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occurs. It is also critical in the transition from terminal airspace to
the airport surface, where runway clearances must be assured.
Many air traffic control positions are staffed by two controllers
who work together. This work design not only divides the task but also
provides redundancy in the form of additional eyes and ears to
APPENDIX A
FACTORS POTENTIALLY AFFECTING SEPARATION SAFETY
A-21
maintain situation awareness. However, under low traffic conditions
supervisors will often elect to turn a team activity into an individual
one. Although this practice does reduce staffing requirements it also
removes the advantages of awareness redundancy. It is noteworthy
that a substantial proportion of operational errors occurs during
periods of low workload, when such combined activities are in effect.]
e. Controller response time
[ATC incidents occur in a real-time, dynamic, event-driven
environment. Nevertheless, in today’s environment, air traffic
controllers normally have minutes or tens of minutes available to
decide on actions (although there are exceptions). As a result, there
normally is a long decision horizon and problem solving can extend
over time with the possibility of incremental progress towards a
solution and repeated inspection of progress and adjustment of that
solution. Thus, a controller’s responses are not just the result of a
single decision. The monitoring of the results of the decision can
provide additional diagnostic cues for adjusting future decisions and
responses. However, any change in the airspace that would reduce the
time available to the controller to formulate a plan or to take an
action reduces the opportunity not only to generate an effective
strategy, but also to revise understanding of the situation and to revise
the response to an incident. Failure in making such revisions are
sources of error in ATC problem solving.]
f. Controller workload
[Projected increases in air traffic and proposed reductions in aircraft
separation minima may substantially increase cognitive demands on
the controller. If safety is not to be compromised, controllers should
not be subjected to sustained overload due to high traffic density or
revised patterns of traffic what might lower their performance. While
individual controllers may work most effectively at different workload
levels, for each controller there are upper and lower workload
thresholds outside of which the controllers’ effectiveness diminishes.
These limits may be increased with the introduction of new procedures
and technologies such as more effective displays and decision aids.]
g. Interaction with displays/automation/decision aids
i. Displays
[The switch from hardwired displays to computer-based displays
has, in general, not changed the way the display of information is
conceived.. Too often, displays provide poor graphic images or
alphanumeric data with little consideration for even basic human
factors design. While technological power has generally advanced
our ability to gather and manipulate data, it has rarely been used
successfully to advance our ability to interpret this avalanche of
data; i.e., to extract meaning from the display of data to support
user goals/tasks. In short, we have to be careful that as we
SEPARATION SAFETY MODELING
A-22
increase the technological sophistication of computer-based ATC
systems, we do not overload the information processing capacity of
the controller. We have to be careful that as the demands of ATC
performance increase, increases in data acquisition and
manipulation do not overload memory or add new tasks that
increase workload. We must always be mindful that a successful
ATC display technology must help the controller extract relevant
information under conditions of actual task performance (e.g.,
time pressure, risk, uncertainty, competing tasks) primarily by
means of more integrated displays.]
ii. Automation
[Automation has the potential for improving ATC efficiency,
handling increased traffic, and possibly enabling reduced
separation minima while maintaining or improving safety. While
automation has led to numerous benefits, including more efficient
performance, elimination of some error types, and reduced
controller workload in some cases, several potential costs have
been noted. These include increased workload, increased
monitoring demands, reduced situation awareness, unbalanced
trust (mistrust and overtrust), new error forms, and loss of team
cooperation. These costs, as well as the fact that perfectly reliable
automation (or software) is not immediately possible, will
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a concept paper for separation safety modeling(67)
