The Air Traffic Control 

Operational Errors Severity 

Index: An Initial Evaluation

Larry L. Bailey

David J. Schroeder

Julia Pounds

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

April 2005

Final Report

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1

THE AIR TRAFFIC CONTROL OPERATIONAL ERROR SEVERITY INDEX: 

AN INITIAL EVALUATION

Currently, the FAA Air Traffic Investigations Division 

oversees and coordinates the Operational Error (OE) reporting process governed by the FAA Air Traffic Quality 

Assurance Order 7210.56c (FAA, 2002) that details the 

conduct of incident reporting processes. The need for a 

formal, system-error reporting program was recognized 

early in the FAA’s evolution. Recommendations were 

developed regarding the conduct of incident investigations and the use of the resulting information, many 

of which were incorporated into the FAA Order 8020 

(O’Connor & Pearson, 1965). In 1964, a six-month test 

of an expanded operational error/operational deviation 

(OE/OD) reporting form was conducted at two en route 

centers located in Miami and Jacksonville, Florida. By July 

1965, FAA Order 8020, the AT System Error Reporting 

Program, was revised “to provide more complete error occurrence information and simplify typing with a resultant 

savings in time” (League, 1965). In addition to many 

items resembling those used in the current report, this 

form included a matrix of points assigned to horizontal 

and vertical separation. This information was used to 

categorize the event as major, moderate, or minor, using 

horizontal and vertical proximity. Although this matrix 

was later discontinued, the system was continually revised 

to become the current operational error reporting system 

(FAA Order 7210.56c). The error reporting system has 

demonstrated the relatively infrequent nature of OEs. 

For example, in fiscal year 2000 alone, the U.S. air traffic system handled 166,669,557 operations with an OE 

rate of .69 per 1000,000 facility activities. (FAA, 2002). 

However, concern has been expressed that the error rate 

has gradually increased from .49 in 1997.

In response to a recommendation in the 2000 report 

by the U.S. Department of Transportation’s Office of 

Inspector General (OIG), the FAA developed a method 

to assign severity values for operational errors that occur in 

flight. A memorandum of understanding was negotiated 

between the National Air Traffic Controllers Association 

(NATCA) and the FAA to support this effort. Having 

a tool to group OEs by severity would facilitate the prioritization and allocation of resources to focus attention 

more efficiently on OEs in the different severity groups. 

Later, in 2003, an OIG report (hereafter called the OIG 

report) led to a scientific evaluation of the OE Severity 

Index (SI) and its component parts. We report on the 

results of that evaluation. 

Operational Error Severity Index (SI)

A study of OEs throughout the National Airspace 

System (NAS) served as the basis for development of a 

model that would characterize their severity (Rodgers & 

Nye, 1993). Information from the initial investigation 

was used as the basis for development of the current 

SI. The index is based on the following factors: vertical 

and horizontal separation distances, relative flight paths, 

cumulative closure rates, and the level of ATC awareness 

(hereafter referred to as ATC Control). Since these factors 

are based on operationally relevant criteria associated with 

the provision of ATC services, they allow for the calculation of the severity of air traffic control separation losses. 

These factors are allocated various point values that total 

100 and thereby allow for an assessment of relative safety 

across operational errors.

The current Severity Index formula is computed from 

data that, for the most part, can be objectively determined 

by post hoc incident investigation. Points are assigned for 

varying levels of vertical separation, horizontal separation, 

closure rate, flight paths, and the amount of control. 

Determination of whether the event was controlled or 

uncontrolled is the only variable that requires ATC expert 

judgment to determine a point value. 

Vertical and Horizontal separation are defined as the 

minimum separation based on the radar data just prior 

to aircraft divergence. Together, these factors make up 

to 50% of the SI point total. As shown in Tables 1a 

and 1b, the maximum point values (25 points each) are 

assigned for both vertical and horizontal distances for 

events that culminate in proximities of less than 500 

ft vertical and less than 1⁄2-mile horizontal separation. 

Given existing radar tolerances and measurement tools, 

further division of minimum separation distance would 

be of limited value. Accepting the notion that the safety 

margin decreases geometrically, as opposed to a steady 

arithmetic decline as aircraft get closer, points assessed 

for close encounters remain higher. Conversely, as vertical 

and horizontal separation increase, the potential threat to 

safety is assumed to be lower, and their associated point 

assignments drop off rapidly.

