Federal Aviation Administration

Research, Engineering and

Development Advisory Committee

Separation Standards Working Group

Final Report

September 20, 2006
i

Executive Summary

The Federal Aviation Administration (FAA) and Joint Program Development Office (JPDO) estimate that by 2025,

the number of operations that the Air Traffic Management (ATM) system must accommodate will grow between

100 and 200 percent, depending on the mix of aircraft in use. The JPDO determined today’s system must be

transformed to accommodate the increased air traffic. This transformation will require that modern technologies be

fully leveraged to enable higher aircraft densities while maintaining or improving current levels of safety. These

technologies give us every reason to believe that aircraft to aircraft separations can be safely reduced to

accommodate increased demand.

The National Airspace System (NAS) Operations Subcommittee of the FAA’s Research, Engineering and

Development Advisory Committee (REDAC) established the Separation Standards Working Group (SSWG) to

examine how to reduce separation standards, without jeopardizing safety, and recommend Research and

Development (R&D) that would facilitate the reductions. The SSWG first examined the basis for current separation

standards, and reviewed past and ongoing studies of separation requirements. The Working Group then considered

improved methodologies for establishing separation standards, as well as changes that new technology may bring.

The process resulted in ten findings, with accompanying recommendations.

The current aviation system, to the great credit of the pioneers and the practical application creators, is both practical

and safe. Refinements to the standards have preserved safety even as air traffic levels grew.

However, the development of many existing separation standards was largely empirical, based on the experience and

judgment of operations experts. Analytical and probabilistic studies have been occasionally used to inform and

quantify those judgments. Recent work shows that modern analytical and statistical studies, based on extensive field

data, are essential for developing new standards, but must still be complemented by operational trials and human

factor considerations.

Judgment and experience will continue to be needed to address rare, unpredictable events. However, the practice of

compensating for “diabolical” threats or similar anomalies by setting arbitrary performance standards can impede

legitimate initiatives. For example, there was only anecdotal evidence about blunders for Precision Runway 

Monitor (PRM) and over ocean operations, and thus no way to validate or challenge the government’s performance

standards, a frustrating outcome for system developers and aircraft operators.

In summary, it is clear that a more data driven, objective methodology is needed to address the challenge of

separation reductions if the forecast traffic levels are to be met.

i
ii

Findings1 and Recommendations2

Finding 1. The current system, based on the separation standards that have evolved over 

the last 50 years, is safe, but still unable to meet projected demand. The separation 

standards (and the approach to establishing separation standards) now need to be 

reconsidered in order to meet the demand for increased capacity.

Finding 2. Most current separation standards have been developed empirically based on 

judgment, extrapolation of past experience, and limited analysis. In recent years, a more 

analytical approach has been applied. The current standards are not based on a consistent 

philosophy, varying from one part of the airspace to another; using varied analytical 

approaches and assumptions about behavior.

Finding 3. Some separation standards are strongly influenced by the possibility of gross 

deviations, or blunders. However, little is known about such blunders: their frequency of 

occurrence, their magnitude, under what circumstances they are most likely to occur. 

Existing information about blunders is primarily anecdotal.

Finding 4. Mathematical analyses require substantial data to accurately characterize 

reality. Historically, sufficient data has not been available. The result of insufficient data 

is overly conservative separation standards.

Finding 5. New separation standards may be developed by comparison with a reference 

system or by evaluating system risk against a threshold level. Comparing to a reference 

system is an appropriate method to support incremental changes to the current system. To 

evaluate the major changes in separation standards that will be required for the Next 

Generation Air Traffic System (NGATS), the evaluation against a threshold methodology 

may be necessary.

A disciplined process for identifying and analyzing risk when developing or revising 

separation standards is of vital importance. Analytical and probabilistic studies are 

essential in the determination of safe standards, but, by themselves, are not enough. They 

should be used together with judgment. Their role is to inform and quantify judgment.

1

Findings 1 though 5 all deal with the historic establishment of separation standards, and the limitations of some of 

the methodology used. They are treated together in this section in order to establish a consistent set of 

recommendations.

2 Recommendations are presented in two time frames, Immediate and Longer Term. The Immediate

recommendations are those that should or can be undertaken immediately. Some of them could increase efficiency,

if applied, in the current National Airspace Space system. Longer Term recommendations are must be preceded by 

other work or do not apply to today’s system.

ii
3

Guarding against unrealistic or diabolical phenomena should not be a basis for the 

establishment of separation standards.

Recommendation:

Establish an R&D program that will lead to consistent and safe reduction of separation 

standards and that will support NGATS. The process outlined below for setting separation 

standards should be adopted. This R&D program should include, but not be limited to:

• Immediate

o Establish a research program to develop an understanding of the nature and frequency of blunders.

• Performance Data Analysis & Reporting System (PDARS) appears to be a possible source for

needed data.

• Develop new systems, if needed, for automated reporting of such anomalies.

o Establish data needs for establishment of separation standards early in NGATS development so

opportunities, such as demonstrations, can be used collect data.

o If conservative separation standards are put in place, such as RNP Parallel Approach Transition

(RPAT), establish a data collection process early in the implementation so operational data collected to

reduce separations in the future.

• Longer Term

o Conduct research to develop consistent approaches for the development of separation standards with

all assumptions stated concisely.

o Conduct research to improve the methodology for evaluating separation standards against an absolute

threshold (target level of safety). In particular, there needs to be a consistent, credible way to take into

account the response of humans to rare events.

Finding 6. The next generation air transportation system will have:

- new roles and responsibilities for pilots and controllers and the automation that 

supports them,

- increased shared situational awareness on board the aircraft that will provide more 

timely and accurate information including intent of nearby vehicles,

- the potential, through good system design, for fewer unexpected deviations, and

- new backup systems to deal with system/subsystem failures, possibly accepting 

lesser performance capability than the system being backed up.

As surveillance, navigation, and communication performance increases, including 

communication of intent, separation standards will be driven more by the need to 

accommodate system failures than by variations in nominal system performance.

Recommendations

• Longer Term

o Establish a research program to develop an understanding of the roles of the human and automation in

dealing with failures and the implication of those roles on separation standards.

o Managing failure gracefully is perhaps the most difficult design aspect of the NGATS. Specific and intense

research into the human and automated alternatives will be required.

iii
4

Finding 7. New technologies (e.g. GPS, ADS-B, CDTI, Datalink) offer the potential for 

reducing required separations. In particular, GPS-based RNP, together with the concept of 

containment, provides much more precise control and knowledge of an aircraft’s intended 

trajectory, and ADS-B permits the pilot of other aircraft, as well as the air traffic controller, to 

monitor the flight path of a proximate aircraft and rapidly sense deviations from its intended 

path.

Recommendations

• Immediate

o As more and more aircraft use RNP-based navigation, monitor their performance, and gather and analyze

data to develop a statistical understanding of the performance of RNP-based systems in various flight

regimes.

o Re-examine the design of parallel and converging approaches and departures based on an appropriate

probability distributions (may not be Gaussian) or on data gathered using RNP-based navigation.

o The Performance-Based Advisory Rulemaking Committee (PARC) should redefine the definition of

“established on approach” to include LNAV and VNAV. The requirement to be aligned with the

runway centerline should be studied for possible elimination.

o Research into potential reduction of Arrival/Departure and Departure/Departure separations due to

RNP guided missed approaches and departures should be pursued.

• Longer term

o Develop (recommendations for) new separations standards based on the improved navigation,

surveillance, communication, control, and automation technologies, which will be part of NGATS.

Utilize lessons learned during the analysis of other standards.

o When the nature and frequency of blunders off an ILS course are better understood using data

ILS/RNP parallel runway separation should be reevaluated. RNP/RNP parallel approach separation

should be established.

o The No-Transgression Zone (NTZ) role for ILS operations should be re-defined based on real blunder

information. Then, if still required, appropriate dimensions and shapes should be established.

o The role of the NTZ in RNP/RNP separations should be established. The NTZ may not be needed.

Finding 8. In designing NGATS, an air-based independent (from ATM system) backup 

collision avoidance system (similar to TCAS or perhaps a modified TCAS) will be required.

Back-up safety systems in the aircraft and air traffic control facilities have been set to 

prevent collision while minimizing false alerts when aircraft are operating at today’s 

separation standards. As separation standards are reduced, procedures and alerting logic 

must be reexamined to optimize the balance between collision avoidance and false alerts.

Recommendations

• Longer Term

o Research is required for the future independent airborne collision avoidance system in the context

of the ATM system construct and the associated separation standards.

o Research and analysis of alerting systems, such as Traffic Alert and Collision Avoidance System

(TCAS), Terrain Awareness and Warning Systems (TAWS), Minimum Safe Altitude Warning

(MSAW), and Conflict Alert (CA) function, should be initiated to minimize false alerts as

separation standards are reduced and revised.

iv
5

Finding 9. Evaluating the controllers’ performance by distribution (stochastic control) rather 

than a hard limit may be able to increase capacity and effective throughput without 

compromising safety.

Recommendation

Immediate

Research into the practicality of stochastic control in terminal operations (specifically landing spacing) should be

initiated. Research should pursue the question of practicality and unintended consequences. This is an important

area for research because it offers the prospect of some near term improvement in landing rates, and because

stochastic control is more appropriate than deterministic control in automated systems such as NGATS.

Finding 10. In considering the possibilities for reducing separations standards, wake 

turbulence becomes the driving consideration. For NGATS, wake turbulence could become 

the primary limiter of capacity.

Recommendations

• Immediate

o Full support of existing research and implement program should continue.

o Commission a team to conduct in-depth annual technical and programmatic reviews of the wake research

and implementation program. The reviews should include the objectives, technical approach, schedule, and

funding. The team should be composed of external experts knowledgeable in the areas of wake vortices in

normal operating configurations, advanced Light Detection and Ranging (LIDAR) and other sensors that

may be useable in detecting the strength of a wake vortex, aircraft behavior in the presence of wakes, and

how this information can be used in the flight deck and air traffic facilities. This team should be structured

along the lines of the Department of Defense Science Board and report to ATO leadership.

• Longer Term

o Investigate advanced instrumentation such as LIDAR or other sensing methods to obtain direct

measurements of vortex strength.

o Investigate the feasibility and practicality of wake vortex sensing/tracking to provide the flight crew an

indication of encroaching wake vortex location, strength and upset risk.

Reducing separations standards, while preserving safety, is an intricate process. An evaluation of the overall system risk

is necessary when the proposed separation is much different from the current. Mathematical analysis, real time

simulations, field demonstrations, risk assessments, judgment, and a structured introduction should all be utilized.

v
1

Table of Contents

Executive Summary.............................................................................................................................................................i

Findings and Recommendations....................................................................................................................................ii

Table of Contents................................................................................................................................................................1

Introduction.....................................................................................................................................................................2

Section 2..............................................................................................................................................................................4

Need and Opportunity for Separation Standards Change .............................................................................................4

Section 3..............................................................................................................................................................................6

History of Separation Standards.....................................................................................................................................6

3.1. Target Level of Safety..............................................................................................................................6

3.2. Enroute Separations (Containment).........................................................................................................6

3.3. Oceanic Separation...................................................................................................................................7

3.4. Radar Separation Standards .....................................................................................................................7

3.5. Obstacle Clearances .................................................................................................................................8

3.6. Successive Arrival Separation .................................................................................................................8

3.7. Parallel Approach Separation Standards .................................................................................................8

3.8. Lessons Learned from Historical Record.................................................................................................9

Section 4............................................................................................................................................................................12

Current State of the Art to Establish Separation Standards........................................................................................12

4.1. ICAO Guidance on Separation Standard Analysis ...............................................................................12

4.2. Examples of Successful Separation Reductions....................................................................................13

4.3. Introduction of Required Navigation Performance (RNP) operations.................................................13

4.4. FAA Safety Risk Management Process.................................................................................................13

Section 5............................................................................................................................................................................15

Discussion.....................................................................................................................................................................15

5.1. Influence of New Technologies on Separation Standards ....................................................................15

5.2. Potential Impact of Automation.............................................................................................................16

5.3. TCAS and other alerting system implications.......................................................................................17

5.4. Stochastic Separation Standards ............................................................................................................18

5.5. Wake Vortex...........................................................................................................................................18

Section 6............................................................................................................................................................................20

Findings and Recommendations..................................................................................................................................20

Appendix 1........................................................................................................................................................................26

SWAG Members ..........................................................................................................................................................26

Appendix 2........................................................................................................................................................................27

Terms of Reference ......................................................................................................................................................27

Appendix 3........................................................................................................................................................................28

Summary of Presentations and list of presenters.........................................................................................................28

Appendix 4........................................................................................................................................................................30

Current MITRE Research Related to Separation Standards.......................................................................................30

Current NASA Research Related to Separation Standards.........................................................................................32

Appendix 5........................................................................................................................................................................33

List of IFR Separation Standards.................................................................................................................................33

Appendix 6........................................................................................................................................................................37

PRM for Simultaneous Parallel Approaches...............................................................................................................37

Appendix 7........................................................................................................................................................................40

ILS/RNP Separation Standard .....................................................................................................................................40

1
2

Section 1

Introduction

The Federal Aviation Administration (FAA) and Joint Program Development Office (JPDO) estimate that by 2025,

the number of operations that the Air Traffic Management (ATM) system will have to accommodate will grow

between 100 and 200 percent, depending on the type of aircraft that will be used. Initial analysis by the JPDO has

shown that the current system cannot accommodate this increase in demand, leading to the conclusion that a

transformation of today’s system will be required. Technology can be deployed that increases efficiency by

separating aircraft closer to existing separation standards. However this will not be sufficient to achieve the needed

capacity increase. One of the means for achieving increased capacity, particularly at some of the nation’s busiest

airports, is through a reduction in separation standards.

