its goal.

The flight crew should prioritize the active tasks. Factors that can influence task priority include the following:

1. The importance and urgency of the task's goal.

2. The importance and urgency of other active tasks' goals.

3. The current and projected status of the task.

4. The current and projected statuses of other active tasks.

Prioritization can be defined as a pairwise comparison of tasks based on these factors and others, that results in an ordering of active tasks.

As a result of the previous steps, the flight crew must then allocate resources to tasks in order of priority. This is an assignment of resources to tasks, with preference given to high-priority tasks, so that the tasks may be executed. The flight crew should initiate newly activated high priority tasks to make them active in progress. They should interrupt low-priority tasks that are active in progress when high-priority tasks requiring the same resources become active. When the high priority tasks finish and resources become available again, the flight crew should resume interrupted tasks, returning them to the active in progress state. These steps result in a set of tasks in the process of execution.

This process causes changes in the set of pending and active tasks and changes in task status and priority. The flight crew should update the agenda to reflect these changes and repeat the process.

To determine the nature and significance of CTM in flight operations, we set out to

The remainder of this page is based on Chou et al (1996).

We developed an initial CTM error taxonomy consisting of seven general CTM error categories corresponding to the functions of CTM described above (Chou & Funk, 1990). Each category was further described in terms of specific error classes. Use of the initial taxonomy in preliminary analyses of accident and incident reports showed some of the error classes to be redundant and the taxonomy, as a whole, to be difficult to apply consistently.

As a result we revised the taxonomy to include the the following CTM error categories: a task may be initiated or terminated too early, too late, under incorrect conditions or for incorrect reasons, or it may not be initiated or terminated at all. Furthermore, a task may be given too high or too low a priority.

This revised taxonomy served as the basis for our accident and incident studies, described below.

The underlying causes of aircraft accidents usually fall into the three broad categories of mechanical factors, weather, and pilot error. However, these labels should not be used to mark the end of further analyses for human and other system performance errors since aircraft accidents are usually the outcomes of a number of contributing factors. In an effort to determine if some instances of pilot error could be explained in terms of CTM, and thereby begin to understand the significance of CTM to flight safety, we reviewed a set of aircraft accident reports (Chou, 1991).

Our analysis reflects the examination of the abstracts of 324 NTSB aircraft accident reports concerning accidents occurring between 1960 and 1989. After reviewing the 324 National Technical Information Service (NTIS) abstracts of these reports, accidents that were obviously unrelated to this study were removed from the screening process. For example, this included accidents due primarily to weather and mechanical failures. This elimination process left 76 accident reports for further analysis.

Following the initial screening, we selected a representative set of cases for further study, based on the following considerations. First, we chose the cases so as to include a complete set of CTM errors as described above. Secondly, we chose cases involving conditions we felt we could reconstruct in a simulated environment. For each accident in this set, we carefully studied the data and conclusions of the NTSB investigators and constructed an operational task context. Each context was a graphical representation of cockpit activities during the time leading up to the accident. It included the number and type of concurrent tasks competing for the flightcrew's resources, the state of each task (pending, active, interrupted, or terminated), and selected system state variables (e.g., aircraft altitude, speed, etc.). With the insights gained from this detailed analysis and using the data and conclusions in the accident abstracts and full reports, we identified and classified 80 CTM errors in 76 of the 324 accident reports. That is, we found that CTM errors occurred in about 23 per cent of the accidents reviewed. These errors, summarized by category, are presented in Table 1.

Although we cannot state categorically that CTM errors were the sole or even primary causes of these accidents, we do believe that they played significant roles. Had the errors been prevented, the accidents probably would have not occurred. We conclude that the moderately high incidence of CTM errors in the accidents -- 76 (23 per cent) of 324 accidents -- is supportive evidence that CTM is a significant factor in flight safety.

