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possibility that a pilot may ignore a correct warning, believing it
to be false. (High false alarm rates may also result in cases of
“creative disablement.”) Thus, to be effective in enhancing safety,
these systems must have very high integrity: The display of traffic
must be very accurate, and the rate of detection of impending
conflicts and collisions must be very high, while the false alarm
rate must be relatively low.
For a given technology, increasing detection rates generally
covary with increasing false alarm rates. Even for sensitive
warning systems, with high detection rates and low false alarm
rates, the likelihood that a warning signal is indicative of a true
collision/conflict could be quite low as a result of where the
system’s alerting threshold is set (lenient or strict) and the
frequency of occurrence of actual collision/conflict events (the a
priori probability of a conflict or conflict base rate). As a result,
only a small proportion of warnings may represent true
collision/conflict events. These and other factors, such as the fact
that these systems cannot anticipate unexpected maneuvers by
other aircraft can also conspire to reduce the effectiveness of
collision/conflict warning systems.]
On-board detection and warning systems also affect the nature
of air traffic control and controller-pilot relationships, as
described below.
1) TCAS
SEPARATION SAFETY MODELING
A-18
[TCAS provides flight crews with a visual display of traffic as
well as aural warnings and commands. This has been one
approach to mitigating the consequences of separation errors
and avoiding midair collisions, particularly in congested areas
(e.g., terminal areas). Despite its important alerting function,
TCAS also changes the nature of the interactions between
controllers and pilots, since the controller is not privy to the
cockpit information provided by the system. In terms of
automation, TCAS illustrates how an automated system can
isolate critical items of situation awareness information that
are required by both controllers and pilots. Accordingly, more
effective use of such systems may require the need for
increased pilot-controller interaction to share this
information.
Although TCAS is proving to be a valuable tool for
collision avoidance, it can add uncertainty to air traffic control
and reduce the level of team-work achieved. TCAS also
provides a good example that the false alarm question is not
trivial. TCAS suffered considerably in its early development
from problems of excessive false alerts that resulted in mistrust
and lack of pilot usage. Although its detection and warning
algorithms have been refined, TCAS integrity is still limited by
the limitations in position information available from its
antennas and vertical position and rate information derived
from the 100-foot-quantized altitude reports of Mode C.].
2) CDTI
[An important technology to appear on the flight decks of the
future is the Cockpit Display of Traffic Information (CDTI).
The form this display finally takes will depend on pilot
responses to the various kinds of displays and the application
of dynamic display design principles. Without expert attention
to human factors, CDTI technology could induce new pilot
errors and thus diminish rather that improve safety. For
example, pilots have the most difficulty in assessing the
likelihood that two aircraft are on a collision course when the
two flight paths are curved and when the other aircraft is
approaching from a large relative bearing. Further, pilots
show a strong tendency to execute maneuvers in the horizontal
rather than the vertical plane. This tendency could be designinduced
if the display emphasizes the horizontal layout of
traffic rather than the vertical layout. Other factors to be
considered include the division of responsibility for traffic
separation and collision avoidance between ground-based
controllers and flight crews.
APPENDIX A
FACTORS POTENTIALLY AFFECTING SEPARATION SAFETY
A-19
Overall, the evidence tends to favor CDTI as an
improvement to flight safety. It is obviously important that
pilot performance with systems such as CDTI be studied
carefully before they become standard operational equipment
on flight decks. The impact on the pilot-controllers
relationship and on separation safety must also be evaluated.]
i. Certification standards
j. Training
[There is the general realization that proper teamwork, reflected in
verbal communications among the crew, is a critical component in
maintaining safety in flight.. As in other technologies, a high
　
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