In air traffic control, “separation” is defined as the spacing of aircraft to achieve their safe and orderly movement in

flight and while landing and taking off. “Separation Minima” are the minimum longitudinal, lateral and vertical

distances by which aircraft are spaced through the application of air traffic control procedures. In this study,

“separation standards” refer to the minimum distances by which aircraft are spaced in all phases of flight.

Establishing appropriate separation standards is an important element of achieving increased National Airspace

System (NAS) capacity and flexibility, especially in terminal airspace. Three principal elements of required interaircraft separation, navigation accuracy, efficient communications, and surveillance capability, have improved

markedly since the current separation standards were established. However the question still remains whether it is

possible to reduce inter-aircraft separation without an acceptable degradation of safety.

The NAS Operations Subcommittee of the FAA’s Research, Engineering and Development Advisory Committee

(REDAC) established the Separation Standards Working Group (SSWG) to examine this question and recommend

Research and Development (R&D). The SSWG looked at the basis for current separation standards, and reviewed

past and ongoing studies of separation requirements. The Working Group also examined improved methodologies

for establishing separation standards. The process resulted in ten findings, with accompanying recommendations

that include research topics. The Terms of Reference for the Working Group is shown in Appendix 2, and the

membership is in Appendix 1.

Study Process

The SSWG solicited inputs from many members of the aviation community with a stake in improving capacity by

reducing separation standards or with subject matter expertise. These included airlines, manufacturers, FAA NASA and

The MITRE Corporation.

A listing of these presentations is shown in Appendix 3. To the extent available, copies of the presentations by these

organizations are included in the “Appendices” section of the enclosed CDROM.

In addition to these inputs, SSWG members contributed substantially from their own knowledge and experience.

Structure of the Report

The report begins with a description of the need for separation standard reductions and a discussion of the

opportunity presented by new and currently available technologies.

A short history of separation standard establishment is presented followed by a summary of current methodologies

used by FAA and ICAO for establishing or revising separation standards.

The Discussion section presents a series of discussions of various topics all of which are relevant to the

establishment or modification of separations standards. These topics are also relevant to the transformation of the

NAS as envisioned by the JPDO.

Comment [MEP1]: Sarah and

Working Group members. We have not

agreed to this approach, but it gives

serious readers all the background

information that we have, and doesn’t

make our report so thick as to be

forbidding.

2
3

Findings are embedded throughout the report adjacent to the relevant discussion. The final section of the report

repeats the Findings and includes the SSWG’s Recommendations for R&D and other actions to address each

Finding.

The report contains a number of appendices pertinent to the Working Group effort.

3
4

Baseline 

Values

Section 2

Need and Opportunity for Separation Standards Change

FAA and JPDO projections indicate an average annual revenue passenger mile growth rate of 3.9% from now to

2016. The JPDO expects enplanement demand in 2025 to be between 1.5 and 2.5 times the 2004 demand. While 

the crystal ball for 2025 demand is somewhat cloudy, the JPDO estimates that, depending on the type of aircraft that

will be used, the number of operations that the Air Traffic Management (ATM) system will have to accommodate

will grow between 100 and 200 percent (see Figure 1).

~3X

Note: Not to scale

Biz shift

• 2% shift to micro jets

Flights 

1.4-3X

~2X

Passengers 

1.8-2.4X

Biz shift

• Smaller aircraft, more airports

Increase of over 10

passengers per flight

Shift in passengers per flight

(e.g., A380, reverse RJ trend,

higher load factor)

1X

2014 and later Baseline analysis 

will use OEP & FACT Capacities

2004 2004 Baseline

• Current Flight Schedule

• Current Capacities

2014 2025 20??

Figure 1 – JDPO Demand Projection Range

The current air traffic control system has a remarkable safety record and has met the growing capacity demand for 

the last 50 years. This has been achieved by careful conservatism in the operation and modification of the hardware

and procedures. There have been improvements to the system in response to safety concerns (e.g. terrain and

aircraft collision avoidance systems, improved sensors and displays, and weather radar), and in response to capacity

demands (e.g. precision runway monitor, and re-sectorization). And some separation standards have been changed

(reduced vertical separation between Flight level 290 and 400, separation reductions in oceanic airspace, and wake

vortex driven landing and takeoff spacing). But there has been no change in the fundamental operating concept of

the system since the introduction of radar about 50 years ago.

The current approach to accommodating increasing capacity requirements in the en route airspace is to subdivide

control sectors so as to maintain an acceptable number of aircraft in each controller’s purview. The price of this is

that each such subdivision increases the number of sector transitions that the aircraft must make as it flies its route.

Notionally, if the number of sectors is doubled to accommodate the capacity demand, the number of sector

transitions also doubles. When the sectors are large, this does not pose a major problem; but in the high-density

airspaces, such as the East Coast Corridor (Boston/New York/Philadelphia/Baltimore-Washington), the sectors have

become so small that the sector transition rate cannot be increased without imposing an unacceptable load on the

controllers and pilots who must service each transition.

4
5

This diminishing returns aspect of re-sectorization has reached the point where we cannot accommodate the

projected increase in demand without some fundamental change in the air traffic control system.

In the terminal area, the increasing capacity demand has been met by strictly structuring both the arrival and

departure streams, and by utilizing multiple runways. The limiting factor in the terminal area is most often wake

vortex, although runway utilization is also limited by provisions for safe resolution of blunder events. Further

increases in terminal area capacity (other than building new independent runways at new airports or by expanding

existing airports) will require new operational concepts (such as synchronized paired approaches and departures

1

),

and a solution to the wake vortex problem (such as a separation system which is responsive to real time

determination of the actual wake vortex situation).

Today, several large airports (e.g. Chicago O’Hare, LaGuardia) are capacity limited and require some form of

demand management. En-route airspace is also showing signs of capacity limitations, particularly in the

northeastern U.S. Projected demand increases are exacerbated by the growing shift to smaller aircraft and the

impending arrival of a potentially large number of Very Light Jets.

Initial analysis by the JPDO has shown that the current ATM/ATC approach cannot accommodate the projected

increase in demand and have concluded that a transformation of today’s system will be required. Technology can be

deployed that increases efficiency by reducing the “buffer” between actual separations and existing separation

standards. However this will not be sufficient to achieve the needed capacity increase. One of the means for

achieving increased capacity, particularly at some of the nation’s busiest airports, is through a reduction in 

separation standards. The challenge is to design and implement the new approach without compromising safety.

A number of modern avionics and ground and space based technologies, many of them already available today, offer

opportunities to reduce separation standards. These include:

- better ability to control the path of flight and the associated reduction in flight technical error;

- the ability for an Air Navigation Service Provider and aircraft (pilot, cockpit automation, or both) to have

more precise position knowledge

- the ability for an aircraft to know where nearby aircraft are

- better prediction of an aircraft’s future position

- pilot knowledge of the intent of nearby aircraft

- potential for coordinated separation assurance maneuvers

- better information regarding terrain and runway location

- higher bandwidth, lower latency communication between the ground and the cockpit

- the ability to detect and avoid wake vortices

In the longer term, the move towards the Next Generation Air Traffic System (NGATS) will change the control

paradigm and make new airspace structures possible. For example, 4D trajectories allow new ways to fly – not

necessarily on pre-charted airways.

Finally, an improved ability to collect and manage data leads to improved process. We have the ability to collect

data to better understand the frequency and severity of flight path deviations. This will allow a more informed

approach to the design of blunder tolerant systems.

Finding 1. The current system, based on the separation standards that have evolved over the 

last 50 years, is safe, but unable to meet projected demand. The separation standards (and the 

approach to establishing separation standards) now need to be reconsidered in order to meet 

the demand for increased capacity.

1 NASA’s “Virtual Airspace Modeling and Simulation System-Wide Concept Report”, Volume 1

5
6

Section 3

History of Separation Standards

In considering new approaches to the establishment of separation standards in aviation, it is necessary to understand the

basis of current standards and to rationally assess system changes that permit safe changes to those standards. The

following section describes the background and derivation of current separation standards, and the initial attempts to

derive separation standards in an analytical manner.

2 A list of today’s Instrument Flight Rule separation standards is

located in Appendix 5.

3.1. Target Level of Safety

A widely used approach to separation involves establishment of a "target level of safety" based on rational

numerical analysis, judgment and comparison to achieved safety records of other applications. The British Air

Registration Board (ARB), the United Kingdom’s (UK) certifying authority, first invoked this approach in the late

1950's. The issue was approval of an automatic "all-weather" landing system for passenger aircraft.

The ARB started by looking at history to establish the landing accident rate then being achieved. Based on a study

of nearly twenty airline-landing accidents (mostly propeller aircraft), they found there had been about one accident

for each million landings. They reasoned that introduction of a new capability such as automatic landing should be

designed with the motive of improving the safety record, but could, in no event, allow safety to deteriorate. They

called for a design that would have a predicted (and, to the extent possible, demonstrated) failure rate of no more

than one in ten million landings, ten times better than the rate experienced in normal operations. They imposed a

further assumption that any failure of the automatic landing system would result in a fatal accident.

Many people and organizations tested the idea of establishing a target level of safety, especially the idea of one

accident in ten million events. A study done by the International Civil Aviation Organization’s ( ICAO) Review of

the General Concept of Separation Panel in 1975, using UK mortality rates, showed that the risk of mortality in the

healthiest age groups was six in ten million person hours (yet another yardstick), while an analysis of observed fatal

accident rates from 1965-1973 yielded a figure of 10.5 fatal accidents in 10 million (10

7

) flying hours - similar to the

postulation of the U.K. Air Registration Board.

ICAO published further corroborating information in a comprehensive study by several countries. This study

looked at fatality rates in manufacturing, railway work, and public road vehicles, mortality rates in the general

populations; and a variety of air accidents from landing to midair collisions. The bottom line finding, using many

logical choices but only a few assumptions, was that an appropriate target level of safety might be between one and

six fatal accidents in 10 million aircraft flying hours, with the resulting risk appropriately shared among mechanical

failures, midair collisions, and other accident causes. Again, the target of a few chances of a fatal accident in 10

million flying hours seemed credible.

3.2. Enroute Separations (Containment)

Before radar of any kind was used for air traffic control, separation depended on dead reckoning and pilot reports.

The controller, using flight strips to “see” his targets, separated aircraft by feeding them into certain routes with time

separation, knowing that designated aircraft speeds over the route distance would keep them apart by “procedural

control”. Pilot reports by radio, when available, were used to update positions. Of necessity, separation distances

were quite large since little was known about winds aloft and the like. The earliest standards for ATC separation

between aircraft, usually for longitudinal spacing, were entirely based on time separation along a pre-established

2 Some of this material draws on History and Evaluation of Separation Standards, S.B. Poritzky, prepared for The

Boeing Company by Applied Techno-Management Systems, Inc., under Contract JE5476, July 31, 1996.

6
7

route, using best estimates of the physical and environmental vagaries. Early uses of navigation aids, starting with

light beacons and later radio beacons, still used time as the basic separation tool.