Fortunately, aircraft accidents are very rare events. Unfortunately, a set of accidents like the one we studied might be a very biased sample of the operating environment. Therefore, inferences made from a set of accidents may have little relevance to reducing the likelihood of future accidents. For that reason, we next turned our attention to aircraft incidents (Madhavan, 1993).

AIncident means an occurrence other than an accident, associated with the operation of an aircraft, which affects or could affect the safety of operations.@ (Federal Aviation Regulations, 1994). Although incidents by definition do not involve death, serious injury or substantial aircraft damage, in retrospect, most airline accidents were foreshadowed by clear evidence that the problems existed long before, as incidents. Our specific objective in analyzing aircraft incidents was to determine the significance of CTM in flight operations more representative of normal conditions.

We used as a source of aircraft incident information NASA's Aviation Safety Reporting System (ASRS). The ASRS database consists of anonymous reports filed by pilots and air traffic controllers describing events in which accidents nearly occurred or in which flight safety was seriously compromised.

Our preliminary analysis of CTM errors focussed on aircraft incident reports relating to in-flight engine emergencies (99 reports) and controlled flight toward terrain (CFTT, 205 reports). We found CTM errors in 19 per cent and 54 per cent respectively of these reports. The high incidence of CTM errors in the CFTT reports as well as the fact that over 49 per cent of all airline accidents occur during approach and landing (Boeing, 1993), caused us to focus further attention on the terminal phases of flight. At our request the ASRS office furnished us with 243 additional reports pertaining to these phases.

 As in most ASRS studies, we used the narrative section of the reports for our analysis. The narrative is the section of the report in which the reporter states in his/her own words what happened and why it happened.

In the narratives, we focussed on activities directly related to task management only. Incidents involving crew personality differences and other sociological factors were excluded. Where narratives were unclear about the specific errors committed (i.e. no categoric admission of the errors by the reporters), some inferences were made about the errors based on our knowledge of standard operating procedures, as gleaned from aircraft operations manuals, accident reports, incident reports, and other aviation literature. Explicit statements in the narratives such as "... forgot ...", "... omitted ...", "... memory lapse ...", "... oversight ..." etc., enabled us to home in quickly on the error classification.

From the 540 ASRS incident reports we obtained, we eliminated duplicates. We then applied the CTM error taxonomy to the remaining 470 unique reports. We found CTM errors in 231 (49 per cent) of the 470 ASRS incident reports. The results of the analysis are presented in Table 2.

Task initiation appears to be the most significant CTM error category, accounting for 42 per cent of the CTM errors identified. Task initiation errors included early descents, late configurations, and failures to tune navigation and communication radios. Task prioritization errors accounted for 35 per cent of the CTM errors and included distractions by weather and traffic watches. The remaining 23 per cent of the CTM errors were in the task termination category. These included early autopilot disengagements, altitude overshoots, and improperly continued landings under unsafe conditions.

While task initiation appears to be the largest CTM error category, that may be somewhat misleading. The failure to start a task on time (or at all) or the decision to start a task too early may often be explained as misprioritization. That is, excessive priority placed on one task may delay the start of a second task or cause the flightcrew to start the first task before they should. Similar arguments can be made for task prioritization verses task termination. Although the initiation and termination categories are useful for understanding errors, their causes, and their consequences, task prioritization should perhaps draw our greatest attention for the development of countermeasures.

We conclude that the high incidence of CTM errors in the incident reports -- 231 (49 per cent) of 470 reports -- is supportive evidence that CTM is a significant factor in flight safety.

 From our accident and incident studies, we determined that CTM is significant enough to warrant further study. However, we felt that a different approach was needed to better understand the nature of CTM behavior. Aircraft accidents are rare events, thus providing few opportunities for developing insights into error processes, which are, in any case, very difficult to reconstruct. By the same token, though ASRS incident reports can provide first-hand information on abnormal cockpit operations, they are subject to self-reporting biases and other problems. Therefore, controlled experimentation provides a useful alternative, serving to compensate for the drawbacks noted above and to provide an opportunity for objective observations. An additional advantage of the simulation method is that it enables observation of how human operators manage tasks under normal conditions.

