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from the traffic. Relief staff may be late. Other controllers in the unit may be handling traffic less efficiently
than expected, and consequently, they cannot accept the additional traffic a controller wants to pass to
them.
External threats to the ATS provider
2.7 Airport layout and configuration can be a source of threat to ATC operations. A basic airport
with just a short taxiway connecting the ramp with the middle of the runway will require ATC to arrange for
backtracking of the runway by most of the arriving and departing traffic. If a taxiway parallel to the runway
were available, with intersections at both ends as well as in between, there would be no requirement for
aircraft to backtrack the runway. Some airports are designed and/or operated in such a way that frequent
runway crossings are necessary, both by aircraft under their own power, and by towed aircraft or other
vehicles.
2.8 Navigational aids that become unexpectedly unserviceable (e.g. because of maintenance) can
pose a threat for ATC since they may cause inaccuracy in navigation and affect separation of aircraft.
Instrument Landing Systems (ILS) available for both directions of the same runway are another example of
this category of threat. Usually only one of the ILS is active, so with a runway change, the ILS for the current
runway direction may not yet be activated when ATC is already clearing aircraft to intercept it.
2.9 Airspace infrastructure/design is another potential source of threat for ATC. If manoeuvring
space is restricted, it becomes more difficult to handle a high volume of traffic. Restricted or Danger Areas
that are not permanently active may be a threat if the procedures for communicating the status of the areas
to the controllers are inadequate. Providing an ATC service to traffic in Class A airspace is less open to
threats than, for example, in Class E airspace where there can be unknown traffic that interferes with the
traffic controlled by ATC.
2.10 Adjacent units. Controllers from adjacent units may forget to coordinate a traffic handover. The
handover may be coordinated correctly, but incorrectly executed. The airspace boundaries may not be
respected. A controller from the adjacent centre may not accept a proposal for a non-standard handover,
forcing the need for another solution. Adjacent centres may not be able to accept the amount of traffic that a
unit wants to transfer to them. There may be language difficulties between controllers from different
countries.
Airborne threats
2.11 Pilots who are unfamiliar with the airspace or airport can pose a threat to ATC. Pilots may not
advise ATC of certain manoeuvres that they may need to make (e.g. when avoiding weather) which can be
Chapter 17. Air Traffic Services (ATS)
Appendix 3 17-APP 3-3
a threat to ATC. Pilots may forget to report passing a waypoint or altitude, or they may acknowledge doing
something that they subsequently will not do. In the TEM framework, an error by a pilot is a threat to ATC.
2.12 Aircraft performance. Controllers are familiar with the normal performance of most aircraft types
or categories they handle, but sometimes the performance may be different to that expected. A Boeing 747
with a destination close to the point of departure will climb much faster and steeper than one with a
destination that is far away. It will also require a shorter take-off roll. Some new-generation turboprop aircraft
will outperform medium jet aircraft in the initial stages after take-off. Derivative aircraft types may have a
significantly higher final approach speed than earlier series.
2.13 Radiotelephony (R/T) communication. Readback errors by pilots are threats to ATC. (Similarly,
a hearback error by a controller is a threat to the pilot.) R/T procedures are designed with the aim to detect
and correct such errors (thus avoiding threats), but in actual practice, this does not always work to
perfection. Communications between pilots and controllers may be compromised by language issues. The
use of two languages on the same frequency, or two or more ATC units sharing the same frequency are
also considered threats under this category.
2.14 Traffic controllers are familiar with the normal traffic flows in their areas and how these are
usually handled. Additional traffic such as aerial photography flights, survey flights, calibration flights
(navaids), parachute jumping activities, road traffic monitoring flights and banner towing flights are threats to
the handling of normal traffic. The earlier a controller is aware of the additional traffic, the better the
opportunity to adequately manage the threat.
Environmental threats
2.15 Weather is perhaps the most common category of threat to all aspects of aviation, including ATC
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Safety Management Manual (SMM) 安全管理手册(137)