With the introduction of more sophisticated navigation aids, particularly VOR, computations of probable

displacement from desired paths were introduced. The FAA, in its air route and separation computations, based

them on a concept of “system use error.” It created airway (route) width designations, on a “95% containment

probability” basis, based on root-sum-square (RSS) combination of ground station error (at the greatest usable

distance from the facility), airborne navigation system and display error, and a pilotage factor. It assumed that the

error distributions were normal.

A “95% containment probability” means that aircraft flying along a designated route:

• may be expected to remain within the airway confines on 95% of the occasions on which observations are

made; and

• that any particular aircraft will remain within the airway confines 95% of the time that it is flying along the

airway.

In practical terms, the meaning often assumed was that the navigation capability of the system is such that when

masses of aircraft are observed flying along a route, they will be essentially normally distributed about the route

centerline, and 95% of the observations will show aircraft to be within the airway width assigned.

3.3. Oceanic Separation

Application of risk modeling was attempted in the 1970's on the North Atlantic aircraft organized track separation

system. There had been no collisions over the oceans at the 120-mile track separation (the 120 mile number being a

figure selected by The International Air Transport Association (IATA) in 1954 - it was the distance traversed by a

four-engine propeller transport aircraft flying orthogonally to its course for a half hour). The task was to try to

"prove" the safety of a smaller (90 mile) spacing, and to consider improved navigation and cockpit capabilities.

After months of discussions between the United Kingdom, the United States and the International Federation of Air

Line Pilots Associations (IFALPA), the mathematicians and the practical problem solvers agreed on a compromise

“collision risk model”. It was realized at the time that the mathematics and the meaning of the numbers were 

suspect because many of the assumptions and variables were unproven. However, an analysis that was acceptable to

reasonable and knowledgeable experts was needed.

As noted by Robert Machol, one of the participants for IFALPA, in the North Atlantic separation discussions, "the

actual mechanism that was finally agreed on for arriving at a target level of safety (for the oceanic system) was to 

take the total rate of accidents of all aircraft (not just those over the North Atlantic Region), and shoot for something

ten times as safe. Putting numbers into this turned out to be rather difficult; it was never possible to arrive at a 

single number, but we finally agreed that the target level of safety lay between 0.45 and 1.2 accidents due to

collision per 10 million flying hours." 3

3.4. Radar Separation Standards

Radar separation standards have traditionally required air traffic controllers to maintain at least three nautical miles

(NM) between aircraft that are within 40 NM of the radar and, typically, the airport. Aircraft further than 40 NM

from the radar must be separated by at least 5 NM. 5 NM separation is also required when the two aircraft are not

observed by a common radar. These requirements were based on maintaining, “green in between” the targets as 

seen on a plan view display of primary radar target video. Mode S and Monopulse Secondary Surveillance Radar

(MSSR), deployed in the 1980s, provide reliable surveillance data that is more accurate than the primary radars, by a

3 “An Aircraft Collision Model ” Robert E. Machol, Northwestern University, The Institute of Management

Sciences, Vol. 21, No. 10, June 1975.

7
8

factor of ten. However, the FAA did not initially exploit those benefits. Recently, the need to maintain three NM

separations within large terminal areas such as Southern California, or Potomac, has caused the FAA to change the

standards, allowing 3 NM separation out to 60 NM from the radar.

3.5. Obstacle Clearances

The FAA Terminal Instrument Procedures (TERPS) have, for many years, been based on a technique similar to that

used for airways, except that, as noted above, the containment criterion is based on 99.7% for aircraft-to-terrain

separation vs. the 95% for aircraft-to-aircraft separation. The “99.7% probability” means that the minimum distance

of any obstruction must be three times the standard deviation of assumed navigation system use error.

In the development of safe procedures, it is recognized that the planes and surfaces protected against are seldomcontinuous physical barriers; instead of a surface, the problem obstruction may be a hill, an apartment block, or a

tree. To remain reasonable in the application of standards, the FAA has developed a collision risk model, which is

used to make informed decisions in developing approach obstruction clearance criteria.

Frequently, there are obstructions close to airports in the U.S. that violate the obstacle clearance criteria in the final

approach area. Often such obstructions have existed for many years, and vast numbers of safe arrivals and 

departures have been made. The decisions to allow such deviations from standards have been made in part because

the obstructions are single incursions into the obstruction planes called out in TERPS, judged by operations experts 

to be benign, and frequently offset by other improvements such as obstacle lighting to provide an overall “equivalent

level of safety.”

3.6. Successive Arrival Separation

The standard radar separation in the terminal area is 3 NM. Several studies in the 1970s investigated the feasibility

of reducing the minimum separation on final approach to 2.5 NM in order to increase Instrument Flight Rules (IFR)

arrival capacity. Two issues required special consideration: wake vortex effects and runway occupancy time.

Additional separation is needed behind a Heavy or B757 aircraft to avoid wake turbulence. Wake turbulence behind

Large aircraft was not considered to be a hazard at 2.5 NM for another aircraft that was also in the Large category or

heavier, based on operational experience with visual separations that typically are less than 3 NM for these aircraft.

For those aircraft pairs with the reduced separation, the lead aircraft would still need to exit the runway before the

trail aircraft crossed the runway threshold. A maximum average runway occupancy time (ROT) that would be

compatible with the 2.5 NM minimum separation needed to be specified in order to prevent an increase in the rate of

go-arounds to avoid simultaneous occupancy. After analysis of current operations and some discussions with

controllers, it was decided that a mean ROT of 50 seconds or less would be needed for 2.5 NM separations.

When the reduced separation was initially implemented, there was an additional requirement that the runways

needed to be clear and dry. Here the concern was that wet runways would increase ROT and the rate of go-arounds

would increase. After collecting ROT data in actual operations and conducting specific demonstrations, the FAA

recognized that wet runway occupancy times do not significantly differ from dry runway times and this requirement

was eliminated.

3.7. Parallel Approach Separation Standards

In 1962, the FAA approved a minimum spacing between parallel-runway centerlines of 5000 feet to conduct

independent instrument approaches. According to a 1971 FAA paper

4

:

4 Internal FAA paper, “Executive Summary – The establishment of a more definitive basis for the determination of

lateral spacing criteria for IFR approaches to multiple runways”, received by MITRE on May 13, 1971.

8
9

“In 1962, data was recorded and analyzed of the lateral deviations of aircraft conducting IFR approaches to

the Chicago O’Hare airport. The current lateral spacing criteria for conducting independent simultaneous

IFR approaches to parallel runways [i.e., 5000 feet runway spacing] were based on the results of that data.”

A MITRE briefing5, circa 1973, recommended reducing the minimum spacing to 4300 feet. MITRE’s

recommendation was based on increasing the No Transgression Zone (NTZ) between the runways from 2000 ft to

2700 ft and decreasing the size of the Normal Operating Zone (NOZ) around the localizer path, due to improved

navigation performance. The NOZ is sized so that the likelihood of any normally operating aircraft being observed

outside the NOZ is very small. However, the FAA decided to retain the 2000 feet NTZ. In 1975, MITRE refined its

analysis6 and calibrated it to the FAA’s implementation of 4300 ft “as the accepted standard of system performance

and safety.” The FAA approved 4300 feet in October 1974, presumably based on MITRE’s input as well as a report

by Resalab7 (1972) that justified assuming smaller navigational errors.

The “MITRE Model” for parallel runway spacing calculated the required runway spacing based on providing a safe

separation in the event of a “worst case” blunder, a sudden 30-degree turn towards the other approach. The

blundering aircraft is assumed to not respond to controller intervention. Because surveillance and navigational

errors increase with distance from the radar and or localizer antennas respectively, it was assumed that the blunder

occurred far from the airport, when the aircraft first lost altitude separation.

More recently, real-time simulations and mathematical analysis were employed to calculate the overall risk of a

collision due to a blunder, sudden deviation from the planned approach. A blunder is an extremely rare event. Even

though simultaneous parallel approaches have been conducted for more than forty years, the FAA has no official 

data on the causes, the frequency, or other characteristics of blunders. However, there was a decision that the ATC

system needed to protect against a 30-degree non-responsive blunder: an aircraft that turns 30 degrees towards the

other approach, whose pilot does not respond (or cannot respond) to commands to return to course.

Finding 2. Most current separation standards have been developed empirically based on 

judgment, extrapolation of past experience, and limited analysis. In recent years, a more 

analytical approach has been applied. The current standards are not based on a consistent 

philosophy, varying from one part of the airspace to another, using varied analytical 

approaches and assumptions about behavior.

Finding 3. Some separation standards are strongly influenced by the possibility of gross 

deviations, or blunders. However, little is known about the occurrence of such blunders: their 

frequency, their magnitude, under what circumstances they are most likely to occur. Existing 

information about blunders is primarily anecdotal.

3.8. Lessons Learned from Historical Record

There are many difficulties associated with deriving an absolute measure of risk. The U.K. Air Registration Board, 

in its deliberations in the 1950’s, was aware that the mathematical risk and collision risk analysis that it imposed had

little or no value in offering guidance as to the absolute safety to be achieved. It was valuable in evaluating several

5 The MITRE Corporation, ca. 1973, Reduction of the Separation Standard for Simultaneous ILS Approaches to

Parallel Runways, unpublished briefing.

6 Haines, A.L., January 1975, Requirements for 3500 Foot Spacings for Simultaneous Parallel IFR Approaches,

MTR-6841, The MITRE Corporation, McLean VA.

7 Resalab, Inc. July 1972, Lateral Separation, FAA-RD-72-58, Volumes I & II.

9
10

alternatives as to their comparative safety value. Nonetheless, while applying it was difficult and easy to misuse in

the wrong hands, the idea of Target Level of Safety (TLS) was, and still is, attractive.

Absolute measure of risk is difficult because most risks in aviation are extremely rare events. As a consequence,

• the probability of a risk event cannot be easily described by standard statistical distributions

• separate risks may not be completely independent, since a risk event may have an underlying cause that

would affect the probability of a different risk event

• defining a meaningful TLS is difficult

Consideration of separation standards must of necessity make assumptions about the performance of people and

systems. But while experts can use judgment about the validity and value of assumptions, mathematical models and

processes need absolute values if a result is to be derived. Because there is frequently little data available to support

assumptions about extremely rare events or new systems (e.g. aircraft deviation due to blunders on a final approach

to parallel runways, or blunder deviation data in over-ocean operations), many assumptions are open to question and

possibly invalid. It is therefore essential that analysts clearly state the assumptions that are used in their safety/risk

analysis and state the basis of the assumptions.

There has been much debate about the uses and limitations of mathematical risk analysis and collision risk 

modeling. An important point is that the requirement to conduct such analysis imposes a discipline on designers and

system evaluators. It requires that designs and procedures be critically examined in a structured way for potential

failure paths. The value of such discipline in testing designs and procedures is far more important than the fact that

the analysis methods and models may not be perfect.

The attempt to assess the safety of aircraft operations and innovative ATC procedures by depending primarily on

analytical means is attractive and convenient and seems sound. But there are serious difficulties. The catastrophes

that might be predicted by a simple probability analysis simply have not happened. Some of the reasons that simple

analysis has proven to be overly conservative are:

• navigation error assumptions are conservative, even perhaps pessimistic

• obstacles and adjacent route boundaries in the past were frequently represented in computations as neat

geometrical shapes such as "walls" or sloping surfaces when often they are single or discontinuous

protuberances

• supplemental methods of navigation or other cross checks are used which can catch blunders or large deviations

before they can cause harm

• radar observations often exist and affect the situation

• pilots recognize the existence of and take special care to give a wide berth to obstructions known to them

• simple representation of overlapping distribution curves for aircraft navigation implies that collisions will occur

if one or both aircraft on adjacent air routes drift into the adjacent route area. This does not consider that 

aircraft on the routes are not flying in solid nose-to-tail streams: aircraft are spaced longitudinally as well as

laterally.

The important conclusion is that while the simple probability computation has proven useful and valid in the

combination of navigation system error elements, the relation of such errors to actual collision risk when other

factors are also at work is not yet completely understood.

While drawing conclusions from experience in purely probabilistic terms may be difficult, reasonable people can,

will, and have drawn valid inferences from such experience. Experience on a valid sample in a practical operational

environment has been a useful guide to decision-makers.

10
11

As noted above, it has been a common practice to try to develop probability numbers to show the safety of systems

and procedures in aviation. The British Air Registration Board was very cautious about the meaning of such

numbers (10⎯7 as their ‘acceptable’ landing accident figure). FAA describes 10⎯7 as an “extremely remote” event,

but recognizes the difficulty of attempting to prove such values.

