5 Feb 2008

**SUMMARY**

**Safety Assessment To Support the Use of 50 NM Lateral Separation
Standards in the WATRS Plus Control Areas**

**Introduction.** The FAA Separation Standards Analysis Team (AJP-7141)
at the William J. Hughes Technical Center in Atlantic City, New Jersey,
USA developed the "Safety Assessment To Support the Use of 50 NM Lateral
Separation Standards in the WATRS Plus Control Areas". The following
provides a summary of WATRS Plus Safety Assessment document. The
document can be obtained by contacting: <RGrimes@cssiinc.com>. Phone: +1
202-863-3692; Fax +1 202-863-2398.

**Major Inputs To Collision Risk Modelling.** The risk of collision in
WATRS Plus Control Areas was analyzed by AJP-7141 using the ICAO
endorsed Collision Risk Modeling safety analytical tool. The primary
factors contributing to the collision risk estimate are listed below.
AJP-7141 safety analysts evaluated each in developing the WATRS Plus
Safety Assessment:

1\. [Aircraft adherence to track.]{.underline} This is a projection of
normal, expected navigation performance in the airspace. The WATRS Plus
aircraft population is projected to navigate within 7nm of cleared route
centerline on a 95% containment basis. This projection was developed
considering that approximately 40% of aircraft will meet the minimum
navigation requirement of RNP 10 (10nm/95% containment) while
approximately 60% will navigate to within 4nm of route centerline on a
95% containment basis, primarily due to GPS equipage.

2\. [Frequency of Gross Navigation Errors (GNE).]{.underline} The
analysts examined the U.S. FAA Air Traffic Quality Assurance and
Aviation Safety Reporting System databases for the periods March 2002 to
September 2007 to determine the number of navigation errors equal to or
greater than 25nm (one-half the 50nm lateral separation minimum). Safety
data collected between June 2006 and September 2007 was used to estimate
the frequency of GNE occurrence in the current operational system. GNEs
of 25nm or greater were estimated to occur at a rate of one event every
48,700 flights.

3\. [Lateral occupancy.]{.underline} Lateral occupancy is a measure of
relative packing of aircraft within the airspace. It was projected
through simulation modeling using an upper 95 percent confidence limit
based on a period 5 years in the future with traffic demand increased
according to FAA Aviation System Forecasts. Vertical and horizontal
occupancy estimates were calculated by counting the number of aircraft
that passed each other in opposite and same directions within the new
WATRS Plus route structure with 50nm lateral separation applied.

**Estimation of Lateral Risk and Comparison to the TLS.**

1\. Under the assumptions outlined in the Safety Assessment and using
the collision risk model parameter estimates contained therein, the
estimate of lateral collision risk is 0.32 x 10^-9^ fatal accidents per
flight hour due to the loss of planned lateral separation for the WATRS
Plus airspace when a lateral navigational performance of 7nm/95%
containment is used. Given the current range of operations seen in the
WATS Plus airspace, this estimate of lateral collision risk yields an
expectation of between 3,500 - 4,400 calendar years between accidents.

2\. If it is assumed that aircraft population performance just meets RNP
10 performance

(10nm/95% containment), the resulting estimate of lateral collision risk
is 1.11 x 10^-9^ fatal

accidents per flight hour due to the loss of planned lateral separation.
Likewise, under the same

set of assumptions, this estimate of lateral collision risk yields an
expectation of between 1,000 --

1,250 calendar years between accidents.

**Conclusion.** The Safety Assessment concluded that both estimates of
lateral collision risk discussed above satisfied the ICAO endorsed
Target Level of Safety of 5 x 10^-9^ fatal accidents per flight hour due
to the loss of planned lateral separation.