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

number of causes of accidents during final approach, such as
structural failure, engine failure, or midair collision. An
initial estimate is that there are nine possible causes of
accidents on final approach. A tenth possible accident cause, a
collision with aircraft on an adjacent approach, is created with
the implementation of parallel approaches.
For simplicity of model development, it is assumed that the risks
of the 10 potential accident causes are approximately equal. Thus
the contribution of any one of the accident causes would be
approximately one-tenth of the total accident rate. Therefore,
the target safety level for midair collisions on simultaneous
parallel approaches is 4 x 10-8, or:
1 ACC
25 mill APP
To begin the evaluation, CPA analyses indicated that controllers
had the greatest difficulty in maintaining a 500 ft spacing
between aircraft in the event of a 30 degree blunder. Twenty
(20) and 10 degree blunders were all resolvable. The simulation
also demonstrated that only blunders which simulated a lack of
response by the blundering aircraft were sometimes unresolvable.
The pilot's inability to respond may be due to a conflict with
another radio transmission, weather conditions, or a malfunction
of the aircraft. Other studies (Precision Runway Monitor Program
Office, 1991) have estimated that only one percent of the
aircraft blundering 30 degrees off course would be unable to
respond to controller commands.
It is assumed that pilots will be able to resolve conflicts
during visual flight rules (VF'R) conditions, therefore only IMC
conditions are used in this analysis. Based upon these findings
and assumptions, a worst case blunder (WCB) is defined as a 30
degree blunder, under IMC conditions, in which the blundering
aircraft's pilot is unable to respond to the controller's
directions and enters the NTZ.
A factor needed in the risk assessment is the probability of a 30
degree blunder in which the pilot of the blundering aircraft was
unable to comply with ATC instructions (i.e., a WCB). As
mentioned earlier, previous research estimated that the
probability of a no-response blunder was 1/100. Therefore, the
ratio of WCB's to 30 degree blunders is:
1 WCB
100 30-Degree Blunder
The longitudinal alignment of the aircraft, relative to the
threshold, on adjacent ILS approaches, was found to be an
important factor in conflict resolution. The probability of a
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