Finding 4. Mathematical analyses require substantial data to accurately characterizes 

reality. Historically, sufficient data has not been available. The result of insufficient data 

is overly conservative separation standards.

11
12

Section 4

Current State of the Art to Establish Separation Standards

Over the years, the FAA has established safe separations standards for many operations and has introduced new

procedures to take advantage of new technology. The safety of these operations was established using analyses that

have grown increasingly more detailed and sophisticated, also taking advantage of new technology for modeling and

simulation. IFR Separation Standards are described in Appendix 5 along with the critical factor that sets the 

standard.

4.1. ICAO Guidance on Separation Standard Analysis

A good general discussion of the current understanding of separation minima may be found in ICAO Doc 9689-

AN/953, Manual on Airspace Planning Methodology for the Determination of Separation Minima. Although this

overview tends to emphasize oceanic operations in its examples, many of the principles are generally applicable.

ICAO sets forth a general process for developing a new separation standard or any revision to the air traffic system.

The steps in this process are:

- Identify the need for change

- Describe the current system

- Determine the proposed system

- Identify method of safety assessment

- Evaluate risk

- Determine changes to the proposed system, if safety criteria are not satisfied, and re-evaluate

- Implement and monitor the new system

In addition, two general techniques for evaluating risk are presented:

- Comparison with a reference system

- Evaluation of system risk against a threshold

4.1.1.Comparison with a Reference System

The first technique provides a relative measure of risk. The proposed system is compared to a substantially similar

system that has already been judged to be acceptably safe. In order to be “substantially similar” for this purpose,

ICAO presents four criteria that ensure that the risk of the new system will be no worse than the existing system.

Most significantly, “separation minima must not be less in the proposed system.”

This criterion would seem to eliminate the comparative technique from consideration for evaluating any possible

reductions to separation standards. However, ICAO acknowledges the proposed system may not always be

“sufficiently similar,” and allows for a trade-off between parameters, such as navigation performance and separation

minima, in such cases. Nonetheless, this comparative process tends to produce overly conservative results.

4.1.2.Evaluation Against a Threshold

The second technique identified by ICAO produces an absolute measure of risk, by explicitly evaluating every

relevant risk associated with the proposed system. Such an approach is required when the new system is radically

different from any existing system.

Parts of the proposed system that are little changed from the existing system may be excluded from the evaluation of

risk, under the assumption that the risk level has not changed. This is feasible if the target level of safety (TLS) to 

be achieved also does not consider the excluded part of the system.

12
13

4.2. Examples of Successful Separation Reductions

4.2.1.2.5 Nautical Mile Separation on Final Approach

In the analysis of reduced longitudinal separation on final approach, various potential hazards were identified.

However, it was determined that most of these hazards did not present an increased risk at 2.5 NM separation

compared to 3.0 NM and the two procedures would be “substantially similar.” Thus, this separation reduction was

achieved by general conformance with the first method set down by ICAO: comparison with a reference system. (A

more detailed description of the reduction can be found in Section 3.6.)

4.2.2.Simultaneous Parallel Approaches using a Precision Runway Monitor (PRM) Surveillance System

The approach used to develop runway spacing requirements for simultaneous parallel approaches using a PRM

surveillance system followed the second ICAO method: evaluation of system risk against a threshold. Real-time

simulations and mathematical analysis based on field measurements were employed to calculate the overall risk of a

collision due to a sudden deviation from the planned approach, commonly referred to as a blunder.

PRM and the Final Monitor Aid, used in Denver, are designed to prevent a collision when one aircraft deviates into

the adjacent approach path, an event termed a blunder. PRM testing included both 15-degree and 30-degree

blunders initially, but it was quickly determined that the 15-degree blunder could consistently be resolved

successfully. The overall risk analysis therefore focused on the 30-degree blunder, which might occur at various

points along the final approach path.

The PRM was assessed using three methods, now believed to be essential for future initiatives that propose

separation reductions. The three methods were:

- Mathematical analyses, based on field data, to estimate how well PRM would prevent a collision due to a rare

deviation from the approach course

- Simulations that included real-time cockpit simulators to understand pilot responses, and focused on

discovering unanticipated procedural or training issues.

- Field demonstration flights for airline pilots and controllers.

These are further described in Appendix 6.

ICAO and FAA have accepted closely spaced parallel approaches with the PRM as meeting the TLS. Extensive

demonstrations and tests, consultations among the concerned parties, and sound judgments by operations experts

finally led to reductions in the required spacing between parallel runways. The required spacing has now reached

3400 feet for parallel approaches, or 3000 feet if one of the approaches is offset by 2.5 degrees.

4.3. Introduction of Required Navigation Performance (RNP) operations

The current effort to integrate Required Navigation Performance (RNP) operations into busy terminal areas 

illustrates how the Comparison to a Reference Method, when applied to new technology, does not take advantage of

the new capability. A full description can be found in Appendix 7, ILS/RNP Separation Standard.

4.4. FAA Safety Risk Management Process

The Safety Risk Management (SRM) process is part of the Safety Management System (SMS) being implemented

by the FAA. SRM parallels the ICAO steps for evaluating risk. At the highest level, the SRM process consists of

five steps:

- Describe System: including scope and objectives, stakeholders, and the risk evaluation criteria

13
14

- Identify Hazards: a structured review of the system to produce an understanding of everything that might go

wrong

- Analyze Risk: estimate the frequency and consequences of the hazards, and the overall risk of the system,

together with ways to control the risks

- Assess Risk: review the hazards and identify those that can or should be addressed

- Treat Risk: including risk treatment plans and monitoring of the system after implementation

As part of the SMS, an SRM analysis is required for any reduction in separations, and must be recorded in a Safety

Risk Management Document (SRMD). The explicit consideration of risks and hazards in the SRM process has not

always been quite as prominent in previous changes to the separation standards. Nevertheless, the FAA has

attempted to manage the risk associated with new or revised procedures through careful analyses, demonstrations,

and reviews, as in the PRM program summarized above.

Finding 5. New separation standards may be developed by comparison with a reference 

system or by evaluating system risk against a threshold level. Comparing to a reference 

system is an appropriate method to support incremental changes to the current system. To 

evaluate the major changes in separation standards that will be required for the Next 

Generation Air Traffic System (NGATS), the evaluation against a threshold methodology 

may be necessary.

A disciplined process for identifying and analyzing risk when developing or revising 

separation standards is of vital importance. Analytical and probabilistic studies are 

essential to the determination of safe standards, but, by themselves, are not enough. They 

should be used together with judgment. Their role is to inform and quantify judgment. 

Guarding against unrealistic or diabolical phenomena should not be a basis for the 

establishment of separation standards.

14
15

Section 5

Discussion

This section contains discussions of relevant topics. The individual topics, though disparate, are linked by their

applicability to the problem at hand: reduction of separation standards.

5.1. Influence of New Technologies on Separation Standards

Separation standards are influenced by a number of factors, including the accuracy of the navigation system in use,

the ability of the aircraft to adhere to the desired course, the ability of the air traffic service provider to monitor the

aircraft’s adherence to its intended track, and the ability of the air traffic service provider to intervene if a hazardous

situation is developing.

Technological advances have provided improved capabilities in all of these areas.

Electronic navigation systems have evolved from low frequency ranges to VOR/DME augmented with Inertial

Reference Units, and most recently to GPS-based satellite navigation. Instead of the miles or fractions of a mile

accuracy of VOR/DME positioning, flight paths can now be defined with an accuracy of a few feet, or at most a few

10’s of feet, i.e. within the dimensions of most aircraft. In addition, GPS signals provide essentially perfect time

information to each user. Additionally aircraft alerting tells the pilot when the navigation or total system error has

exceeded the Required Navigation Performance (RNP).

The real power of RNP is its independence from an extensive ground-based infrastructure and the fact that its

performance distributions are not Gaussian. For example, the monitoring and alerting capability that is required in

all RNP systems truncate the distribution at the 10-5 level, which means that the crew has high confidence that the

error is less than 1 * RNP when the system is operating normally. The crew also knows when the performance is

inadequate, and at that time, can apply the appropriate operational procedures, thereby truncating the distribution at

that level.

Electronic flight management systems currently allow the pilot to program in and adhere very closely to the intended

course. In the future, the intended course could be transmitted to the air traffic service provider so that there is no

uncertainty as to the aircraft’s intent. Not only does the FMS enable closer adherence to the specified flight track,

including time along track, but a data link could transmit the selected flight track to the controller, confirming the

aircraft’sintent.

The ability of the controller to maintain track of the aircraft position has evolved from voice position reports to highprecision monopulse radars. The implementation of ADS-B will further enhance the surveillance capability,

providing the controller with more precise aircraft position information, and other aircraft state parameters. In

addition, other suitably equipped aircraft will be able to receive the ADS-B reports, giving them knowledge of the

position and intent of the other ADS-B-equipped aircraft.

Using the more precise monitoring of aircraft position and intent afforded by ADS-B and enhanced automation aids;

the controller will be able to more rapidly sense and react to deviations of an aircraft from its intended flight path.

Eventually the ground-based computer will itself be able to take over the task of resolving routine separation issues.

In addition, ADS-B will allow other aircraft to also monitor aircraft positions and deviations from intent, and so

participate in the separation process.

Fundamental to these capabilities is reliable digital communication between aircraft and between aircraft and the

ground-based control system. The limited data link capability inherent in ADS-B will be augmented with a general15
16

purpose link to provide connectivity between the computers on board the aircraft (especially the FMS) and the

ground-based automation system.

The combination of these enhanced capabilities will allow aircraft to be routinely spaced more closely than they 

have in the past, because their flight paths can be more precisely defined and adhered to, and the control system will

more quickly be able to sense and respond to deviations.

Finding 6. New technologies (e.g. GPS, ADS-B, CDTI, Datalink) offer the potential for 

reducing required separations. In particular, GPS-based RNP, together with the concept 

of containment, provides much more precise control and knowledge of an aircraft’s 

intended trajectory, and ADS-B permits the pilot of other aircraft, as well as the air traffic 

controller, to monitor the flight path of a proximate aircraft and rapidly sense deviations 

from its intended path.

5.2. Potential Impact of Automation

It is generally agreed that humans will have a significant role in managing traffic within NGATS. This role will,

however, be fundamentally different than in today’s system. The human controller function will shift from tactical

separation of aircraft to strategic management of traffic flows. “Overall objectives for increased capacity, safety,

predictability and efficiency are met through two primary transformations: implementation of trajectory-based

operations in concert with application of performance-based services” (page 2-1, NGATS CONOPS, V-0.2, July,

2006). “The NGATS capitalizes on human and automation capabilities to increase airspace capacity, improve

aviation safety, and enhance operational efficiency. This is based on building processes and systems that support

humans in doing what they do best—choosing alternatives and making decisions, while automation accomplishes

what it can do best—the acquisition, compilation, monitoring, evaluation, and exchange of information. Research

and analysis will determine the appropriate functional allocation of tasks between Air Navigation Service Provider

(ANSP), Flight Operators, and automation. It will determine when decision support tools are necessary to support

humans (e.g., identify conflicts and recommend solutions for pilot approval) and when functions should be

completely automated without human intervention.” (page 1-9, NGATS CONOPS, V-0.2, July, 2006).

NGATS will be a 4D trajectory-based system with a Trajectory Agreement between the aircraft operator and the

ANSP. The trajectory is defined with only the level of specificity necessary to meet the performance requirements

of the proposed operation. A “flexibility volume is assigned by” the ANSP “represents the extent to which an

aircraft is authorized to deviate from the assigned path laterally, vertically, and in time. The flexibility volume

defines the operator’s flexibility to maneuver without negotiating a new Trajectory Agreement, and is assigned

based on the density and complexity of traffic in the volume of airspace plus the operations being performed by the

aircraft.” (page 2-8, NGATS CONOPS, V-0.2, July, 2006).

“Performance-based services align ATM assets with user demand. New kinds of flight operations such as

autonomous operations in which aircraft manage their own tactical separation from each other, and ANSP flow

operations in which precise execution of agreed trajectories permit much higher traffic throughput than is possible

today dramatically improve en-route productivity and capacity. Even with two to three times today’s traffic,

trajectory-based operations enables control of the number and complexity of conflicts, ensuring safe separation can

always be maintained, whether separation assurance is the responsibility of the ANSP or the flight crew. Conflict

detection and resolution, both airborne and ground-based, is highly automated, allowing for reduced and encounterspecific separation standards.” (page 2-2, , NGATS CONOPS, V-0.2, July, 2006).

