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

3.4 ESTIMATING COLLISION RISK
The final approach, whether precision, non-precision, Or VFR is
not just a single operation, but is composed of several
operations requiring certain systems for their successful
completion. In order to determine the risk of collision with an
aircraft on the adjacent glidepath, it is necessary to determine
each operation which must be performed and each system which must
function to successfully complete the approach without an
accident. The following is a list of events which could produce
an accident during an instrument approach:
1. Collision with an obstacle during
the instrument portion of the approach.
2. Collision with an obstacle during the
visual portion of the approach.
3. Pilot failure due to mental or
physiological malfunction.
4. Failure of aircraft systems except engine,
structures, electronic.
5. Aircraft structural failure.
6. Engine failure.
7. Failure of approach guidance electronics,
ground and air.
8. Natural environmental phenomenon.
9. Midair collision.
10. Midair collision with an aircraft on an
adjacent approach.
Event 9 represents a collision with another aircraft which is not
established on an adjacent, parallel approach, while event 10
represents a collision with an aircraft which is, or has been (in
case of a blunder),established on a parallel approach.
Using these events as the ones which could produce an accident if
they occur, then the probability of an accident on the ILS
approach, P (A) , would be:
P(A) = 1 - P(A').
The probability that an accident will not occur, P(A1) is the
probability that event 1 does not occur, P(l'), and event 2 does
not occur, P(2'), ... , and event 10 does not occur, P(10').
Assuming that the events are independent, the probability that an
accident does not occur is:
L-6