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diagram with lines linking factors that might potentially interact. The difficulty with this is
that if all factors are included, the diagram becomes illegible. Another form of constructing
large-scale models, where hundreds of variables play a role, is the use of causal diagrams.
Causal diagrams are used in the Systems Dynamics methodology developed by J. Forrester
SEPARATION SAFETY MODELING
7-2
in the early sixties. This technique is particularly well suited for models where all the
variables in the model have time dependencies.
One final remark with respect to the independence of factors. Often, it appears that two
or three factors are independent of each other¾that the occurrence of one has no effect
on the (probability of) occurrence of others. There may, however, be another, supposedly
exogenous factor, that can influence all of the others. Consider the case of an airline with
poor management, one which spends as little as possible on training and maintenance.
This airline may hire a maintenance company with similar predilections, may postpone
maintenance and repair, and may not adequately train its pilots or ground crews. In such a
case, a ground crew error, for example, might suggest greater than usual probabilities of
maintenance problems and pilot errors: “Corporate Culture” counts!
The following pages present our current outline of factors that might influence the risk of
a midair collision. The outline is a “work in progress”¾ it undoubtedly will be revised
over time. This outline is also reproduced, with annotations, in Appendix A. The
annotations provide invaluable insights into the reasons some factors are important and
also provide additional, useful information.
7.2 FACTORS THAT MAY INFLUENCE SEPARATION SAFETY
Interactions among these factors must also be considered, as must be the possibility that
"exogenous" factors might impact more than one of the primary safety-related factors.
A. Relative aircraft positions and velocities (encounter geometry)
1. "Blind-flying" risk [factors that affect risk¾no intervention]
a. Horizontal and vertical positions and closing angles
b. Aircraft velocities and accelerations
c. Climb/descent rates and accelerations
d. Vertical path separation at crossing point
2. Pilot intervention - factors that affect timely pilot detection and correction
a. Closure rate
b. Relative bearings and aspect angles in relation to cockpit field of view
restrictions, the horizon.
c. Rate of change of the above angles (zero for linear collision courses)
d. Aircraft attitudes and bank angles
e. Meteorological conditions and background conditions, including
location of the sun (affecting ability to perceive other aircraft and their
relative distance, velocity, and trajectory)
f. Natural lighting conditions (e.g., day, night, dawn, dusk)
g. Threat aircraft size, skin color, and lighting
h. Condensation trails
i. Empty visual field
j. Night accommodation
k. Party line
FACTORS POTENTIALLY AFFECTING SEPARATION SAFETY
7-3
l. Reliance on ground-based surveillance and procedures
m. Cockpit workload, staffing, automation, and procedures
n. Flight crew training, skill, teamwork
o. TCAS/ACAS responsiveness (affected by aircraft bank angle, ...)
It should be noted that, in accordance with the guidance given in
ICAO Annex 11, the carriage of ACAS by aircraft within a region
should not be used to justify a reduced separation minimum.
However, the presence of such systems may be relevant when
contemplating the application of reduced separations as changes to
the ACAS systems may be required in order to avoid an
unacceptable rate of false alerts.
3. ATC intervention - factors that affect the probability of timely and effective
ATC intervention
a. Air traffic service provided
b. Climb/descent rate and acceleration (affects ATC computer projections)
c. Horizontal velocity and acceleration
d. Turn rate and turn acceleration (change in turn rate)
e. Airspace complexity
f. Traffic complexity and density
g. Proximity to an airspace boundary (e.g., SUA)
h ATC coordination (e.g., involving an aircraft in hand-off or point-out
status.)
i. Air traffic management tools for reducing controller workload and
improving controller intervention capability
1) Automated controller planning tools including trajectory projection,
conflict probe, and conflict resolution
2) Automated out-of-conformance alerts (3 D) (which alert ATC to any
deviation of an aircraft from its nominal path).
3) Controller display quality: picture, information, and presentation of
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a concept paper for separation safety modeling(51)