Clearly, there will be instances when things go wrong, when equipment or software fails on the ground, in the

aircraft, or in space. In the current ATC system, human intervention to resolve problems happens quite naturally

because the system relies on air traffic controllers who maintain a continuous internalized understanding of the

airspace situation. When something goes amiss, they can intervene quickly based on that understanding. In

NGATS, such a backup mechanism for providing separation assurance will no longer be possible since the 

controller no longer has a picture of all aircraft in his or her head. Also, since airspace is less structured in NGATS,

16
17

it may be more difficult for humans to intervene during malfunctions than in a more structured system. As research

begins to identify roles and responsibilities for humans and for automation in NGATS, the research will also have to

identify what potential failures might occur and how people and machines work together to deal safely with failures.

If appropriate recovery mechanisms cannot be show to be safe and effective, proposed roles and responsibilities

during normal operation will have to be redefined.

Any proposed IFR separation standard in the NGATS environment must provide a safe distance between aircraft

during normal operations, when automation, pilots, and controllers are exercising due care and diligence. The

separation standard must also provide a margin of safety in the event of failures by providing enough time for some

automated system to sense and successfully resolve the situation.

Finding 7. The next generation air transportation system will have:

- new roles and responsibilities for pilots and controllers and the automation that 

supports them,

- increased shared situational awareness on board the aircraft that will provide more 

timely and accurate information including intent of nearby vehicles,

- the potential, through good system design, for fewer unexpected deviations, and

- new backup systems to deal with system/subsystem failures, possibly accepting 

lesser performance capability than the system being backed up.

As surveillance, navigation, and communication performance increases, including 

communication of intent, separation standards will be driven more by failure analysis 

rather than system performance.

5.3. TCAS and other alerting system implications

The Traffic Alert and Collision Avoidance System (TCAS) is designed to reduce the risk of mid air collisions by

sensing when aircraft are likely to violate existing separation standards and providing flight crews with situational

awareness and, where necessary, vertical maneuver advisories. It is also an independent backup for any failure of

the ATC system to ensure safe separation.

The TCAS threat logic operates using only surveillance data, e.g., the range to and altitude of other aircraft, and its

own altitude to achieve a balance between maintaining separation while minimizing nuisance alarms. It does not

have access to intent, nor air traffic control procedures. It also uses azimuth estimates for displaying traffic

advisories.

As new procedures, involving further reductions in separation are implemented, TCAS will produce more

unnecessary or nuisance alerts if left in the Resolution Advisory (RA) mode. Further, there will be an increased

chance that the multi-aircraft logic within TCAS will be exercised as well, causing additional false or nuisance

advisories.

In order to insure that TCAS does not become such a nuisance that flight crews ignore it, the use of TCAS and the

design of its threat logic must be examined with respect to each new procedure that reduces aircraft separation. It is

also recommended that new procedures be reviewed to insure they do not needlessly provoke TCAS RAs.

In addition to TCAS, Terrain Awareness and Warning Systems (TAWS), Minimum Safe Altitude Warning

(MSAW), and Conflict Alert (CA) function, should be initiated to minimize false alerts as separation standards are

reduced and revised.

17
18

Finding 8. In designing NGATS, an air-based independent (from ATM system) backup 

collision avoidance system (similar to TCAS or perhaps a modified TCAS) will be required.

Back-up safety systems in the aircraft and air traffic control facilities have been set to 

prevent collision while minimizing false alerts when aircraft are operating at today’s 

separation standards. As separation standards are reduced, procedures and alerting logic 

must be reexamined to optimize the balance between collision avoidance and false alerts.

5.4. Stochastic Separation Standards

Currently controllers are held to a set of minimum landing separations. If the controller’s action results in a

separation that is less than the stated minimum, this is considered to be a violation. This encourages the controllers

to work at an average separation larger than the minimum to ensure that natural variations will not produce a

violation. A stochastic approach would hold the arrival controller to a distribution of arrival separations (during a

shift or a push, say) that had an appropriate mean and standard deviation. If the actual means are too small or the

deviations too large, then the controller would be faulted. The threshold mean and standard deviation are chosen to

achieve the required level of safety.

Research into the use of stochastic control in terminal operations (specifically landing spacing) has shown a capacity

benefit (i.e. the controllers can give up the “cushion” that protects them from a violation), but there is still the

question of practicality. What unintended consequences are likely to emerge? This is an interesting area for 

research not only because it offers the prospect of some near term improvement in landing rates, but also because

stochastic control is more appropriate than deterministic control in automated systems such as NGATS.

There is anecdotal evidence that the stochastic approach is in informal use at some heavily utilized airports.

Finding 9. Evaluating the controllers’ performance by distribution (stochastic control) 

rather than a hard limit could increase capacity and effective throughput without 

compromising safety.

5.5. Wake Vortex

Interest in potential capacity gains from Wake Vortex research has increased in recent years, as traffic congestion 

has increased. Although current standards have proven to be safe it is thought that these separation distances may be

over-conservative and thereby unnecessarily reduce capacity. There is not, at this time, an acceptable way to 

mitigate wake-vortex constraints and obtain the desired increases in airport capacity.

There are at least four approaches being considered for increasing capacity while mitigating the vortex encounter

risk.

One approach for closely spaced parallel runways relies on wake observations to build a solid statistical

basis to validate the assertion that vortices do not travel from one runway to the other under any operational

wind conditions. This approach is being pursued for St. Louis now, for Cleveland in the near future,

possibly followed by a national rule change.

A second approach is to determine safe distances for an aircraft to trail behind a leading aircraft as a

function of wind conditions. This approach can be applied to single runway or multiple runway operations.

Another approach being considered for approaches to parallel runways is to apply a stagger between the

aircraft so that the trailing aircraft on the adjacent approach stays in front of the leading aircraft’s vortices.

18
19

A fourth approach considers static reductions of wake separation under the premise that there is some

agreed-upon wake severity that would allow harvesting of potential capacity with better knowledge of

wake vortex/aircraft encounter behavior.

NASA, working with the FAA, has a research program to enable an increase in NAS capacity through novel

concepts for wake vortex constrained terminal area operations. The project is following a three-phase approach.

1.Procedural Approach

The first phase determines the feasibility of reducing current standards under certain conditions. The

objective is to safely increase capacity without the need for infrastructure changes. The FAA is leading the

procedural changes while NASA is responsible for providing data and data analysis to validate that the

proposed procedures do not lead to an unacceptable risk of a vortex encounter.

2.Wake Transport Approach

The second phase addresses the feasibility of safely increasing capacity by accounting for knowledge of the

winds and the effect that winds have on moving a vortex out of the path of a trailing aircraft for single

runway operations, or, ensuring the vortices can not move into the path of a trailing aircraft on a parallel

runway. This is a joint activity between FAA and NASA.

3.Application of Advanced Technologies

The third phase will examine the application of new technologies to provide improved awareness of the

presence of vortices and methods for mitigation of vortex encounters thereby reducing the risk of a vortex

encounter while attaining a substantial increase in capacity. The third phase is being led by NASA but

followed by the FAA. Technologies being considered include improved wake vortex prediction

algorithms, wind persistence and prediction algorithms, wake vortex detection technologies, and weather

measurement and forecasting technologies. The third phase will also address integration of ground and

airborne systems for dynamic spacing, the development of flight deck visualization systems, and wake

mitigation technologies. Safety analysis will be included throughout the effort.

Although the community generally supports the current program, the importance of this work for separation

reductions requires that it be periodically reviewed for technical content, progress and support from NASA and the

FAA.

Finding 10. In considering the possibilities for reducing separations standards, wake 

turbulence becomes the driving consideration. For NGATS, wake turbulence could become 

the primary limiter of capacity.

19
20

Section 6

Findings8 and Recommendations9

Finding 1. The current system, based on the separation standards that have evolved over 

the last 50 years, is safe, but still unable to meet projected demand. The separation 

standards (and the approach to establishing separation standards) now need to be 

reconsidered in order to meet the demand for increased capacity.

Finding 2. Most current separation standards have been developed empirically based on 

judgment, extrapolation of past experience, and limited analysis. In recent years, a more 

analytical approach has been applied. The current standards are not based on a consistent 

philosophy, varying from one part of the airspace to another; using varied analytical 

approaches and assumptions about behavior.

Finding 3. Some separation standards are strongly influenced by the possibility of gross 

deviations, or blunders. However, little is known about such blunders: their frequency of 

occurrence, their magnitude, under what circumstances they are most likely to occur. 

Existing information about blunders is primarily anecdotal.

Finding 4. Mathematical analyses require substantial data to accurately characterize 

reality. Historically, sufficient data has not been available. The result of insufficient data 

is overly conservative separation standards.

Finding 5. New separation standards may be developed by comparison with a reference 

system or by evaluating system risk against a threshold level. Comparing to a reference 

system is an appropriate method to support incremental changes to the current system. To

8

Findings 1 though 5 all deal with the historic establishment of separation standards, and the limitations of some of

the methodology used. They are treated together in this section in order to establish a consistent set of

recommendations.

9 Recommendations are presented in two time frames, Immediate and Longer Term. The Immediate

recommendations are those that should or can be undertaken immediately. Some of them could increase efficiency,

if applied, in the current National Airspace Space system. Longer Term recommendations are must be preceded by

other work or do not apply to today’s system.

20
21

evaluate the major changes in separation standards that will be required for the Next 

Generation Air Traffic System (NGATS), the evaluation against a threshold methodology 

may be necessary.

A disciplined process for identifying and analyzing risk when developing or revising 

separation standards is of vital importance. Analytical and probabilistic studies are 

essential in the determination of safe standards, but, by themselves, are not enough. They 

should be used together with judgment. Their role is to inform and quantify judgment. 

Guarding against unrealistic or diabolical phenomena should not be a basis for the 

establishment of separation standards.

Recommendation:

Establish an R&D program that will lead to consistent and safe reduction of separation 

standards and that will support NGATS. The process outlined below for setting separation 

standards should be adopted. This R&D program should include, but not be limited to:

• Immediate

o Establish a research program to develop an understanding of the nature and frequency of blunders.

• Performance Data Analysis & Reporting System (PDARS) appears to be a possible source for

needed data.

• Develop new systems, if needed, for automated reporting of such anomalies.

o Establish data needs for establishment of separation standards early in NGATS development so

opportunities, such as demonstrations, can be used collect data.

o If conservative separation standards are put in place, such as RNP Parallel Approach Transition

(RPAT), establish a data collection process early in the implementation so operational data collected to

reduce separations in the future.

• Longer Term

o Conduct research to develop consistent approaches for the development of separation standards with

all assumptions stated concisely.

o Conduct research to improve the methodology for evaluating separation standards against an absolute

threshold (target level of safety). In particular, there needs to be a consistent, credible way to take into

account the response of humans to rare events.

Process for Separation Standard Reduction

The PRM program was only one example of the successful implementation of a new ATC procedure through a

process of careful analysis and testing. Based on this experience, the following general process should be the

guideline for evaluating and implementing future separation reductions. An evaluation of the overall system risk is

necessary when the proposed system is much different from the current system – as is frequently the case when new

technologies are introduced.

Mathematical Analysis. Such analyses may be simple at first, considering only the most common scenarios for the

proposed procedure. Based on actual data wherever possible, such analyses could help to refine the procedure and

identify scenarios wherein the proposed procedure would not produce satisfactory performance (a collision being

only one example of a negative outcome). More sophisticated analyses, such as Monte Carlo simulations, should be

employed when well constructed models describing the performance of systems and humans, derived from field

measurements, are available.

Real Time Simulations. The refined procedure should be given an operational evaluation through real-time,

human-in-the-loop simulations that are as realistic as feasible. Mathematical analyses will have identified the

21
22

factors, which most affect the probability of unsatisfactory performance; these factors should be incorporated into

the simulation with the highest degree of realism. In the process, it is likely that additional factors will be identified

as potentially significant.

Performance data collected during the real-time simulations can be used to refine and extend the mathematical

analyses. Also, insights obtained during the simulations provide important data for the later risk assessment of the

procedure.

Field Demonstrations. Additional realism is provided by field demonstrations, clearly. But the expense involved

necessitates that the procedure be thoroughly tested prior to the demonstration phase. Demonstrations can help to

identify additional operational concerns, as well as help to validate the simulation results for a sample of conditions.

