Published Report (DOT/FAA/CT-94-36)

lunder rate may be computed. The speeds used in the simulation
ranged from 120 kts to 227 kts. The 120 kts was due to a
blundering aircraft and the 227 kts was due to an evading
aircraft. If is assumed that either aircraft could be traveling
at any speed between these two numbers, then the ratio of speeds
ranges from 120/227 = .53 to 227/120 = 1.89. Using these two
ratios as the minimum and maximum speed ratios and using a
maximum blunder angle of 30°, the maximum window length is 2279
feet. This occurs at the 1.89 ratio. Therefore, the probability
of correct alignment, assuming 3 miles longitudinal separation,
is given by
2279
1
= 0.125 = -
3 x 6076 8
Analysis of the data using the equations derived for the window
of risk, indicated that the number of at-risk aircraft was 186
with two resultant TCVs. Using a .99 confidence interval to
compute the upper bound for the binomial probability, the upper
bound would be 0.049. This would lead to the following ratio of
TCV's to at-risk aircraft:
Another factor needed is the ratio of Worst Case Blunders to 30'
blunders. This really means, the ratio of 30' blunders in which
the pilot of the blundering aircraft is unable to respond to
instructions by the controller. In previous studies, the ratio
of worst case blunders to 30' blunders, based on conversations
with controllers and pilots, has been estimated to be 1/100.
Recent conversations with controllers indicate that the 1/100
ratio may be too large and that the actual rate may be
significantly lower. A more conservative approach would be to
increase the 1/100 ratio, already considered conservative by the
responding controllers, to 1/10. Because of the uncertainty of
the ratio, both conservative estimates will be considered. Since
the target risk is given in accidents per approach, factors must
be introduced to correct for the number of approaches taking
place during a triple approach and for the fact that one
collision is equivalent to two accidents. The equation will be
displayed with appropriate units for the convenience of the
reader.
Using 1/100 as the estimate, the number of acceptable blunders to
achieve the target probability of 4 x 10 or 1 fatal accident in
25 million approaches, becomes
1 ACC 8WCB 102algnWCB 3app lTCV 10030"Bl -
130' Blunder
X- x-x
25 mill app 1 algn WCB 5 TCV 1 triple 2 ACC 1 WCB 1021 triple approaches
L-13