When wake turbulence separation standards govern 

aircraft around an airport, SI point values for vertical 

separation are not calculated. Instead, in-trail separations point values are used, as shown at the bottom of 

Table 1b.
2 

Respective point values for closure rates and flight paths 

were developed to objectively include these parameters 

as components of the Severity Index (Tables 1a-1b). It 

should be noted that faster aircraft closure rates, coupled 

with converging, opposite-direction flight paths reduce 

the safety margin more so than slower closure rates and 

diverging flight paths. Also, a significant assumption 

is made that aircraft on diverging or non-intersecting 

paths have little impact on the overall safety margin and 

consequently do not receive Severity Index points. However, head-on encounters at high rates of speed, coupled 

with minimum radar data separation distance prior to 

divergence, account for the greatest total point value. 

The highest possible point values assigned to closure rate 

and flight path determinations make-up 10% and 20%, 

respectively, of the index.

A final factor, ATC Control, completes the assessment 

of severity for each airborne operational error. While the 

main purpose of the ATC system is to preserve safety, 

determining the adequacy of ATC services assists in understanding shortfalls that occasionally occur; therefore, 

ATC Control is included in the model of error severity. As 

with the other factors, an initial assumption is made that 

maximum point values are assigned when the controller 

was unaware of the conflict, took no corrective action, 

and/or became aware of the conflict but did not have 

sufficient time to effectively mitigate the loss of separation (20% of maximum index score). For encounters in 

which a controller is aware of the impending conflict and 

issues control instructions insufficient to attain required 

aircraft separation distances, minimum point values are 

assigned for this factor, as it is considered a controlled 

event (4% of maximum index score). For events in which 

a Traffic Alert and Collision Avoidance System Resolution Advisory (TCAS- RA) is initiated and the controller 

was taking actions to mitigate the event, an intermediate 

point value is assigned (15% of maximum index score). 

The ATC Control component of the Severity Index was 

introduced in April of 2002. 

While an initial estimate of the overall point total for 

an OE is determined as part of the preliminary report, a 

specialist at FAA Headquarters reviews each of the events 

and assigns the final rating. The point total for an event 

can range from 12 through 100. As a part of this effort, 

three severity levels were initially established. High Severity 

events have a point range from 90 to 100. Moderate events 

range from 40 to 89 points, and Low Severity events have 

a rating of 39 or less. Later, the Moderate category was 

further divided into Low and High Moderate, primarily 

based on whether the controller was aware (Low Moderate) 

or not aware (High Moderate) that an error was occurring 

at the time of the closest loss of separation. 

The following is an example of how the point total 

is used. If an aircraft is in en route airspace at a higher 

altitude, standard separation calls for the minimum separation to involve either a horizontal distance of 5 miles 

or a vertical distance of 2,000 ft. As separation between 

aircraft is reduced to less than the required distance, points 

are assigned for the moment when minimum separation 

occurred. If the aircraft pair was separated by less than 

500 ft vertically (25 points) and between 1 to 1.499 miles 

(24 points), the subtotal would reach 49 points. At a 

closure rate of 300 to 699 knots, an additional 8 points 

would be added. If the flight paths were converging on a 

crossing course, another 18 points would be added. The 

final factor included in the algorithm involves a decision 

regarding the controller’s awareness of the event. If it was 

uncontrolled, a situation where the controller was not 

taking action to avoid the loss of separation, another 20 

points would be added. This is the only factor of the 

Severity Index where subjective judgment plays a role. 

All other information is available from objective sources. 

For this particular event, the point total is 95, placing 

this event in the High Severity category

As part of an evaluation of the FAA’s SI, this report 

focuses on three objectives: (1) to evaluate the SI and 

identify the relationships between components of the SI, 

(2) to examine the aircraft safety margin associated with 

SI point values, and (3) to examine the cut score used to 

differentiate between High Moderate and High Severity 

OEs. The analyses were designed to evaluate the relationships between loss of separation from the prescribed 

standards and the Severity Index, and to identify ways in 

which the index can be used to support efforts to develop 

strategies that will mitigate the severity of OEs.

