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6.7 ICAO REQUIRED NAVIGATION PERFORMANCE (RNP)
Required Navigation Performance (RNP) is a method used to specify the navigation
performance required by aircraft for a particular operation. The method was originally
developed by ICAO and applied to oceanic, en route, and terminal area operations [R6.9].
It also is being developed for application to approach and landing operations [R6.10].
Purpose and application
As applied in the lateral separations between established routes, RNP specifies the 95%
lateral navigation performance accuracy. This value is then used to determine the spacing
(separation) between parallel routes, where the larger the 95% value, the greater the
required spacing. Also factored into the spacing is the rate at which large navigation
errors occur.
As applied to approach and landing operations, RNP specifies both a 95% performance
accuracy and an outer boundary, which specifies aircraft performance with a 10-7
probability. For approach, these limits are applied to both the lateral and vertical
dimensions. The outer boundary is related to the obstacle clearance surfaces. Unlike en
route, there has not been a proposal to use the approach RNP to establish separation
requirements.
EXISTING MODELS AND MODELING TOOLS
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6.8 THE REDUCED AIRCRAFT SEPARATION RISK ASSESSMENT
MODEL (RASRAM)
Overview
The Reduced Aircraft Separation Risk Assessment Model (RASRAM) is a collision risk
analysis tool that describes scenario outcomes in a fault tree framework that includes
dynamic, time-budget, and probabilistic computations in addition to the static risk factors
normally included in fault trees [R6.12]. The objective of RASRAM is to link aircraft
separation standards and intervention procedures to quantitative safety risk. The model
provides an analysis framework suitable for comparing the risks of current procedures
with risks associated with applications of new technology and reductions in aircraft
separation standards. Fault tree presentations make it easy to understand how accident
and incident risks are distributed among the contributing elements.
RASRAM development is being performed for NASA, in coordination with the FAA, as
an integral part of NASA's Terminal Area Productivity (TAP) program. Initial safety risk
assessments have been performed for flight scenarios related to final approach, landing,
and roll-out for parallel and single runway operations. These scenarios are the lateral
blunder scenario, runway occupancy/incursion scenario, and the wake vortex scenario.
The final result for each scenario is a consolidated risk of incident and accident from all
sources directly applicable to the scenario.
Good agreement has been obtained among several of the predicted risks and actual
incident and accident statistics. This provides a level of confidence that the baseline safety
results can be used to provide a relative comparison of the safety of proposed new
procedures with the safety of current operations and technologies. The safety associated
with independent parallel approaches using the Precision Runway Monitor (PRM) has
been quantified previously. Using RASRAM, the safety of further separation reductions
for parallel approaches based on the application of new technologies can be analyzed. The
RASRAM computational methodologies that resolve conflict geometries in the lateral
blunder scenario can be generalized to scenarios anywhere in the terminal area or en route
airspace.
RASRAM produces a variety of risk measures. In the lateral blunder scenario, there is a
fairly well established baseline safety level for the risk of a Near Midair Collision
(NMAC). For possible en route scenarios there is, as yet, no consensus on target
measures of safety. The approach used in RASRAM quantifies the connection between a
number of risk measures. In this way, RASRAM can remain useful as the aviation safety
community explores different alternatives for target measures and levels of safety.
SEPARATION SAFETY MODELING
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Computing Miss Distances for a Blunder into a Stream of Traffic
The RASRAM static risk factors are computed and presented in Excel spreadsheets. A
commercially available mathematical analysis program called Mathcad is used to compute
the probabilities of time-dependent events. A prototype graphical user interface (GUI) is
available to allow a user to interact with the model and vary the parameters of the
scenarios. All of the software runs on a laptop personal computer.
The physical quantity of interest for the lateral blunder scenario is miss distance, which is
the distance between two aircraft at their point of closest approach. The Mathcad
equations model a fixed flight path for a blundering aircraft that violates a separation
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a concept paper for separation safety modeling(47)
