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Other sectors
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Aircraft crew
Figure 1: The pivotal role of the air traffic controller in information transfer.
The interfaces shown are those that bear directly on aircraft management in the
airspace system. (Adapted from Billings and Cheaney, 1981:87)
Air traffic control developed from its initial role of communicator and traffic advisor
to separation estimator and flight path decision-maker. With the coming of radar,
decision-making and judgement functions of ATC increased to the extent that today it
has also taken on the role of traffic flow director. The nature of the job and the
problems encountered differ with the service being provided. Tower controllers are
concerned with separating landing and departing aircraft, the surface movements of
aircraft and vehicles, local weather conditions, and the runways in use. They deal
with runway incursions, emergency landings and emergency services call-outs, wrong
turns, ground conflicts, airfield maintenance teams, and so on. Terminal area
controllers, using radar, sequence arriving aircraft into a landing order, accept
departing aircraft and establish them on their initial departure tracks, and control all
aircraft operating within the vicinity of the airport. They must handle problems such
as sequencing disorders, runway changes, aircraft deviations from trajectories (course,
speed, altitude), aircraft not on proper approach and departure paths, airspace
penetrations, and in-flight emergencies. Enroute controllers deal with aircraft cruising
or transiting to or from the cruise conditions and establish holding patterns. Problems
of route changes for weather avoidance, flight plan changes, level changes, holding
pattern “stacking”, climbing, descending and crossing conflicting traffic, and in-flight
emergencies must be resolved. Flow controllers analyse the inbound traffic stream,
determine the landing order and communicate it to the enroute and terminal area
controllers. They must deal with the mix of aircraft types, re-routing, speed
adjustments, issuance of holding instructions to controllers, runway changes, and
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changing priorities due to emergencies or medical flights. In some cases (e.g.
Sydney), even before departure, pilots will be told the time that they must arrive in
order to be guaranteed a landing ‘slot’. Flight service officers provide a traffic
information and flight information service to pilots outside of controlled airspace. Air
traffic controllers and flight service officers together provide air traffic services.
Such job descriptions fail to convey the complexity of the four-dimensional (space
and time) conundrum. Hopkin (1995:153) writes that “air traffic control is complex,
more so than it seems at first...To an uninformed observer, most of air traffic control
is not inherently meaningful, and it has only become meaningful to the controller
because of training”. Muller (1996) describes the controller’s job as a “strange and
specialised one”—on one hand the controllers are expected to apply strictly defined
procedures and abide by countless regulations, yet at the same time, they are
confronted with new situations requiring substantial flexibility in their response.
There are several distinguishing features to air traffic control:
• Three-dimensional nature of movement: The three-dimensional nature of aircraft
trajectories can only be displayed on a two-dimensional radar screen or, more
awkwardly, on a two-dimensional procedural display console. The controller must
think in three-dimensions and predict a fourth.
• Speed and stress: Mastering the three-dimensional movement is further
complicated by the speed at which it occurs. This reduces the time to recognise,
evaluate and react to unexpected problems. It is a matter of reaching quick,
workable decisions and not of looking for a perfect solution but finding it too late.
Often heuristic thinking is required, not algorithmic.
• Limited correction possibilities: There is little leeway for correction. Safety
tolerances are usually large but the rapid sequence of events reduces the time
remaining to register or correct errors. Controllers must be able to concentrate and
react rapidly.
• Great significance of small errors: Minor errors or slips can cause serious
accidents yet these are difficult to detect. Human error has been called “the
relentless threat to aviation safety” (Maurino, Reason, Johnston and Lee, 1995).
• Constant changes: The aviation system is in the vanguard of technical
development. ATC procedures are in a state of virtually constant change which
　
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