METHODS AND PROCEDURES

Index Scores

The OE database contained 2,390 OEs that occurred 

between April 2001 and May 2003. Missing values in 394 

cases left a total of 1,996 OEs with valid index scores. 

Of the 1,996 cases, 572 involved the En Route 2,000 ft 

requirement, 723 for the En Route 1,000 ft requirement, 

679 were Terminal and En Route Single Site Radar (TESingle Site), and 22 were Wake Turbulence.

Severity Index Variables

The OE SI is a function of five variables. Vertical 

Separation (feet), Horizontal Separation (miles), Closure 

Rate (knots), Flight Path (converging – opposite course, 

converging – crossing course, same course, diverging/

nonintersecting), and ATC Control (Uncontrolled, Controlled no TCAS RA, and Controlled with TCAS RA). 

The only subjective component is ATC Control.
3

Statistical Analysis

Descriptive statistics were used to assess the distribution 

of the SI components. Correlations between each of the 

rating categories that comprise the SI and the SI scores were 

computed to better understand the relationships between 

the ratings and the overall SI score. For these analyses, 

we used the actual vertical and horizontal separation distances and the ratings for the closure rate category. Flight 

Paths were categorized as follows: Converging– Opposite 

Courses (1), Converging – Crossing Courses (2), Same 

Course (3), and Diverging/Non-Intersecting (4). The 

ATC Control Factor events were categorized in a similar 

fashion: Uncontrolled (1), Controlled with TCAS RA 

(2), and Controlled with no TCAS RA (3). 

To examine the aircraft safety margin, ten loss-ofseparation categories were created based on the percentage of available separation (10% or less through 91% to 

100% of the separation standard) for horizontal and for 

vertical separation. Next, we determined the number and 

percentage of the OEs that fell within each of the ten 

loss-of-separation categories. These were determined for 

each of the three separation standards (En Route 1,000 

ft, En Route 2,000 ft, and Terminal and En Route Single 

Site Radar). Finally, we calculated the average SI score for 

the OEs that were present in each of the ten categories 

defined above. To illustrate these differences graphically, 

we collapsed the ten loss-of-vertical separation categories 

into five categories to simplify the illustration. We then 

plotted the percentage of OEs in those categories against 

the ten horizontal loss-of-separation categories. 

Discriminant analyses were conducted to determine 

whether there was subjective bias associated with differentiating between High Moderate and High Severity 

OEs. The goal of the discriminant analysis was to develop 

an equation that classified OEs into these two categories 

based on the SI components. The classification results 

were then compared with the results of classifying OEs 

using the SI. A high degree of correspondence between 

the comparisons would indicate that the SI is consistent 

with the objective data. Similarly, a low correspondence 

between the comparisons would indicated that the SI 

was inconsistent with the objective data. Moreover, the 

weights assigned to each of the predictors were compared 

with the weights used in the current SI. A high degree of 

correspondence between the comparisons would indicate 

that the SI system of weighting was consistent with empirically derived weights. Objective values of vertical and 

horizontal separation, closure rate, and flight path, along 

with a subjective measure of ATC Control (no objective 

measure exists), were used to classify OEs. However, in 

contrast to the values used in computing correlations, the 

ATC Control factor was dichotomized: 1 = controlled, 

and 2 = uncontrolled. Separate analyses were conducted 

for the three separation conditions: En Route 2,000 ft, 

En Route 1,000 ft, and TE- Single Site. 

It should be noted that the results of the discriminant 

analyses cannot be directly compared with the results of 

the correlations. This is because the two statistics are computed over different portions of the SI. The correlations 

are calculated across the entire SI range. In contrast, the 

discriminant analyses are conducted for SI values greater 

that 79. Since the area of interest for the discriminant 

analysis is the cut score separating High Moderate OEs 

from High Severity OEs, it is important that the focus of 

the discriminate function should be on OEs within the 

region of dispute. If the total range of High Moderate 

(70-89) is used, the classification results will be inflated 

due to the high base rate associated with this category. 