Demonstrations are not a substitute for the simulations, but can provide confidence that the simulations provided

valid results.

Risk Assessment. The FAA SRM process should form the basis for such assessments. The results of the analyses,

simulations, and demonstrations should inform the risk assessment process. All assumptions that are part of a system

risk assessment should be stated, all assumptions should be accepted by consensus, and the sensitivity of the results 

to the assumptions should be evaluated. Here the different risk elements and their consequences are evaluated and a

determination is made whether or not to proceed with development or implementation of the procedure. The risk

assessment should be transparent and involve a range of users and operators.

Evaluation against a threshold or application of the target level of safety poses tough problems, but it is based on a

solid premise: that any new design, whether it is an engine, flight control system, or wing structure, must be at least

as good or better than its predecessors. This is primarily due to the fact that most risks in aviation are due to

extremely rare events.

Judgment and Experience. Judgment and experience will be needed to address rare, unpredictable events.

Attempting to compensate for “diabolical” threats or similar anomalies in the system is impractical.

A “diabolical” threat in the context of maintaining separation between two aircraft is a threat in which one of the

aircraft begins to maneuver in such a manner as to maximize the probability of collision. Designing a system that

would detect such a maneuver in time to effect a successful evasion by the burdened aircraft is extremely

demanding. The concept is not a practical basis for designing a separation maintenance system but it is a useful

construct for thinking about the vulnerability of various approaches to separation assurance.

Structured Introduction. Finally, the new procedure should be introduced to general usage in a stepwise manner,

with opportunities to observe actual operations, review performance, and make modifications (or halt the procedure)

if necessary. For example, a new approach could be demonstrated in visual conditions, then allowed in lower and

lower weather conditions until the intended minima are achieved. For another example, one airline could pioneer 

the new procedure at an airport, and then the procedure could be implemented at other airports or by other carriers.

Such a process of analysis, testing, observation, and modification requires an extended commitment of time and

resources. However, it is a necessary investment that can pay off in operational benefits while maintaining safety.

Finding 6. The next generation air transportation system will have:

- new roles and responsibilities for pilots and controllers and the automation that 

supports them,

- increased shared situational awareness on board the aircraft that will provide more 

timely and accurate information including intent of nearby vehicles,

- the potential, through good system design, for fewer unexpected deviations, and

- new backup systems to deal with system/subsystem failures, possibly accepting 

lesser performance capability than the system being backed up.

22
23

As surveillance, navigation, and communication performance increases, including 

communication of intent, separation standards will be driven more by the need to 

accommodate system failures than by variations in nominal system performance.

Recommendations

• Longer Term

o Establish a research program to develop an understanding of the roles of the human and automation in

dealing with failures and the implication of those roles on separation standards.

o Managing failure gracefully is perhaps the most difficult design aspect of the NGATS. Specific and intense

research into the human and automated alternatives will be required.

Finding 7. New technologies (e.g. GPS, ADS-B, CDTI, Datalink) offer the potential for 

reducing required separations. In particular, GPS-based RNP, together with the concept of 

containment, provides much more precise control and knowledge of an aircraft’s intended 

trajectory, and ADS-B permits the pilot of other aircraft, as well as the air traffic controller, to 

monitor the flight path of a proximate aircraft and rapidly sense deviations from its intended 

path.

Recommendations

• Immediate

o As more and more aircraft use RNP-based navigation, monitor their performance, and gather and analyze

data to develop a statistical understanding of the performance of RNP-based systems in various flight

regimes.

o Re-examine the design of parallel and converging approaches and departures based on an appropriate

probability distributions (may not be Gaussian) or on data gathered using RNP-based navigation.

o The Performance-Based Advisory Rulemaking Committee (PARC) should redefine the definition of

“established on approach” to include LNAV and VNAV. The requirement to be aligned with the

runway centerline should be studied for possible elimination.

o Research into potential reduction of Arrival/Departure and Departure/Departure separations due to

RNP guided missed approaches and departures should be pursued.

• Longer term

o Develop (recommendations for) new separations standards based on the improved navigation,

surveillance, communication, control, and automation technologies, which will be part of NGATS.

Utilize lessons learned during the analysis of other standards.

o When the nature and frequency of blunders off an ILS course are better understood using data

ILS/RNP parallel runway separation should be reevaluated. RNP/RNP parallel approach separation

should be established.

o The No-Transgression Zone (NTZ) role for ILS operations should be re-defined based on real blunder

information. Then, if still required, appropriate dimensions and shapes should be established.

o The role of the NTZ in RNP/RNP separations should be established. The NTZ may not be needed.

Finding 8. In designing NGATS, an air-based independent (from ATM system) backup 

collision avoidance system (similar to TCAS or perhaps a modified TCAS) will be required.

23
24

Back-up safety systems in the aircraft and air traffic control facilities have been set to 

prevent collision while minimizing false alerts when aircraft are operating at today’s 

separation standards. As separation standards are reduced, procedures and alerting logic 

must be reexamined to optimize the balance between collision avoidance and false alerts.

Recommendations

• Longer Term

o Research is required for the future independent airborne collision avoidance system in the context

of the ATM system construct and the associated separation standards.

o Research and analysis of alerting systems, such as Traffic Alert and Collision Avoidance System

(TCAS), Terrain Awareness and Warning Systems (TAWS), Minimum Safe Altitude Warning

(MSAW), and Conflict Alert (CA) function, should be initiated to minimize false alerts as

separation standards are reduced and revised.

Finding 9. Evaluating the controllers’ performance by distribution (stochastic control) rather 

than a hard limit may be able to increase capacity and effective throughput without 

compromising safety.

Recommendation

Immediate

Research into the practicality of stochastic control in terminal operations (specifically landing spacing) should be

initiated. Research should pursue the question of practicality and unintended consequences. This is an important

area for research because it offers the prospect of some near term improvement in landing rates, and because

stochastic control is more appropriate than deterministic control in automated systems such as NGATS.

Finding 10. In considering the possibilities for reducing separations standards, wake 

turbulence becomes the driving consideration. For NGATS, wake turbulence could become 

the primary limiter of capacity.

Recommendations

• Immediate

o Full support of existing research and implement program should continue.

o Commission a team to conduct in-depth annual technical and programmatic reviews of the wake research

and implementation program. The reviews should include the objectives, technical approach, schedule, and

funding. The team should be composed of external experts knowledgeable in the areas of wake vortices in

normal operating configurations, advanced Light Detection and Ranging (LIDAR) and other sensors that

may be useable in detecting the strength of a wake vortex, aircraft behavior in the presence of wakes, and

how this information can be used in the flight deck and air traffic facilities. This team should be structured

along the lines of the Department of Defense Science Board and report to ATO leadership.

• Longer Term

o Investigate advanced instrumentation such as LIDAR or other sensing methods to obtain direct

measurements of vortex strength.

24
25

o Investigate the feasibility and practicality of wake vortex sensing/tracking to provide the flight crew an

indication of encroaching wake vortex location, strength and upset risk.

25
26

Appendix 1

SWAG Members

•Sarah Dalton- Chair-Alaska Airlines

•Jim White -DFO - FAA

•Gloria Dunderman -Administration -FAA

•Bill Swedish - MITRE

•Dallas Denery - UCSC

•Dres Zellweger - JPDO

•Glenn Morse – Continental Airlines

•Jim Duke - ALPA

•JohnFielding-Consultant/Raytheon

•Mark Cato - ALPA

•Michael Perie - Consultant

•Paul Drouilhet – Lincoln Labs

•Ray LaFrey-Consultant

•Siegbert Poritzky- Consultant

26
27

Appendix 2

Terms of Reference

FAA’s Research, Engineering and Development Advisory Committee

Separation Standards Working Group

Established August 2005

•Establishing appropriate separation standards is an important element of achieving increased 

NAS capacity and flexibility, especially in terminal airspace. Two principal elements of required 

interaircraft separation, navigation accuracy and surveillance capability, have improved 

markedly since the current separation standards were established. It is important to understand 

how these improvements, plus other technology advances, can lead to adjustment in required 

interaircraft separation without any degradation of safety.

•The Working Group will examine the basis for current separation standards, and review past 

and ongoing studies of separation requirements. It will consider improved methodologies for 

establishing separation standards, and will outline a recommendation R&D program for the FAA 

to determine to what degree separation standards can be reduced using current and future 

technologies.

27
28

Appendix 3

Summary of Presentations and list of presenters

PRESENTATION LIST

DATE PRESENTER TITLE

10/18/2005 Brian Colamosca, FAA The Reduced Vertical

Separation Minimum (RVSM):

Implementation Criteria

10/18/2005 John Andrews, MIT/LL Validation of New Separation Standards:

• Closely Spaced Parallel Runways

• General Horizontal Separation

Standards

General Horizontal

Separation Standards:

Validation Issues

10/19/2005 Bill Swedish, MITRE A Breif History

of Separation

Standards

10/19/2005 Dr. Andrew

Zeitlin, MITRE

TCAS and Separation

10/19/2005 Suzanne Porter, MITRE RNAV and RNP

Track Spacing:

Opportunities for

Benefit

10/19/2005 S. R. Jones, MITRE Determination of

Requirements for ADS-B

Separation Requirements

10/19/2005 Randy Bone, MITRE Automatic Dependent

Surveillance Broadcast

(ADS-B) Applications

Review

ATO DRAFT ADS-B

Separation Standards (3 NM

Separation - Terminal Area)

prepared by John Marksteiner,

FAA

11/15/2005 Russ Paielli, NASA Tactical

SeparationAssisted Flight

Environment

(TSAFE)

11/16/2005 Steve Fulton,

NAVERUS

Informal talk

on position

performance

11/16/2005 Tom Imrich, Boeing

Requiered Navigation

Performance

Briefing not

available

1/10/2006 Sherry Borener, FAA NGATS

Briefing not available

1/10/2006 Don Pate, FAA TERPS –

Using

Performance

Modeling to

Set Standards

1/10/2006 Steve Barnes System Management System

(SMS) –briefings not available

1/10/2006 David Lankford, FAA History of Blunder Resolution
29

1/10/2006 Wayne Bryant, NASA Joint FAA/NASA Wake Vortex

1/10/2006 Steve Lang, FAA Research Program

1/10/2006 George Donohue Aircraft Separation Standards

George Mason University

1/11/2006 Irv Statler, NASA Aviation System Monitoring and

Modeling

(ASMM) Project

1/11/2006 David Holl, ATAC Corp Applications of the Performance Data 

Analysis & Reporting System to 

Separation Standards Development 

1/11/2006 Kevin Corker,San Jose State 

University 

Application of Human Performance

Modeling to Separation Standards 

Developments and Hazard 

Assessment 

1/11/2006 Steve Bradford, FAA Required Surveillance Performance to

Support 3-Mile Separation in the

National Airspace System 

Briefing not available

1/11/2006 Dr. James Yates, ISI Target Level of Safety (TLS)

28
30

29
31

Appendix 4

Current MITRE Research Related to Separation Standards

RNP/RNAV Track Spacing

POC: Walt Scales, wscales@mitre.org

MITRE CAASD is working with the FAA to develop the operating procedures and standards needed for RNP-2 and

RNP-1 en route and terminal applications.

The current lateral separation criterion for en route RNAV routes (Q routes with radar separation) is 8 NM track-totrack. Even wider Q-route spacings have been applied in the Gulf of Mexico. The thrust of the current effort is to

explore methods for reducing parallel route separations for Q-Routes, T-Routes, RNAV SIDs and RNAV STARs.

Spacing reductions could potentially be based on RNP containment, on radar separation, or on combinations of the

two (e.g., using RNAV and/or RNP to limit controller intervention rates while using radar for separation assurance).

Requirements for Simultaneous Parallel Approaches

POC: Vince Massimini, svm@mitre.org

MITRE CAASD is reviewing the major assumptions and analytical techniques used during previous simultaneous

approach studies, and will provide the FAA an assessment of which ones potentially could be modified in light of

new technology, procedures, testing or data availability. CAASD will also provide recommendations for data

analysis, analytical techniques, and testing that could be accomplished to provide a basis for modifying current

simultaneous approach standards.

As one specific example, CAASD will provide an analysis of the feasibility of substituting RNAV vertically-guided

approaches (i.e., GLS, LPV, LNAV/VNAV and RNP) for ILS and MLS approaches during the conduct of

simultaneous, dependent, SOIA, and RPAT approaches.

