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operations. Managing this threat is made easier by knowing the current weather and the forecast trend for at
least the duration of a controller’s shift. For example, changes in wind direction may involve runway
changes. The busier the traffic, the more crucial becomes the timing for a runway change. A controller will
plan strategies to make the change with a minimal disruption to the traffic flow. For en-route controllers,
knowing areas of significant weather will help to anticipate requests for re-routings or circumnavigation.
2.16 Appropriate knowledge of local weather phenomena (e.g. turbulence over mountainous terrain,
fog patterns and intensity of thunderstorms) and/or sudden weather occurrences such as wind shear or
microbursts contributes towards successful weather threat management.
2.17 Geographical environment. Threats in this category comprise high terrain or obstacles in the
controller’s area of responsibility. Less obvious threats can be posed by, for example, residential areas that
must not be overflown below certain altitudes or during certain hours. At some airports, runway changes are
mandatory at specified times of the day for environmental reasons.
3. ERRORS IN AIR TRAFFIC CONTROL
3.1 Errors may be defined here as “actions or inactions by the ATCO that lead to deviations from
organizational or ATCO intentions or expectations”. Unmanaged and/or mismanaged errors frequently lead
to undesired states. Errors in the operational context thus tend to reduce the margins of safety and increase
the probability of adverse events.
17-APP 3-4 Safety Management Manual (SMM)
3.2 Errors can be spontaneous (i.e. without direct linkage to specific, obvious threats), linked to
threats, or part of an error chain. Examples of errors would include not detecting a readback error by a pilot;
clearing an aircraft or vehicle to use a runway that was already occupied; selecting an inappropriate function
in an automated system; and data entry errors.
4. UNDESIRED STATES IN AIR TRAFFIC CONTROL
Undesired states are defined as “operational conditions where an unintended traffic situation results in a
reduction in margins of safety”. Undesired states that result from ineffective threat and/or error management
may lead to compromising situations and reduce margins of safety in ATC operations. Often considered the
last stage before an incident or accident, undesired states must be managed by ATCOs. Examples of
undesired states would include an aircraft climbing or descending to another level than it should, or an
aircraft turning in another direction than it should. Events such as equipment malfunctions or flight crew
errors can also reduce margins of safety in ATC operations, but these would be considered threats.
Undesired states can be managed effectively, restoring margins of safety, or the ATCO’s response(s) can
induce an additional error, incident or accident.
An undesired state is often the first indication to a controller that an
earlier threat or error was not adequately managed.
5. THREAT AND ERROR COUNTERMEASURES
5.1 As part of the normal discharge of their operational duties, ATCOs employ countermeasures to
keep threats, errors and undesired states from reducing margins of safety in ATC operations. Examples of
countermeasures would include checklists, briefings and SOPs, as well as personal strategies and tactics.
Flight crews dedicate significant amounts of time and energy to the application of countermeasures to
ensure margins of safety during flight operations. Empirical observations during training and checking
suggest that as much as 70 per cent of flight crew activities may be countermeasures-related activities. A
similar scenario is likely in ATC.
5.2 All countermeasures are necessarily ATCO actions. However, some countermeasures to threats,
errors and undesired states that ATCOs employ build upon “hard” resources provided by the aviation
system. These resources are already in place in the system before ATCOs report for duty and are therefore
considered as systemic-based countermeasures. The following would be examples of “hard” resources that
ATCOs employ as systemic-based countermeasures:
a) minimum safe altitude warning (MSAW);
b) short-term conflict alert (STCA);
c) SOPs;
d) briefings; and
e) training.
Chapter 17. Air Traffic Services (ATS)
Appendix 3 17-APP 3-5
5.3 Other countermeasures are more directly related to the human contribution to the safety of ATC
operations. These are personal strategies and tactics, and individual and team countermeasures, which
typically include canvassed skills, knowledge and attitudes developed by human performance training, most
　
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