RESULTS

Descriptive Statistics

Descriptive information regarding the components of 

the OE SI is provided in Table 2. There was considerable 

variability in vertical separation, mean of 621 ft and SD 

of 484.49. In contrast, the horizontal separation had a 

SD of 1.34 around the mean of 2.932 miles.

Nearly one-half of the OEs involved a Diverging/

Non-Intersecting flight path where no points were added 

for either closure rate or flight path. With the exception of 

OEs where the closure rate exceeded 699 knots (2.51%), 

the percentage involving the three other closure rate categories was relatively consistent, ranging from 13.53% 

to 18.69% of the OEs. Only a small percentage of the 

OEs involved converging, opposite-direction flight paths 

(5.21%) or Same Course headings (9.12%). Nearly 75% 

were considered to be Uncontrolled events (70.84%). A 

small percentage was considered as a Controlled event 

that involved a TCAS RA (2.15%). Diverging flight 

paths involved a higher percentage of Controlled-No 

TCAS RA events (42.8%).

When severity scores are used and only those OEs that 

have been assessed by this metric to date utilized, the 

distribution reflects a somewhat negative skew as shown 

in Figure 1. A majority of the errors (73.1%) fell in the 

Moderate Severity group. The High Severity category involved 5.47% of the OEs, and the low comprised 21.44% 

of the OEs. This is similar to the sample used for the OIG 

report: 8% high, 72% moderate, and 22% low. 

Correlations between the SI score and the rating 

categories that comprise the index under the three separation standards and across all conditions (overall) are 

presented in Tables 3a through 3d. The small number 

of instances where the OEs involved Wake Turbulence 
4 

made the use of those correlations of limited utility. For 

the three separation standards, correlations between the 

rating categories were generally low to moderate. The 

exception was the consistently high correlations between 

Flight Path and Closure Rate (ranging from -.85 to -.97). 

This is understandable when one considers that the highest 

closure rates occur when aircraft are flying toward each 

other on opposite courses. The negative relationship is 

simply an attribute of the coding system used to classify 

the rating categories in the data analysis. A value of 1 

was assigned to the highest Closure Rate, and a value of 

4 was assigned to a Flight Path that was on a diverging/

non-intersecting course. Another factor that contributed 

to the high correlation is the restricted range of points 

associated with Closure Rate and Flight Path. Nearly half 

of the OEs analyzed received no points for Closure Rate 

and 4 points for Flight Path.

Statistically, all of the correlations between the SI rating categories and the SI scores were significant (p<.01). 

Flight Path exhibited the highest correlations with the SI 

scores, ranging from a -.77 for the TE-Single-Site separation condition to -.78 for En Route 1,000 ft and 2,000 ft 

separation. Correlations between Closure Rate and the SI 

scores were similar to those for the Flight Path measure 

(ranging from .71 to .77). Overall, the lowest correlations 

between the SI rating categories and the SI scores were 

found for vertical separation (ranging from a -.14 to a 

-.41). The higher correlations between the SI and the 

Flight Path and Closure Rate variables suggest that they 

are relatively more important in determining the overall SI 

score. However, the overlap of these two variables suggests 

that once the contribution of one has been considered 

there would be little additional information provided by 

the other. Additionally, correlations between horizontal 

separation and the other SI measures were consistently 

higher than those between those measures and vertical 

separation. This suggests that the horizontal separation 

dimension may play a more prominent role in predicting 

the SI score of an OE than does the vertical separation.

Safety Margin

Figures 2-4 graphically illustrate the number of OEs 

that fell within each of the combined loss of separation 

standards categories. A scan of the En Route 1,000 ft and 

2,000 ft separation figures (Figs. 2 and 3) reveals that a 

majority of the errors lie on the right-hand side involving 60% or more of the horizontal separation. For the 

TE-Single-Site separation condition (Figure 4) the errors 

exhibit greater spread across the two dimensions. Only a 

small number of OEs, from 0.7% (4/571) for En Route 

2,000 ft to 10.5% (71/671) for TE-Single-Site separation, involved 30%, or less, of prescribed horizontal and 

vertical separation. As one could expect, due perhaps to 

the more restricted range of altitudes available for vertical 

separation, there was a higher percentage of OEs under 

the TE-Single-Site separation condition that involved 

less than 50% of the prescribed standards (36.3%) when 

compared with the En Route 1,000 ft (12.3%) and En 

Route 2,000 ft conditions (7.2%). 

