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depart Hi. FAA-RD-78-76

VORTEX ADVISORY SYSTEM SIMULATION OF
CHICAGO O'HARE INTERNATIONAL AIRPORT

Barry E. Keeffe

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TRANSPORTATION LIBRARY

NOV 9 197R
NORTHWESTERN UNIVERSITY

JULY 1978
FINAL REPORT

Document is available to the U.S. public through

the National Technical Information Service,

Springfield, Virginia 22161.

Prepared for
U.S. DEPARTMENT OF TRANSPORTATION

FEDERAL AVIATION ADMINISTRATION

Systems Research & Development Service

Washington, D.C. 20590

Tl_ 574.W3 V957

3 5556 021 008 958

NOTICE

The United States Government does not endorse products
or manufacturers. Trade or manufacturer's names appear
herein solely because they are considered essential to
the object of this report.

Technical Report Documentation Pag

1. Report No.

-FAA-RD-78-76

2. Government Accession No.

4. Title and Subtitle

LVORTEX ADVISORY SYSTEM SIMULATION OF
CHICAGO O'HARE INTERNATIONAL AIRPORT

7. Author's)

Barry E. Keeffe

3. Recipient's Cotolog No.

5. Report Date

July 1978

6. Performing Organization Code

8. Performing Organization Report No.

FAA-NA-78-12

9. Performing Organization Name and Address

.^Federal Aviation Administration

-National Aviation Facilities Experimental Center
Atlantic City, New Jersey 08405

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

084-451-500

12. Sponsoring Agency Name and Address

U.S. Department of Transportation
Federal Aviation Administration
Systems Research and Development Service
Washington, D.C. 20590

13. Type of Report and Period Covered

Final
March 1977 to June 1977

14. Sponsoring Agency Code

15. Supplementary Notes

16. Abstract

This report evaluates, in simulation, the procedural implications of the Vortex
Advisory System (VAS) on the Chicago O'Hare terminal air traffic control environ-
ment. It also attempts to demonstrate any cost benefits/capacity gains which may
accrue using reduced VAS aircraft separation criteria on the final approach course
based upon meteorological assurance of vortex dissipation. Utilizing the National
Aviation Facilities Experimental Center (NAFEC) Digital Simulation Facility (DSF) ,
a real-time simulation of the Chicago O'Hare International Airport airside opera-
tions was conducted between March 28 and June 24, 1977. There were 105 data runs
of 1 hour and 20 minutes duration completed during this period. Two favored runway
configurations were identified (based on O'Hare usage data during 1977) and exer-
cised in the VAS green, standard red, and transition conditions; that is, the
green condition reducing separation between all aircraft classes to 3 nmi, the red
condition maintaining 3-, 4-, 5-, and 6-nmi standard heavy- jet separation; and the
transition between the two conditions under instantaneous and 5-minute warning
transitions were evaluated. Three vortex clear zones were exercised: middle marker
to touchdown, outer marker to touchdown and 20-nmi fix to touchdown. Test results
indicate that (1) no procedural implications emerged which would deter the implemen-
tation of VAS at Chicago O'Hare Airport, and (2) arrival rate increases are suffi-
cient to support previous cost/benefit analysis studies conducted by Landrum and
Brown Aviation Consultants and Mitre Corporation.

17. Key Words

Vortex Advisory System
Airport Safety
Flight Safety
Wind Dissipation

19. Security Classif. (of this report)

Unclassified

18. Distribution Statement

Document is available to the public
through the National Technical
Information Service, Springfield,
Virginia 22151

20. Security Classif. (of this page)

Unclassified

21. No. of Poges

22. Price

Fo«n DOT F 1700.7 (8-72)

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TABLE OF CONTENTS

Page

INTRODUCTION 1

Purpose 1

Background 1

System Feasibility 2

System Description 2

SYSTEM EVALUATION 5

Details of Approach 5

Test Measures 6

RESULTS 7

Series I 7

Series II 8

Series III 10

Series IV 11

SUMMARY OF RESULTS 13

CONCLUSIONS 15

REFERENCES 16

APPENDICES

A - Comparison Table of Arrival Aircraft Classes and Pairing Mix

B - Speed Profiles (knots) at Threshold and Outer Marker for
Small, Heavy, and Large Weight Classes

C - Predicted Gains (in Arrivals per Hour)

xxi

LIST OF TABLES

Table Page

1 Average Arrival Operations per Hour in Red Condition 8

2 Baseline (Red Condition) Percentage of Aircraft Pairs and 9

Median Separation

3 Average Number of Arrivals per Hour in Green Condition 9

4 Comparison of Percentage of Aircraft Pairs and Median 10

Pair Separation for VAS Green and Baseline Red Conditions

5 Average Number of Arrivals per Hour in Green/Red Condition 11

6 Average Number of Arrivals per Hour for Runway Changeover 12

Hour

7 Controller Communications 13

8 Controller ATC Activities 14

9 Separation Criteria Violations for Total Aircraft Pairs 14

INTRODUCTION

PURPOSE.

