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The perception of useful ATC information is confined to the senses of sight and
hearing. Communications between pilots and controllers are a major factor
influencing controller mental workload. Air traffic controllers frequently deal with
information overload. Selectively switching one’s attention between competing
information sources, visual and auditory, is a basic skill that a controller must develop
in order to stay ahead of the situation and not ‘lose the picture’. The need to
selectively attend and divide one’s attention is due to the limitations of working
memory.
Working memory represents a critical component of communications. It temporarily
retains information which is either verbal or spatial. Verbal working memory is the
‘rehearsable’ memory for sounds, such as digits and words, that a controller uses
when receiving a request or readback from a pilot. Information in working memory is
interpreted on the basis of information stored in long-term memory, which retains
the less dynamic aspects of the controllers environment, such as airspace knowledge,
air route structure, aircraft performance, procedures, and standard phraseologies.
Routine actions, decision-making and planning processes draw heavily on knowledge
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in long-term memory, and strategies (or heuristics) picked up by experience or
observation of others are stored there. The exchange of information between the
working and long-term memories is “the process of comprehension that provides a
mental picture of the situation confronting the controller and underlies the controller’s
situational awareness” (National Research Council, 1997:94).
Because working memory is heavily involved in the processing of speech, it is limited
by long messages, which are often misunderstood, and background noise, particularly,
as noted, other verbal activity. This limitation occurs whether the speech is in the
immediate environment or the controller trying to remember a communication while
concurrently speaking or listening. A controller can easily talk to a pilot while
scanning a radar display, but cannot easily talk while listening to another controller.
The length of the message may induce errors: having to produce two kinds of
response to the same message increases the chances of forgetting to do one or the
other response, yet if a controller presents a pilot with two short messages, this also
can lead to forgetting (Morrow and Rodvold, 1998). Working memory also suffers if
it must retain items that are similar to each other, such as similar aircraft callsigns,
names or acronyms. Where control positions have two or more frequencies, the
controller can generally listen to only one transmission at a time with any accuracy
and recall of the information. Experiments show that switching between incoming
auditory information can take place but that only one message at a time can be
listened to precisely (Roske-Hofstrand and Murphy, 1998).
The problem for air traffic controllers is not so much the quantity of information
being received, although this is considerable, but rather the lack of influence they
have over the timing of these events to which they must promptly respond, and the
little time available to mentally process them while they remain in the working
memory. The number of communications events adds much to the complexity of
controllers information processing tasks. Much voice communication is ‘expectationdriven’
which reduces the load on working memory until unfamiliar material (such as
strange names, non-native language) or long communication strings must be retained
for even a few seconds before being translated into action (National Research
Council, 1997).
A further complication arises for the radar controller: visual dominance
phenomenon occurs primarily when auditory information competes with visual
information (Roske-Hofstrand and Murphy, 1998). Visual stimuli offer good
referability through continuous presentation but auditory stimuli are transient. Visual
stimuli are spatial and their display occupies space, but auditory stimuli are temporal
in that their presentation occupies time. Visual stimuli can be presented sequentially
or simultaneously, but auditory ones must be sequential. The dominance of visual
stimuli over the auditory means that a controller concentrating on a visual display of,
say, aircraft rapidly converging , may not hear a concurrently presented auditory
signal such as the sound of a minimum safe altitude warning alert or an aerodrome
terminal information service change.
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3.5 Expectation
Expectations influence perceptions and therefore underlie many other potential errors
　
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