Figures 5 and 6 provide information regarding the 

average SI score based on percent of the required horizontal or vertical separation. With the exception of the 

values for the Wake Turbulence condition, the mean SI 

score declined linearly from an average of 80 to 90 for 

the three separation conditions to an average of 36 to 55 

for horizontal separation (Fig. 5). 

Changes in the average SI score for percent of vertical 

separation presented a much different picture (Fig. 6). For 

the categories involving 0 to 50% of the required vertical 

separation, the average SI scores for the three separation 

conditions (En Route 1,000 ft, En Route 2,000 ft, and 

TE-Single-Site) remain separate and fairly stable. Under 

the TE-Single-Site separation standards, the average SI 

score ranged from 76% to around 69% across the 0-

10% through the 41-50% categories of vertical separation. Across the same categories, the average SI score 

ranged from around 59% to 63% for En Route 1,000ft 

The average SI for En Route 2,000 ft also evidenced a 

slight increase from 53 at 1-10% of vertical separation 

to 56 at 51-60%. From the 51-60% percent of vertical 

separation, the average SI score declined markedly to an 

average of 41-47 at 90% and above. The only exception 

is a score of 68 for the TE-Single-Site condition. This is 

the score for a single OE that fell within the 90% and 

above category.

These comparisons reveal the relatively greater importance of loss of horizontal separation in determining the 

overall SI score. SI scores increase in a relatively linear 

fashion for each 10% loss in horizontal separation. These 

changes are similar for each of the separation standards, 

with the exception of Wake Turbulence. Outcomes suggest that once there has been a 50% loss in the prescribed 

vertical separation, the continued loss of vertical separation 

contributes little to any overall increase in the SI score. 

The results also suggest that the loss of vertical separation may have a different influence in determining the 

SI score than horizontal separation. Additionally, loss 

of vertical separation my operate somewhat differently 

under the two En Route and TE-Single-Site separation 

conditions.
5

Rater Bias

Discriminant Analyses were conducted separately for 

the three separation standards: (1) High En Route (with a 

2000 ft requirement, n = 73), (2) Low En Route (having 

a 1000 ft requirement, n = 126), and (3) Low En Route 

(having a 1000 ft requirement, n = 126),TE-Single-Site 

(n = 185). The predictor variables were Vertical Separation (V), Horizontal Separation (H), Flight Path (FP), 

Closure Rate (CR), and ATC Control (ATCC).

The resulting discriminant functions (L) are presented 

below.

High En Route:L = (1.34)V+(.83)H+(0)FP- (.78)CR+(0)ATCC.

Low En Route: L = (1.09)V+(1.02)H-(.77)FP- (.52)CR+(0)ATCC.

TE-Single-Site: L = (.61)V+(1.03)H-(.14)FP- (.34)CR+(0)ATCC.

As can be seen, the beta weights for elements of the 

SI that were included in the equations for the High En 

Route and Low En Route were similar. The only difference was that Flight Path played a more important role 

for Low En Route than for High En Route. Values for the 

High and Low En Route conditions differed substantially 

from the values included in the TE-Single-Site analysis. 

Horizontal separation played a much more critical role for 

the TE-Single-Site separation condition than in either of 

the other conditions. Overall, classification accuracy was 

85% for both the Low and High En Route conditions. 

For the TE-Single-Site condition, classification accuracy 

was 92%. In all instances, these values were above those 

expected from the population base-rate. It should also 

be noted that weights of zero applied to ATCC and FP 

do not mean that these variables were not important 

classification variables. It just means that for SI values 

greater than 79, these variables became a constant and, 

thus, were not useful for differentiating between High 

Moderate and High Severity OEs.

CONCLUSION

The analyses conducted in an initial evaluation of the 

FAA’s Severity Index (SI) reveal that a complex relationship exists among the individual components comprising the SI and the different separation standards within 

which the SI is applied. However, a consistent finding 

was the importance of horizontal separation to the overall 

SI score. The more horizontal separation there was, the 

less severe was the OE. This was especially true when 

using the TE-Single-Site Separation Standard. This 

conclusion was further supported by the results of the 

discriminant analyses. Greater weight was assigned to 

horizontal separation when classifying OEs into High 

Moderate and High categories using the TE-Single-Site 

Separation Standard.

