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Goals, Boundaries and Margins – The Structure of Tasks

In Chapter 3,1 outlined some properties of the system, such as boundaries and margins. I now' want to elaborate on the idea of a task and to incorporate the concept into a systems model. Any commercial aviation task can be described as a trajectory from assembly to dispersal. Assembly is the process of bringing together the resources needed to accomplish the task. This will include the airframe, crew, fuel, payload, etc. Dispersal is the reallocation of resources at the end of the specific task. A trajectory will comprise a set of goal states. A goal state is a specific configuration of the aircraft and resources relevant to some aspect of the task and can be described in terms of the set of constraints to be satisfied. Once a goal state has been satisfied

A goal state model

FIGURE 5.2 A goal state model.

the task continues along its trajectory to the next goal state. The status of the system reflects the degree to which the actual state and planned state are congruent.

Task management, then, involves establishing a future goal and then initiating actions that will allow that goal to be achieved. Figure 5.2 shows the main components of a dynamic goal state model. As the trajectory unfolds (represented by the hatched line in Figure 5.2), we move from the current goal state (2 in Figure 5.2) towards the desired future goal state (1). The status (3), as we have said, is the current configuration. If the task management is successful, status and state will be congruent until the next state change is required. As the trajectory transitions through one goal state to the next, the status will diverge until it can be brought back into equilibrium.

Some goal states can be specified in fairly specific terms. Landing the aircraft requires a fairly limited, specific set of constraints to be met if the landing is to be deemed a success. However, other goal states can be better represented as a volume of space and time. For example, we can draw a notional ‘top of descent’ as a mark on a chart, but, in reality, it is an event in time rather than a point in space. The actual descent point will vary according to several factors, such as weather, other traffic and air traffic control (АТС).

The primary role of the crew is to manage the aircraft through these transitions from one goal state to the next, as illustrated in Figure 5.3. For example, during the taxy phase, the runway in use is usually identified by АТС together with the route to follow to arrive at the point at which the aircraft can enter the active runway. The movement from the gate to the runway is more than simply a physical relocation of the aircraft. First, the crew must successfully depart from the terminal area and any associated ground support. Systems are tested and configured for departure. The actual take-off configuration and route are validated through the monitoring of АТС transmissions from the departing aircraft; contingencies are discussed, and crew actions allocated. There are probably four distinct goals buried in this stream of activity: push back, test and configure, manoeuvre and, finally, confirm the next step.

An idealised trajectory

FIGURE 5.3 An idealised trajectory.

The nature of the transitions between the goal states in terms of complexity and time available varies throughout the trajectory. A very short taxy route from the gate to the runway requires an increased work tempo whereas a long taxy at a busy airport, with other aircraft waiting to depart ahead of you, can create a different set of problems. Achieving the immediate goal state will trigger the next set of actions necessary to accomplish the subsequent goal.

Desired goal states are usually associated with a critical point (CP), which is a notional point on the trajectory by which a particular goal state must be achieved before the aircraft can safely (or efficiently) proceed to the next goal state (see Figure 5.4). Convergence between the current status and the desired goal state will vary in terms of rate and the extent to which convergence will occur before the CP. Failure to achieve a goal state by the CP will have implications downstream. Looking at the example of taxying the aircraft, the crew cannot start the journey to the runway until they have successfully finished all the activities associated with the push back. Aircraft have struck ground equipment that had not yet been moved injuring personnel and dragged still-attached equipment and collided with other manoeuvring aircraft during this phase. Taxying to the runway cannot commence until a variety of other activities have been done. Equally, the take-off cannot commence until after the aircraft has been correctly configured. Each element of the move from the terminal to the runway will have a CP by which tasks must have been accomplished.

It is possible to look forward to successive goal states and if a downstream state becomes illegal, all preceding states becoming irrelevant. As an extreme example, when the magnitude 9 earthquake and subsequent tsunami struck Japan on 11 March 2011, aircraft in the area were told that all airports were closed. For those aircraft in the descent or on approach, their goal state instantly became one of ‘where can I land before I run out of fuel’. Everything else was redundant.

I need to distinguish between a system boundary and CP. The boundary is the point at which the system, as a whole, becomes unstable, shifting from a safe to an unsafe state. A CP, on the other hand, is a notional point in a stream of work by which the goal state constraints should be met. A really simple example would be the need to select gear down sufficiently in advance of landing in order to allow time for the undercarriage to deploy. To bring control and predictability to the flight process, airlines develop procedures, many with associated notional ‘gates’, such as a stabilised approach point. These are often, in fact, pseudo-goals in that, although easy to define, they do not necessarily map onto the operator’s cognitive model of the current process, but they, nonetheless, represent CPs set sufficiently in advance of the boundary. The landing system boundary is a broader concept, but in some circumstances, a goal state CP and the boundary may be identical.

The margin is the space available to satisfy the goal state constraints and can be considered synonymous with the use of the term in relation to the broader system. In both cases, it is a volume of space in which action is required. Margins are not constants and are created as the trajectory unfolds. In the example of taxying to the runway, the taxi route length will affect the time available to complete the necessary tasks without incurring a penalty, such as delaying take-off. In the case of the approach, ground speed and the point of interception of the inbound course (height in relation to desired glide slope, distance from touchdown) will shape the margin available. Satisfying the requirement to align the status and goal state by the boundary requires the crew to anticipate and act in such a way that states, transitions and constraints can be satisfied.

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