The purpose of this report is to summarize the results of a National Aviation
Facilities Experimental Center (NAFEC) simulation of the Vortex Advisory
System (VAS) . The simulation was designed to determine the procedural impli-
cations of the VAS on the air traffic control (ATC) system within the Chicago
O'Hare International Airport terminal environment.

BACKGROUND.

The introduction of wide-bodied jet aircraft, coupled with an increase in
airport total operations, has given added significance to the aircraft wake
vortex problem. The trailing wake vortices from a large aircraft can pose a
serious hazard to smaller aircraft. The smaller aircraft encountering one of
these wake vortices may be subjected to extreme rolling movements which can
result in a loss of control, a loss of altitude, and possibly structural
damage. At present, to reduce the possibility of this occurring, and thus
maintain safety standards, separation between selected aircraft pairs has been
increased from previous standards. This, of course, has resulted in a reduc-
tion of operations below theoretical airport capacity.

Until recently, limited knowledge concerning the characteristics and behavior
of wake vortices mandated these larger separation standards. Studies and
tests sponsored by the Federal Aviation Administration (FAA) have shown that
vortex characteristics are established initially by the aircraft's gross
weight, airspeed, flight configuration, and wingspan. Subsequently, it has
been found that vortex characteristics are altered, and eventually dominated,
by interactions between the vortices and the ambient atmosphere. This was
determined from analysis of over 50,000 vortex tracks entered into the data
base at Transportation Systems Center (TSC) , and it has led directly to the
development of the VAS, a system designed to expedite ATC operations while
exercising vortex avoidance.

It has been established that the concept of wake vortex avoidance is based
on three considerations supported by available wake turbulence data.
These are:

1. For a large percentage of the time, prevalent meteorological conditions
exist which cause vortices to move quickly off the flightpath, or decay rapidly
in the approach corridor, thus posing no hazard to aircraft following on the
same flightpath.

2. The duration, intensity, and movement of vortices can be reliably predicted
if adequate knowledge of existing meteorological conditions is obtained.

3. Vortices can be detected and tracked at selected points along the approach
or departure paths through the use of existing sensing techniques.

From these considerations, it became evident that airport operations are need-
lessly limited by using larger separation standards for wake vortex avoidance
when no real hazard exists, since vortex behavior can be predicted with
sufficient accuracy to permit selection of appropriate smaller separation
standards.

From analysis of the extensive data collected on vortex behavior as a function
of meteorological conditions, a wind criterion has been developed and tested
to determine when aircraft separations can be uniformly reduced to 3 nautical
miles (nmi) between all aircraft types, rather than the 3-, 4-, 5-, and 6-nmi
separations currently required between approach aircraft pairs of various
weight differentials. The VAS was designed to take advantage of this wind
criterion. The VAS measures wind magnitude and direction (with respect to each
runway heading) for comparison with the wind criterion. The comparison indi-
cates, via a simple red/green light display in the ATC room, when aircraft
separations can be safely reduced to 3 nmi for all traffic.

SYSTEM FEASIBILITY.

Chicago's O'Hare International Airport was selected for prior system feasibility
tests based on the following criteria: adequate available real estate for the
VAS equipment, operations near saturation during visual flight rules (VFR) and/
or instrument flight rules (IFR) conditions, and a significant percentage of
large aircraft in the traffic mix. The VAS was feasibility tested at O'Hare
by using an instrumentation system to measure vortex positions and ambient
meteorological conditions as a function of time, and correlate these with the
VAS separation criteria. The amount of time that the VAS indicated that reduced
separations could be used was evaluated to determine how many additional opera-
tions could be accommodated if reduced separations were used and the dollar
saving which would accrue from reduced delays. This evaluation was performed
considering all the usable combinations of approach and landing runway scenarios
under both VFR and IFR weather conditions. Results from this previous study
statistically validated that the system could accurately determine when 3-nmi
separation could be used for all aircraft types, and that the criterion
algorithm contained adequate safety margins for all meteorological and vortex
conditions.

SYSTEM DESCRIPTION.