Despite the importance of horizontal separation, the 

results of the discriminant analyses revealed that improvements to the SI might be obtained by developing a different 

weighting system for the three separation standards. For 

example, in Low En Route, the evidence suggested that 

vertical and horizontal separation should be weighted the 

same (as reflected in the current SI). Within High En 

Route, the evidence suggested that more weight should 

be given to vertical separation compared to horizontal 

separation. However, as previously mentioned, within 

TE-Single-Site more weight should be given to horizontal 

separation compared to vertical. It should be noted that 

this suggestion is based on analyses conducted for SI values 

greater than 79. Additional research is needed to explore 

this matter more fully and to determine whether similar 

results can be achieved using the entire range of the SI.

Finally, this evaluation of the FAA’s Severity Index 

revealed that the SI provides a rational approach for 

categorizing the severity of ATC Operational Errors. 

Although questions remain as to the SI cut scores used 

to categorize OEs, it is recommended that they not be 

changed unless objective measures can be developed that 

support those changes. With the exception of the ATC 

Control component, the remaining four components 

are objective and are derived from performance characteristics of the aircraft involved in the OE. Additional 

research is needed to determine if the development of a 

checklist for the classification of the ATC Control factor 

would ensure a higher consistency of classification across 

individual controllers.

REFERENCES

Federal Aviation Administration (June 2003). Administrator’s fact book. Washington, DC: Author.

Federal Aviation Administration (June 1983). ATC system error reporting program (FAA Order 8020.31). 

Washington, DC: Author.

Federal Aviation Administration (2002). Air Traffic Quality Assurance (FAA Order 7210.56c). Washington, 

DC: Author.

League, A. (1965). Revisions to ATC system error reporting 

program (FAA Order, 8020.3). Washington DC: 

Federal Aviation Administration.
6 

O’Connor, W.F., and Pearson, R.G. (1965). ATC system 

error and appraisal of controller proficiency. Washington, DC: Federal Aviation Administration Office 

of Aviation Medicine (Report No. 65-10).1

Rodgers, M.D., and Nye, L.G. (1993). Factors associated 

with severity of operational errors at air route traffic 

control centers. In M.D. Rodgers (Ed.), An examination of the operational error database for air route 

traffic control centers (pp. 11-25). Washington, DC: 

Federal Aviation Administration Office of Aviation 

Medicine (Report No. 93/22).1

1

This publication and all Office of Aerospace Medicine technical 

reports are available in full-text from the Civil Aerospace Medical 

Institute’s publications Web site: 