The main objectives of our experiment were to elicit and observe CTM errors similar to those identified in the accident and incident analyses and to identify the factors leading to such errors. Our approach was to have subject pilots fly a low fidelity flight simulator in several flight scenarios and observe and analyze their behavior in managing and performing concurrent flight tasks.

Our flight simulator consisted of three networked personal computers. The system simulated a generic, two-engine commercial transport aircraft. One computer simulated aircraft dynamics using a very simple aerodynamic model and produced a simple primary flight display showing heading, altitude, airspeed, pitch, and roll. The subject controlled the simulated aircraft by means of a joystick. A second computer simulated the navigation system and presented a moving map display. The subject could use the navigation display for planning and navigating purposes and could control map scale and orientation (north up or track up) by means of mouse-activated controls. The third computer simulated aircraft subsystems, including engines and hydraulic system, and generated a simplified Engine Indicating and Crew Alerting System (EICAS) display. Aircraft subsystem models included failure modes that could be triggered by script files and which required subject interaction by mouse-activated controls to correct.

Twenty-four unpaid subjects from Oregon State University participated in the experiment. The subjects included two engineering faculty members, three undergraduate engineering students, and 19 engineering graduate students. Two of the subjects had private pilot licenses with 120 to 150 hours flight time. The other subjects had no flight experience. Sixteen subjects participated in two pilot studies, and the remaining eight subjects participated in the data collection runs. The pilot studies were used for refining training procedures and flight scenarios.

Subjects received a 60-minute training session prior to each experiment. This session included viewing a training video tape and running a simplified scenario. The scenarios were categorized into 6 different levels by the following independent variables: resource requirements, maximum number of concurrent tasks, and flight path complexity. Following concepts from multiple resource theory (Wickens, 1992) and W/INDEX (North & Riley, 1989), scenarios were created and rated according to the requirements for visual resources (to acquire needed information from simulated visual displays), manual resources (to manipulate simulated controls), and mental resources (to recognize, remember, calculate, and decide). Each scenario received an aggregate resource requirements rating (low or high). The number of concurrent tasks was defined as the maximum number of tasks requiring subject attention at any point in the scenario (three or six). Flightpath complexity (easy or hard) was varied by adjusting the sharpness of turns at waypoints in the flightpath.

A split-plot design was used for the experiment. The latter factors (number of concurrent tasks and flightpath complexity) were crossed to provide four levels for whole unit factors. These four whole unit factors were then crossed with the subunit levels (resource requirements) to provide eight treatments. Given this design, eight subjects were used to provide two responses for each treatment. Each subject performed two levels of the subunit factor (low and high resource requirements), and the assignment of treatments to subjects was randomized to control learning effect. That is, four subjects started with the high resource requirements treatment and then performed the low resource requirement treatment, whereas the other four performed their treatments in the reverse order.

The following performance measures were used:

1. Average response time to system faults,
2. Root-mean-square (RMS) flight path error,
3. Task prioritization score, and
4. Number of tasks that were initiated late.

The response time to a system fault was defined as the time from the occurrence of the fault (such as an electrical bus fault) until a compensating response was initiated. This corresponded to task initiation. The task prioritization score was determined from paired comparisons between tasks, and was used for measuring task prioritization performance. A score of +1 was assigned when a correct prioritization was made by the subject (i.e., attention was first given to the higher priority task), otherwise a -1 was assigned. Scores for the remaining tasks were set to zero. Finally, a task was said to be initiated late if the subject did not respond to the task 60 seconds after it had been activated. This was used to measure task initiation performance.