ADS-B Separation Standard

POC: Stan Jones, sjones@mitre.org

ADS-B provides an alternative to radar for aircraft surveillance. A single ADS-B position report is potentially more

accurate than a radar-based position, depending on the radar type and location. The minimum aircraft separation

based on ADS-B will be affected by this accuracy as well as by other characteristics of ADS-B such as the update

rate, asynchronous updates, and the possibility of a GPS integrity failure.

MITRE has used the ICAO Close Approach Probability (CAP) model to evaluate proposed 3 NM and 5 NM

separation standards when using ADS-B for surveillance. A separation less than the separation standard may result

if one aircraft turns towards the other during the asynchronous reception time, or if there is an undetected GPS fault.

The proposed standards would be acceptable if the minimum distance between aircraft for the ADS-B system was 

no greater than that of the SSR reference system at the same CAP risk level.

The analysis provided performance requirements for the ADS-B system that would allow use of the 3 NM and 5 NM

separation standards. These requirements are likely to be met by an operational system. The CAP approach,

combined with automated conflict detection for the controller, may support reduced separation standards in the 

future.

ADS-B Applications

POC: Randy Bone, bone@mitre.org

30
32

MITRE is working with the FAA to develop concepts and requirements for new advanced ADS-B applications that

will improve overall operations, e.g., safety (improved traffic awareness), capacity, and efficiency. Such 

applications include Cockpit Display of Traffic Information (CDTI) Assisted Visual Separation (CAVS) and parallel

approaches to closely spaced runways.

CAVS is intended to improve pilot traffic awareness during visual approaches in marginal visibility conditions. 

The pilot will use the CDTI to assist visual acquisition of other traffic, and then maintain separation using the CDTI

even if visual contact is lost due to low clouds or haze. United Parcel Service (UPS) has formally requested

operational approval for the use of CAVS. MITRE CAASD has been the lead in analyzing the technical and

operational issues required for certification and operational approval, through concept development and real-time

simulations.

Oceanic Separation Reductions

POC: Cathy Horton, chorton@mitre.org

As part of the continuing transition toward more stringent RNP operations, MITRE/CAASD has been working with

the FAA to reduce horizontal separations in the Pacific. In FY05, MITRE CAASD developed the Safety Case

Hazard Analysis for applying 30 NM lateral separation and 30 NM longitudinal separation (30/30) in the South

Pacific. CAASD also collaborated with the FAA and industry to develop the 30/30 procedures. The 

implementation of these reduced separations for aircraft capable of this more stringent RNP capability, as well as

other CNS elements is part of a worldwide ICAO coordinated effort. Such reduced separations provide more

efficient use of airspace, more optimum wind efficient routings, accommodation of increased traffic, and greater

flexibility.

Wake Turbulence Separation

POC: Jeff Tittsworth, allen@mitre.org

MITRE CAASD has been working with the FAA and NASA to reduce the separation requirements for alternating

arrivals to runways spaced less than 2500 ft apart. Under current ATC procedures, consecutive arrivals to these

runways are given full radar or wake vortex separation even if they are approaching different runways. An

upcoming demonstration project at St. Louis Lambert Field (STL) will allow a smaller separation (1.5 NM) where

the lead aircraft does not present a wake vortex hazard for the trailing aircraft. In FY05, CAASD evaluated the

implementation risk of such a procedure and developed a prototype of a wake turbulence visualization tool for

controller use. This year, CAASD will prepare the hazard analysis and benefits assessment for an enhanced

procedure, where separations between departures are reduced when wind conditions deflect the vortex away from

the path of the next departure.

Separation Standards Analysis

POC: Bill Swedish, swedish@mitre.org

This research project will investigate current separation standards and their basis, and attempt to develop a

consistent analytical framework for developing new standards. Safety analysis (per Safety Management System

requirements) is an integral part of this investigation. The analytical framework is intended to enable the FAA to

analyze the effect of new technologies (e.g., ADS-B, GPS), new procedures (e.g., RNP), and new concepts (e.g.,

UAVs and very light jets) on operations. A comprehensive framework will support development of appropriate

separation standards that expedite operations while maintaining or improving safety.

31
33

Current NASA Research Related to Separation Standards

Wake Vortex:

POC: Wayne Bryant, Wayne.H.Bryant@nasa.gov

Interest in potential capacity gains from Wake Vortex research has been of long term interest but has increased in

recent years. Although current standards have proven to be safe it is recognized that these separation distances are

often over-conservative and in some cases unnecessarily reduce capacity.

NASA working with the FAA has an active research program to enable an increase in terminal area capacity at an

agreed upon level of safety for the National Airspace System through new standards for wake vortex operations. 

The project is following a three phase approach. The first phase is aimed at determining the feasibility of reducing

current standards under certain conditions through the implementation of procedural changes. The objective is to

safely increase capacity without the need for infrastructure changes. The FAA is leading the procedural changes

while NASA is responsible for providing data and data analysis to validate that the proposed procedures do not lead

to an unacceptable risk of a vortex encounter. The second phase is aimed at determining the feasibility of safely

increasing capacity by accounting for knowledge of the winds and the affect that winds have on moving a vortex out

of the path of a trailing airport. This is a joint activity between FAA and NASA. The third phase is aimed at the

application of new technology to provide improved awareness of the presence of a vortex thereby removing the risk

of a vortex encounter while attaining a substantial increase in capacity. The third phase is being led by NASA but

followed by the FAA. Technologies being considered include improved wake vortex prediction algorithms, wind

persistence and prediction algorithms, wake vortex detection technologies, and weather measurement and 

forecasting technologies. The third phase will also address integration of ground and airborne systems for dynamic

spacing, the development of flight deck visualization systems, and wake mitigation technologies. Safety analysis 

will be included throughout the effort.

Aviation System Monitoring and Modeling Project:

POC: Irving Statler, Irving.C.Statler@nasa.gov

The Aviation System Monitoring and Modeling Project has not directly addressed the definition of separation

standards but has included several activities that are relevant to this question.

One such activity is the Performance Data Analysis & Reporting System (PDARS) Project. The PDARS project

collects, processes, and analyzes ATM operational data obtained from Center and TRACON radars. It is currently

operational at all 20 Centers and 13 TRACONS. The project includes the collection, processing, and analysis of

ATM operational data. It also includes tools to support explorative studies. The FAA has recently accepted full

responsibility is continuing expansion of PDARS to include the 35 Operational Evaluation Plan (OEP) airports and

associated TRACONS and the development of metrics for safety, capacity, and cost as a function of runway, airport,

TRACON, and Center. PDARS could be a valuable source of information to gain insight into the nature and

frequency of blunders and their impact on system safety in current operations.

Another activity that has direct relevance is the Modeling and Simulations Project. The Modeling and Simulations

Project is aimed at the development of modeling tools that can be used to predict the system-wide impact and

efficacy of interventions and the use of these models in simulation. It includes model interactions among multiple

human and non-human agents (automation), use of these models to simulate scenarios for which there are no data or

experience in order to identify systemic features conducive to human error. The methods appear to be extendable to

the determination of the likelihood and hazard impact of off-nominal (“blunders”) operations in human-system

interactions.

32
34

Tactical Separation Assisted Flight Environment:

POC Russ Paielli; Russ.Paielli@nasa.gov

The Tactical Separation Assisted Flight Environment activity is a prototype system being developed by NASA for

alerting air traffic controllers to imminent conflicts. Although this activity is directed towards assuring that current

separation standards are not violated, the analysis has included the identification of the nature and cause of several

“blunder” types that have led to operational errors. This analysis could be useful in defining the nature, frequency,

and likelihood of hazardous operations. The work has focused on the En Route airspace.

33
33

Appendix 5

List of IFR Separation Standards

Table 1 is a listing of the IFR separation standards in the radar and Oceanic environments. These standards have been selected for this

table because they have the greatest impact on the capacity of the NAS. The important characteristics of these separations are

summarized in the table. A full description of the standards can be found in FAA Order 7110.65, Air Traffic Handbook (appropriate

paragraphs indicated in the fourth column) and other FAA documents, as indicated.

The fifth column indicates the primary factor, or factors, affecting the separation standard.

Table 1: Critical IFR Separation Standards

Flight Phase Separation Minima SelectedRequirements

1 Reference2 Controlling Factor

OCEANIC LATERAL:

60-120 NM

Depends on speed and route

(North Atlantic and

Caribbean)

¶8-7-4

¶8-8-4

Navigation accuracy, no

radar

or VERTICAL:

2000 ft

Above FL290 (non-RVSM) ¶8-7-2

¶8-8-2

Altimetry accuracy

1000 ft Above FL290 (RVSM) or

at or below FL290

or LONGITUDINAL:

10-60 minutes at track entry

Depends on speed and distance

flown

¶8-3-3.e. Navigation accuracy, no

radar

EN ROUTE within

the U.S.

LATERAL:

5 NM

Below FL 600, if multiple radar

sensors (mosaic mode) radar

or either aircraft more than 40

NM from antenna, and 60

NM for Mode S surveillance

3

¶5-5-4 Radar resolution and

update rate

1 For complete requirements, refer to FAA Order 7110.65 and other FAA documents. 2 FAA Order 7110.65, unless otherwise indicated. 3 An FAA sponsored study determined that the increased accuracy of new Secondary Surveillance Radars permits increasing the range for 3 NM separation to at

least 60 NM and FAA Order 7110.65R, Chapter 5, ¶5-5-4. MINIMA, has been changed accordingly.

33
34

Flight Phase Separation Minima SelectedRequirements

1 Reference

2

Controlling Factor

3 NM Below FL 180, if radar in single

sensor mode and both aircraft

within 40 NM of antenna, and

60 NM for Mode S

surveillance

4

EN ROUTE within

the U.S.

(continued)

or VERTICAL:

2000 ft

Above FL290 (non-RVSM) ¶4-5-1 Altimetry accuracy

1000 ft Above FL290 (RVSM) or

at or below FL290

SUCCESSIVE

ARRIVALS –

Same runway or

parallel runways

spaced <2500 ft

apart

LONGITUDINAL:

3.0 NM

Radar in single sensor mode

and both aircraft within 40

NM of the antenna, and 60

NM for Mode S surveillance

5

¶5-5-4 Radar resolution

6 and

update rate

2.5 NM On final approach, if runway

occupancy is 50 sec or less

and no wake turbulence effect

¶5-5-4.g. Runway occupancy time

4/5/6 NM Behind a Heavy aircraft or

B757 (depends on trailing

aircraft type)

¶5-5-4.f. Wake turbulence

PARALLEL

APPROACHES –

IndependentILS

approaches to dual

runways

Simultaneous operations once

established on final approach

Runways <4300 ft apart

7 ¶5-9-7 Blunder recovery

Radar resolution

8 and

update

Localizer resolution

Runways 3400-4300 ft apart

9 ¶5-9-8

Runways 3000-3400 ft apart

10

4 Ibid.

5 Ibid.

6 Current SSR range and azimuth accuracies, 1.1 mrad RMS and 60 ft RMS respectively, are not available because of limitations in the CD data formats. 7 Requires operational ASR radar system. 8 See footnote 5. 9 Requires high accuracy / high update rate system and Final Monitor Aid (high resolution display and alerting logic), i.e., Precision Runway Monitor system 10 Requires Precision Runway Monitor system and 2.5 degrees localizer offset.

34
35

Flight Phase Separation Minima SelectedRequirements

1 Reference

2

Controlling Factor

PARALLEL

APPROACHES –

Dependent ILS

approaches to dual

runways

2.0 NM diagonal between aircraft

on adjacent runways

Runways <4300 ft apart ¶5-9-6 Blunder recovery

1.5 NM diagonal between aircraft

on adjacent runways

Runways 2500-4300 ft apart Wake turbulence is an

issue below 2500 ft

spacing

CONVERGING

APPROACHES –

Independent

Simultaneous operations once

established on final approach

Missed Approach Points

separated by 3.0 NM, and

TERPS surfaces for turning

missed approaches do not

overlap (TERPS + 3 criteria)

FAA Order

7110.98

Simultaneous missed

approaches

CONVERGING

APPROACHES –

Dependent

Alternating arrivals with specified

separations based on runway

geometry

Converging Runway Display

Aid (CRDA)

FAA Order

7110.110A

Simultaneous missed

approaches

SUCCESSIVE

DEPARTURES –

Same runway or

parallelsspaced

<2500 ft apart

1.0 NM Courses diverge by 15 degrees

or more (not behind

Heavy/B757)

¶5-8-3 Radarseparation

Wake turbulence

2.0 NM increasing to 3.0 NM Courses do not diverge ¶5-5-4 Radarseparation

Wake vortex separation

- distance (see above)

- time (2 minutes)

Behind a Heavy/B757 ¶5-5-4

¶3-9-6

Radarseparation

Wake turbulence

SIMULTANEOUS

DEPARTURES –

Parallel or nonintersecting

runways

Simultaneous operations Parallel runways separated by

2500 ft or more and courses

that diverge by 15 degrees or

more

¶5-8-3.c. Radarseparation

Wake turbulence

Non-intersecting runwaysthat

diverge 15 degrees or more

¶5-8-3.b.