http://www.cami.jccbi.gov/aam-400A/index.html
7

Table 1a. Radar OE severity index en route chart 

VERTICAL SEPARATION 

1,000 feet required 

POINTS HORIZONTAL SEPARATION 

5-mile separation requirement 

POINTS 

Less than 500 feet 25 Less than 1⁄2 mile 25 

500 feet to 599 feet 20 1⁄2 mile to 0.999 mile 25 

600 feet to 699 feet 16 1 mile to 1.499 miles 24 

700 feet to 799 feet 12 1.5 miles to 2 miles 24 

800 feet to 899 feet 6 2 miles to 2.49 miles 23 

900 feet to 999 feet 2 2.5 miles to 2.999 miles 22 

VERTICAL SEPARATION 

2,000 feet required

3 miles to 3.499 miles 20 

Less than 500 feet 25 3.5 miles to 3.999 miles 16 

500 feet to 599 feet 25 4 miles to 4.499 miles 10 

600 feet to 699 feet 24 4.5 miles to 4.999 miles 5 

700 feet to 799 feet 24 CLOSURE RATE POINTS 

800 feet to 899 feet 23 700 knots and greater 10 

900 feet to 999 feet 22 300 knots to 699 knots 8 

1,000 feet to 1,099 feet 20 100 knots to 299 knots 6 

1,100 feet to 1,199 feet 18 Less than 100 knots 4 

1,200 feet to 1,299 feet 16 FLIGHT PATHS POINTS 

1,300 feet to 1,399 feet 14 Converging  Opposite Courses 20 

1,400 feet to 1,499 feet 12 Converging  Crossing Courses 18 

1,500 feet to 1,599 feet 10 Same Course 10 

1,600 feet to 1,699 feet 8 Diverging/Non-Intersecting 0 

1,700 feet to 1,799 feet 6 ATC CONTROL FACTOR POINTS 

1,800 feet to 1,899 feet 4 Uncontrolled 20 

1,900 feet 10 1,999 feet 2 Controlled with TCAS RA 15 

 Controlled with no TCAS RA 4 

Standard separation in the en route environment is 5 nautical miles horizontally and 1,000 ft vertically up to 

29,000 ft, and 2,000 feet vertically above 29,000 feet. All references to miles in this report are nautical 

miles. A nautical mile is equivalent to 6,076 ft. 

TABLES AND FIGURES
8 

Table 1b. Radar OE severity index terminal and en route single-site chart (TE-Single-Site) 

VERTICAL SEPARATION POINTS HORIZONTAL SEPARATION 

3-mile separation requirement 

POINTS 

Less than 500 feet 25 Less than 1⁄2 mile 25 

500 feet to 599 feet 20 1⁄2 mile to 0.999 mile 18 

600 feet to 699 feet 16 1 mile to 1.499 miles 14 

700 feet to 799 feet 12 1.5 miles to 2 miles 10 

800 feet to 899 feet 6 2 miles to 2.49 miles 6 

900 feet to 999 feet 2 2.5 miles to 2.999 miles 2 

CLOSURE RATE POINTS HORIZONTAL SEPARATION 

2.5-mile requirement 

POINTS 

700 knots and greater 10 Less than 1⁄2 mile 25 

300 knots to 699 knots 8 1⁄2 mile to 0.999 mile 20 

100 knots to 299 knots 6 1 mile to 1.499 miles 16 

Less than 100 knots 4 1.5 miles to 1.999 miles 10 

FLIGHT PATHS POINTS 2 miles to 2.499 miles 4 

Converging  Opposite Courses 20 ATC CONTROL FACTOR POINTS 

Converging  Crossing Courses 18 Uncontrolled 20 

Same Course 10 Controlled with TCAS RA 15 

Diverging/Non-Intersecting 0 Controlled with no TCAS RA 4 

* When wake turbulence separation standards are governing, Do Not include any vertical point value. 

Instead use the appropriate in trail separation index below, as well as other applicable factors (FAA Order 

7210.56, section 6-1-5). 

IN TRAIL SEPARATION 

4-mile separation requirement 

POINTS IN TRAIL SEPARATION 

5-mile separation requirement 

POINTS 

3.49 miles or less 60 4.499 miles or less 60 

3.5 miles to 3.999 miles 35 4.5 miles to 4.999 miles 35 

IN TRAIL SEPARATION 

4-mile separation requirement 

POINTS 

5.499 miles and less 60 

5.5 miles to 5.999 miles 35 
9

Table 2. Distribution of OEs across SI categories 

Variable Sub Category Frequency Percent Mean Std. Deviation 

Vertical Sep. 621.14 ft. 484.49 

Horizontal Sep. 2.93 miles 1.34 

Closure Rate 

700 or more 

Knots

50 2.51% 

300699 kts 270 13.53% 

100299 kts 352 17.63% 

Less than 100 kts 373 18.69% 

(Diverging) 951 47.64% 

Flight Path 

Conv.  Opposite 104 5.21% 

Conv.  Crossing 777 38.93% 

Same Course 182 9.12% 

Diverging/NonIntersecting933 46.74% 

ATC Control 

Uncontrolled 1,414 70.84% 

Controlled  

TCAS RA 

43 2.15% 

Controlled  

No TCAS RA 

539 27.00% 

Table 3a. Correlations between SI measures for the En Route 1,000 ft Separation Standard 

SIVERT SIHORZ CTLFAC FLTPATH CRATE 

SIVERT 

SIIHORZ -.118* 

CTLFAC -.134* .368* 

FLTPATH -.047 .333* .300* 

CRATE .058 -.326* -.281* -.935* 

SI INDEX -.216* -.586* -.614* -.782* .758* 

* p<.01 

Table 3b. Correlations between SI measures for the En Route 2,000 ft Separation Standard 

