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APPROACHES TO COLLISION RISK ANALYSIS
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Flight Simulators and Pilots
Flight simulators with airline pilots at the controls are used for evading aircraft during
simulated blunders. Flight simulators assume the configuration of aircraft flying the
localizer course and replace TGF aircraft that are scheduled to enter the simulation.
Approach type (i.e., coupled autopilot, flight director, or raw data) are scripted for the
flight simulators based upon surveys of current airline procedures. Flight simulators and
airline pilots are essential to obtain valid data of pilot and aircraft performance during a
blunder maneuver.
Evaluation of Simulation Results
Before the simulation begins, it is essential that all test parameters that will be used in the
evaluation of the simulation have been determined and that criteria have been established
to determine the acceptability of the operation being tested. In the case of the MPAP
simulations, the primary concern is to estimate the risk of collision during parallel
approach operations. It was decided that a center-of-gravity to center-of-gravity distance
between two aircraft of less than 500 feet would be unacceptable and called a Test
Criterion Violation (TCV). Other factors used in the evaluation include communications
workload, the number of nuisance breakouts (NBO), and an operational evaluation by test
observers.
The most important element in the evaluation process is the TCV. Although a TCV does
not necessarily result in a collision, for simplicity, a TCV is regarded as a collision. The
analysis of the data focuses on the TCV rate. Before a TCV can occur, even without
controller intervention, the two aircraft must be located at appropriate positions relative to
the runway threshold, which is based on the aircraft speeds and the angle the blunder
forms with the extended runway centerline. An interval can be determined mathematically
so that if the evading aircraft is within this interval when the blunder begins and no evasion
action is taken, a TCV will occur. This interval is called the window of risk. Because the
window of risk is short and the number of simulated blunders is small, a simulation could
easily result in zero TCVs if the simulated aircraft are situated at random on the approach
path. This could lead to an erroneous acceptance of an unsafe operation. Therefore,
simulated aircraft are purposely aligned so that the simulated blunder will result in a TCV
without timely controller and pilot action. When the evading aircraft is situated in the
window of risk, it is said to be at-risk.
The probability of a collision can be estimated from the rate of TCVs per at-risk blunder.
Because of the small number of at-risk blunders generated during the real-time simulation,
the estimate of the TCV rate is subject to random fluctuations. An upper confidence limit
is used to find a “worst case” or conservative value for the TCV rate. If this value is less
than the predetermined acceptable TCV rate, the operation can be accepted as safe. If the
raw or observed TCV rate is larger than the acceptable TCV rate, the operation can be
rejected. However, if the acceptable TCV rate is between the upper confidence limit and
the observed TCV rate, it is unclear whether the operation should be accepted or rejected.
SEPARATION SAFETY MODELING
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The fast-time simulation is employed to shorten the confidence interval of the TCV rate
resulting in a more accurate estimate of the actual TCV rate.
5.4.2 Fast-time (Monte Carlo) Simulation Models
The purpose of a fast-time simulation is to provide a more accurate estimate of the
acceptance parameter than the estimate derived from the real-time simulation. In the realtime
simulation, because of budgetary and time considerations, only a small number of
simulated operations can be performed. This means that combinations of controller
response times, pilot response times, and other human factor related parameters have not
been fully explored. Other factors that are not fully explored are related to weather
phenomena such as wind speed and direction and aircraft parameters related to the size
and speed of the aircraft performing the operation. Because of the large number of
combinations left unexplored in the real-time simulation, the actual value of the acceptance
parameter may be different than that observed in the real-time simulation.
A fast-time simulation must be carefully constructed in order to instill confidence in the
estimate of the acceptance parameter. Because of the unique characteristics of aircraft
trajectories, the fast-time simulation model should employ flight dynamics representing
aircraft currently used in commercial operations. The model should employ advanced
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a concept paper for separation safety modeling(31)