DEPARTURE 2.0 NM increasing to 3.0 NM Within 40 NM of the antenna

11 ¶5-8-4 Radarseparation

11 See footnote 5.

35
36

Flight Phase Separation Minima SelectedRequirements

1 Reference2 Controlling Factor

AND ARRIVAL –

Same runway

2.0 NM increasing to 5.0 NM Not within 40 NM of the

antenna

Radar resolution and

update rate

DEPARTURE

AND ARRIVAL –

Parallel or nonintersecting

runways

Simultaneous operations Thresholds are even

12

Runway thresholds are at least

2500 ft apart

Missed approach and departure

courses diverge by at least 30

degrees

Missed approach by a heavy jet

can not overtake departing

aircraft

¶5-8-5 Radarseparation

Wake turbulence

12 Staggered thresholds increase or decrease the runway separation required.

36
37

Appendix 6

PRM for Simultaneous Parallel Approaches

The approach used to develop runway spacing requirements for simultaneous parallel approaches using a

Precision Runway Monitor (PRM) surveillance system followed the second ICAO method: evaluation of

system risk against a threshold. Real-time simulations and mathematical analysis based on field

measurements were employed to calculate the overall risk of a collision due to a sudden deviation from the

planned approach, commonly referred to as a blunder.

PRM and the Final Monitor Aid, used at Denver, are designed to prevent a collision when one aircraft

deviates into the adjacent approach path, an event termed a blunder. Even though simultaneous parallel

approaches have been conducted for more than forty years, the FAA has no official data on the causes, the

frequency, or other characteristics of blunders. However, there was a decision by an industry committee

that the ATC system needed to protect against a 30-degree non-responsive blunder: an aircraft that turns 30

degrees towards the other approach, whose pilot does not respond (or cannot respond) to commands to

return to course.

PRM testing included both 15-degree and 30-degree blunders initially, but it was quickly determined that

the 15-degree blunder could consistently be resolved successfully. The overall risk analysis therefore

focused on the 30-degree blunder, which might occur at various points along the final approach path.

The PRM was assessed using three methods, now believed to be essential for future initiatives that propose

separation reductions. The three methods are:

- Mathematical analyses, based on field data, to estimate how well PRM will prevent a collision due to a

rare deviation from the approach course

- Simulations that include real-time cockpit simulators to understand pilot responses, and focus on

discovering unanticipated procedural or training issues.

- Field demonstration flights for airline pilots and controllers.

ICAO and FAA have accepted closely spaced parallel approaches with the PRM as meeting the TLS.

Extensive demonstrations and tests, consultations among the concerned parties, and sound judgments by

operations experts finally led to reductions in the required spacing between parallel runways. The required

spacing has now reached 3400 feet for parallel approaches, or 3000 feet if one of the approaches is offset

by 2.5 degrees.

Mathematical Analysis of PRM Effectiveness

To estimate PRM effectiveness, the Monte Carlo method was used. This consisted of selecting a blunder

scenario (10 NM final, 15 degree deviation for example), and creating a trial blunder situation using

randomly selected conditions: initial aircraft positions, radar antenna position, monitor controller response,

radio activity, and pilot and aircraft response. The resulting aircraft trajectories for each aircraft were then

calculated and it was noted whether adequate separation was maintained; this process was repeated for one

hundred thousand trials, and the results used to estimate the effectiveness of PRM in maintaining a 500-foot

minimum separation. The Monte Carlo process was then repeated for other blunder scenarios.

To enable this analysis the following field data was collected:

37
38

- The response of twenty five pairs of FAA monitor controllers to blunder scenarios was collected at a

Memphis Experimental Facility during a six month effort.

1 Additional data was collected at a second

test facility at the Raleigh-Durham experimental facility. This work and the pilot response studies

were designed and overseen by an experienced human factors scientist.

- The response of FAA and ALPA pilots to blunders and the breakout scenarios was collected using full

motion B727, DC-102

, B7473 and A3204 simulators.

- Final approach lateral deviations for 7000 approaches at Memphis were recorded and the Total System

Navigation Error analyzed. This included coupled and non-coupled approaches.

5

- The accuracy of a 2.4 second scan secondary surveillance radar (SSR) at the Memphis airport and a 1.0

second scan SSR at Raleigh-Durham airport.

6

- Estimates of radio channel blockage statistics from VHF radio recordings at Memphis and Chicago

O’Hare airports.

Simulations

Data was collected during real-time simulations at Raleigh Durham and the FAA Technical Center near

Atlantic City, NJ. Controllers used prototypes of the PRM digital displays to monitor aircraft on final

approach. Traffic was a combination of computer-generated targets and flight data from full-motion

aircraft simulators being “flown” by airline pilots. Selected aircraft would blunder towards the other

approach course, and the controllers would then act to return the blundering aircraft to its approach path

and to turn the other aircraft out of harm’s way

Data collection included how quickly the controller responded after the blunder began, how quickly the

flight crew responded to the controller’s instructions, and the minimum separation between the two

simulated aircraft. Less than 500 feet horizontally or vertically was considered to be a collision.

During these tests it was discovered that pilots flying an MD80 had difficulty disabling the autoland,

delaying their response. In other tests, pilots attempted to use the go-around mode, but switch to the

autopilot heading mode. This delayed the start of the evasion maneuver. Since safety is enhanced by rapid

response (the risk analysis assumed a hand-flown, standard rate evasion turn), this indicated the need to

address flight technique and pilot training.

During the B727, B747 and A320 pilot response studies, pilot preferences on breakout phraseology, flight

procedures, and training were solicited.

Field Demonstrations

Three hundred flight demonstrations were provided by fifty airline pilots at MEM and RDU. The airline

pilots flew the FAA B727 (N40) and experienced staged blunders and breakout maneuvers in visual

conditions, with PRM radar monitoring provided by FAA controllers. The results of those flight

1 FAA Research and Development Services, Precision Runway Monitor Demonstration Report Final

Report DOT/FAARD-91/5, February 1991 2 Ibid 3

Hollister et al, Evaluation of Boeing 747-100 Performance During ATC-Directed Breakouts on Final

Approach, MIT Lincoln Laboratory Project Report ATC-263, 7 January 1998 4 Hollister et al, Airbus A320 Performance During ATC-Directed Breakouts on Final Approach, MIT

Lincoln Laboratory ATC Project Report ATC-265, 20 November 1998. 5

Owen, The Memphis Precision Runway Monitor Program Instrument Landing System Final Approach

Study, MIT Lincoln Laboratory Project Report ATC-194, 24 May 1993. 6 Reference 1.

38
39

demonstrations and the subsequent pilot surveys indicated that the majority of pilots accepted PRM

operations.

The real-time demonstrations also identified potential issues that had not been previously considered. One

of these was the possibility that the controller’s evasion instructions to the pilot could be blocked or garbled

by other pilot transmissions. This led to the current requirement for a second communications frequency, 

to be monitored by the pilot but used only for transmissions by the monitor controller, to ensure that the

evasion instructions are received promptly.

Overall Risk

A Target Level of Safety for PRM approaches was set prior to the simulations. The FAA decided that the

introduction of PRM should add a collision risk of no more than one in twenty-five million approaches,

which is one-tenth that of the perceived ILS accident risk.

7

The risk analysis considered such factors as:

- the rate of blunders on final approach

- the probability that the blunder will involve a turn of 30 degrees or more

- the probability that the aircraft will be non-responsive

- the probability that another aircraft is aligned with the blundering aircraft (in other words, the two

aircraft will collide if there is no evasion)

- the probability that the collision is not averted by pilot/controller action

Based on the Monte Carlo assessment, real-time simulations and other analyses, it was concluded that the

Target Level of Safety would be met even if the blunder rate were as high as one 30-degree blunder per

2000 pairs of simultaneous ILS approaches. There is no official FAA data on the rate of blunders. However,

in the opinion of controllers familiar with operations at airports like Chicago O’Hare and Atlanta, where

simultaneous ILS approaches have been conducted for many years, blunders occur less frequently than this

threshold rate. Therefore, closely spaced parallel ILS approaches with the PRM were accepted by the FAA. 

These approaches were also accepted by ICAO, after a thorough review of the simulation methodology and

analyses.

7 See reference 1, page v.

39
40

Appendix 7

ILS/RNP Separation Standard

The Comparison with a Reference System method is being used to establish the lateral separation between

an ILS course and a RNP course in the RNP Parallel Approach Transition (RPAT) concept. The purpose of

RPAT is to increase capacity to closely spaced parallel runways (typically 750-4300 ft apart). It allows a

dual aircraft stream to closely spaced parallel runways in weather conditions that currently allow only a

single stream.

Figure 2 illustrates the RPAT concept. One aircraft flies the straight-in ILS course, which is aligned with

the runway centerline. The second flies a parallel RNP approach, offset by 5000 ft. At the PFAF point, the

RNP aircraft executes a guided sidestep towards the other approach to align with the second runway. This

concept is similar to the PRM Simultaneous Offset Instrument Approach (SOIA), in that the aircraft fly

instrument procedures through the clouds and then visual procedures to the runways.

While flying through the clouds, the ILS and RNP courses are monitored by controllers in the same way as

standard simultaneous parallel approaches. Prior to the PFAF, the pilot of the RNP aircraft must visually

acquire the ILS aircraft; once the aircraft are clear of the clouds, they are separated visually. This allows

dual approach streams to closely spaced parallel runways when ceilings are as low as 2000 ft. Typically

such dual approach streams must be halted and only a single stream can be used when the ceilings are 

4500 ft or below.

Figure 2. Simultaneous Operations – RPAT and ILS

The minimum separation between ILS and RNP courses has not previously been established. The

preliminary RPAT concept separates these two courses by 5000 ft. The minimum separation between

parallel ILS courses for simultaneous approaches is 4300 ft (with standard radar), so the RPAT concept

provides 700 ft more separation.

40
41

1500 FEET

NTZ

ILS

1150 ft NOZ

NTZ ≥ 4300 ft

1150 ft NOZ

ILS

Figure 3. Parallel ILS Runway Spacing

The preliminary RPAT concept is based on the 4300 ft minimum runway spacing for parallel ILS

approaches. The ILS aircraft is protected by the standard ILS Normal Operating Zone (NOZ), 1150 ft

wide, and the standard 2000 ft wide No Transgression Zone (NTZ). The NOZ for the RPAT aircraft is

assumed to be one times RNP, or 1800 ft for a navigational performance of RNP .38

. The total separation

between the two courses is thus nearly 5000 ft, so 5000 ft is the proposed standard.

The preliminary RPAT concept placed the NTZ closer to the ILS course than to the RPAT course. This is

inconsistent with the standard for parallel ILS approaches, which states that the NTZ should be equidistant

between the approaches. Therefore, the latest RPAT proposal retains the total 5000 ft separation but

centers the NTZ between the courses. This increases the NOZ for the ILS, and decreases the NOZ for the

RPAT course, to 1500 ft.

ILS

Course

5000 ft

RNP

1500 FEET

Visual

Course Clear of Clouds

(Ceiling Minimum)

Separation

Required

8 RNP .3 is equivalent to .3 nautical miles or 1800 ft. In other words, the aircraft is expected to be within

.3 NM of the course centerline 95% of the time.

41
42

Figure 4. Current proposed Simultaneous ILS/RPAT separation

This is an application of the Comparison with a Reference System method. Some members of industry

have largely accepted the proposed course separations because they are greater than the equivalent ILS

course separations. However others are concerned that this course separation is larger than it needs to be,

so the maximum benefits of the procedure will not be realized. In general, the Comparison with a

Reference System approach to safety results in conservative standards that will not produce the dramatic

capacity increases needed to support NGATS, because it limits the new procedure to be similar to an

existing, accepted procedure.

42