SIVERT SIHORZ CTLFAC FLTPATH CRATE 

SIVERT 

SIIHORZ -.303* 

CTLFAC -.048* .326* 

FLTPATH -.145* .448* .232* 

CRATE .152* -.439* -.222* -.971* 

SI INDEX -.144* -.589* -.623* -.782* .773* 

* p<.01 
10 

Table3c. Correlations between SI measures for the TE-Single-Site Separation Standard 

SIVERT SIHORZ CTLFAC FLTPATH CRATE 

SIVERT 

SIIHORZ .266* 

CTLFAC .125* .350* 

FLTPATH .134* .476* .304* 

CRATE -.056 -.382* -.272* -.860* 

SI INDEX -.411* -.627* -.597* -.770* .713* 

* p<.01 

Table 3d. Correlations between SI measures across all conditions 

SIVERT SIHORZ CTLFAC FLTPATH CRATE 

SIVERT 

SIIHORZ .147* 

CTLFAC .082* .401* 

FLTPATH .119* .539* .337* 

CRATE -.045 -.472* -.307* -.928* 

SI INDEX -.320* -.640* -.635* -.794* .766* 

* p<.01 

0

0.9

5.32

15.22

8.45

14.67 14.42

17.8

14.62

5.47

0

2

4

6

8

10

12

14

16

18

20

1 - 9 10 - 19 20 - 29 30 - 39 40 - 49 50 - 59 60 -69 70 - 79 80 - 89 90 - 99

Severity Index Value

Percent of OEs

Low Low Moderate High Moderate High

0

0.9

5.32

15.22

8.45

14.67 14.42

17.8

14.62

5.47

0

2

4

6

8

10

12

14

16

18

20

1 - 9 10 - 19 20 - 29 30 - 39 40 - 49 50 - 59 60 -69 70 - 79 80 - 89 90 - 99

Severity Index Value

Percent of OEs

0

0.9

5.32

15.22

8.45

14.67 14.42

17.8

14.62

5.47

0

2

4

6

8

10

12

14

16

18

20

1 - 9 10 - 19 20 - 29 30 - 39 40 - 49 50 - 59 60 -69 70 - 79 80 - 89 90 - 99

Severity Index Value

Percent of OEs

Low Low Moderate High Moderate High

Figure 1. Distribution of Severity Index values. 
11

Figure 2. Distribution of operational errors ( n= 719) by percent of required horizontal and 

vertical separation for low altitude en route with a 1000 ft vertical separation standard. 

Low Altitude En Route

0

10

20

30

40

50

0 10 20 30 40 50 60 70 80 90 100

% of Required Horizontal Separation

No. of Operational Errors

10-20%

30-40%

50-60%

70-80%

90-100%

% Required Vertical 

Separation

Figure 3. Distribution of operational errors (n = 571) by percent of required horizontal and 

vertical separation for high altitude en route with a 2000 ft vertical separation standard. 

High Altitude En Route 

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90 100

% of Required Horizontal Separation

No. of Operational 

Errors

10-20%

30-40%

50-60%

70-80%

90-100%

% Required Vertical 

Separation
12 

Figure 4. Distribution of operational errors (n = 671) by percent of required horizontal and 

vertical separation for Terminal and Single-Site Radar separation standard. 

Terminal and Single Site Radar

0

10

20

30

40

50

0 10 20 30 40 50 60 70 80 90 100

% of Required Horizontal Separation

No. of Operational Errors

10-20%

30-40%

50-60%

70-80%

90-100%

% Required Vertical 

Separation

30

40

50

60

70

80

90

100

10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Percent of Required Horizontal Separation

Mean Index Score

En Route 1000ft

En Route 2000ft

TE-Single Site

Wake Turbulence

Figure. 5. Average SI score based on percent of prescribed horizontal separation. 
13

35

40

45

50

55

60

65

70

75

80

10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Percent of Required Vertical Separation

Mean Index Score

En Rroute 1000ft

En Route 2000ft

TE-Singe-Site

Figure 6. Average SI score based on percent of prescribed vertical separation.