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

This is a very high rate, which would not be approached in actual
operations. Since the actual blunder rate seems to be much
smaller, the risk of the operation may be assumed to be much
smaller than the target, acceptable risk.
If it is assumed that 1/10 is the ratio of Worst Case Blunders to
30° blunders the ratio would become
130" Blunder
10,2 10 triple approaches
This rate may be the same order of magnitude as the actual, but
unknown, blunder rate. However, since this rate represents an
acceptable rate which will result in an accident rate no larger
than the target rate, the risk of the operation is still deemed
acceptable.
7.0 SUMMARY AND CONCLUSIONS
Historical accident data indicate that the current probability or
risk of a fatal accident during an ILS approach in instrument
meteorological conditions is about 4 x Since ten primary
causes of a fatal accident during the approach may identified,
including a midair collision with an aircraft on an adjacent
approach, the target or acceptable risk rate may be determined to
be 4 x or 1 fatal accident per 25 million instrument
approaches.
The window of risk may be determined analytically for two
aircraft on adjacent approaches from knowledge of their relative
positions, their speeds, and the blunder angle. Using this
information, the number of at-risk blunders from simulated flight
track data was determined to be 186.
Analysis of the simulated flight track data also indicated that
two TCV's occurred. Confidence intervals for the Bernoulli
probability parameter were used to conservatively estimate the
probability of a TCV given an at-risk blunder to be 0.049.
Conversations with controllers were used to estimate the ratio of
worst case 30' blunders to 30° blunders to be between 1/100 and
1/10. Using the ratio, 1/100, the acceptable blunder rate was
determined to be 1 30' blunder per 1021 triple approaches. Using
the ratio, 1/10, the acceptable blunder rate was determined to 1
30° blunder per 10,210 approaches. In either case, the operation
would meet the target risk rate.
aU.S. GOVERNMENT PRINTING OFFICE: 1994-504-078-00067
L-14