The analysis of variance (ANOVA) results for factors with significant effects are summarized in Table 3. We found the resource requirements level to have a significant effect on the average task response time. That is, higher resource requirements increased delays in initiating a task. However, neither combination of flightpath complexity nor maximum number of concurrent tasks (alone or in combination) had a significant effect on task response time.

During the experiments, subjects were warned if 60 seconds passed after the occurrence of a system fault and no actions were taken. Thus, the definition of a late initiation was failure to initiate the task within one minute following fault occurrence. The analysis of variance results show that resource requirements had a significant effect on late task initiation.

Results from the ANOVA show that both resource requirements and the combination of flightpath complexity and number of concurrent tasks created significant effects on task prioritization. Therefore, task prioritization degrades as either one of these factors increase.

We calculated the RMS of deviations in flight parameters using data obtained from whole mission information. Heading deviations were significantly affected by the combination of flightpath complexity and the number of tasks; changes in mental resource requirements were significant to the altitude deviation. None of the other RMS deviations were significantly affected by either the resource requirements or the combination of flightpath complexity and the number of concurrent tasks. 

We developed a normative theory of Cockpit Task Management and a taxonomy of CTM errors, based on that theory and applied the latter in the analysis of National Transportation Safety Board aircraft accident reports and Aviation Safety Reporting System incident reports. We found CTM errors in 76 (23 per cent) of the 324 accident reports analyzed and in 231 (49 per cent) of the 470 incident reports. In a low fidelity simulator study, we found that resource requirements (visual, manual, and mental) had a statistically significant effect on task initiation and task prioritization performance, and that the number of concurrent tasks coupled with flight path complexity had a statistically significant effect on task prioritization performance.

From our studies of aircraft accidents and incidents, we conclude that CTM is a significant factor in flight safety. And, as Raby and Wickens' (1994) results implied, our experiments confirm that increased resource requirements increase the likelihood of CTM errors, specifically, late task initiation and incorrect task prioritization errors.

We offer four recommendations. First, we recommend that pilots receive instruction concerning CTM and how to avoid CTM errors. More specifically, pilots should be made aware that in periods of high workload, when large numbers of concurrent tasks are competing for their attention, there is danger that they will not initiate important tasks promptly and/or that their attention will be drawn away from safety-critical tasks. Presumably, pilots can be taught to recognize these precursor conditions and to develop personal strategies to avoid CTM errors when these conditions are present. CTM instruction might most naturally fit into existing Crew Resource Management training programs.

This recommendation is based on the assumption that our experimental environment, involving a low fidelity simulator and (mostly) non-pilot subjects is, at a very high level of abstraction, similar enough to the real commercial transport aircraft environment to warrant extrapolation. This assumption should be tested, so our second recommendation is that further studies of CTM be conducted using full-mission scenarios in high fidelity training simulators with line pilots as subjects. The objectives should be to validate our earlier findings, to search for other factors affecting CTM performance, to identify patterns of both good and bad CTM, and to attempt to link CTM errors with human cognitive characteristics, such as short term (working) memory limitations.

Third, we recommend that research be conducted to develop and evaluate formal cockpit procedures to facilitate CTM performance, based on findings from the studies recommended above. Such procedures might, for example, involve memory aids and elaborated versions of the well-known pilots= prioritization maxim: Aaviate -- navigate -- communicate -- manage systems.@

Finally, our fourth recommendation is that research be conducted to develop and evaluate a computational aid to facilitate CTM performance: a Cockpit Task Management System (CTMS). A CTMS might, for example,

1. maintain a current model of aircraft state and current cockpit tasks,
2. monitor task state and status,
3. compute task priority,
4. remind the pilots of all tasks that should be in progress, and
5. suggest that the pilots attend to tasks that do not show satisfactory progress.

We must point out, however, that for any approach to be effective, net pilot workload must not increase. If personal strategies, formal procedures, or computational aids impose additional mental demands, there must be compensatory workload reductions. Otherwise, the supposed aids may actually lead to even worse CTM performance.

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