METEOROLOGICAL TOWERS. The VAS incorporates a network of instrumented meteor-
ological towers positioned to measure the wind in each operating airport corri-
dor. A network of towers is required since variations in atmospheric condi-
tions preclude the use of a single, centrally located sensor for measuring
wind in individual approach corridors. Each tower is instrumented with three
sensors for wind magnitude and direction, one at the 50-foot level and the
other two at the 47-foot level. The 47- foot-level sensors are mounted on
opposite sides of the tower to provide a measurement undisturbed by tower
shadowing. All sensor and communication electronics at each tower are housed
in an environmental enclosure. The towers are free-standing on a concrete
base and are marked and lighted.

TOWER DATA COMMUNICATION SUBSYSTEM. Transmission of the data from the towers
to the centrally located processors is accomplished with standard hardware.
A multiplexer successively samples the sensor outputs and converts them to
digital words which are serialized and transmitted over standard existing FAA
control lines to a central facility. Receivers reconvert the data to a
parallel format for input to a microprocessor. The sampling rate is 160 words
per second, each word being 16 bits in length.

METEOROLOGICAL DATA PREPROCESSING. Individual microprocessors are used to
process the data from each meteorological tower. The microprocessors contain
8k bits of read-only memory and 2k bits of random-access memory. Each micro-
processor is packaged on a single plug-in board. The microprocessors sample
the meteorological data output from each receiver at a sample interval rate
of two samples per second. The sampled wind magnitude (R) and wind direction
(9) are computed as a 64-second (128-sample) running average (R and 0).

The wind magnitude and direction readings from the three sensors on each tower
are compared after each sampling interval. A sensor-failure bit is generated
if the magnitude of any sensor differs by more than 3 knots or if the direction
of any sensor differs by more than 10°. Normally, the 50-foot-level sensor
data are used for the VAS algorithm. If the 50-foot-level sensor fails, the
microprocessor switches to a 47-foot- level sensor and selects the sensor which
is not in the wind shadow of the tower. Failure of two sensors to agree
terminates all data output from that tower. Upon detection of a failure, a
failure word is generated identifying the sensor or tower which has been shut
down.

The microprocessors also calculate the recommended aircraft landing separations
for each runway based on the windspeed and wind direction measured by the
instrumented tower. As shown in figure 1, an elliptical VAS algorithm is used
which includes a buffer or "transition zone." The major and minor axes are
12.5 and 5.5 knots for the inner ellipse, and 14.5 and 7.5 knots for the outer
ellipse. The transition zone is designed to preclude rapid oscillations between
standard (red) and reduced separation (green) conditions, thus avoiding an
unmanageable controller workload while still insuring the required level of
safety from vortex effects.

The criteria for separation and for changing separation from one condition to
another are:

(a) If the wind vector (R, e) is inside the inner ellipse, the condition
is red, and standard 3-4-5-6-nmi separation applies.

(b) If the wind vector (R, 0) is outside the outer ellipse, the green
condition exists, and all aircraft can be separated by 3 nmi.

(c) If the condition is red and the wind is increasing, the requirement
exists for the wind vector to be outside the outer ellipse for

64 seconds before the green condition is indicated.

(d) If the condition is green and the wind vector enters the inner
ellipse region, a change to the red condition takes place immediately.

14% KNOTS

TRANSITION ZONE

7% KNOTS

R = WINDSPEED

S = WIND DIRECTION

78-12-1

FIGURE 1. VAS ALGORITHM WIND CRITERION

SYSTEM EVALUATION

DETAILS OF APPROACH.

LABORATORY REQUIREMENTS.

General. The Digital Simulation Facility (DSF) ATC laboratory at NAFEC
was configured to simulate an Automated Radar Terminal System (ARTS III)
terminal ATC system. Two-way radio communication between controllers and
pilots as well as coordination lines between control positions were simulated
using the Bell 300 communication system.

Controller Positions. Two arrival, one departure, one tower/monitor, and
two enroute feeder positions were established. The arrival positions displayed
video for a radius of 35 nmi, the tower and departure positions displayed video
for a 20-nmi radius, and the two enroute positions displayed a ■ 50-nmi radius.
Keyboard functions were available at all positions.

Simulator Pilot Positions. Eighteen simulator pilot positions were
required. Controlled aircraft were assigned to all 18 positions. Keyboard
functions were available at all pilot consoles.

Geography. The area simulated was the Chicago O'Hare International
Airport Approach Control Area, with parallel approaches to the southeast on
runways 14L and 14R, dual approaches to the west and northwest on runways
27R and 32L, and parallel approaches to the east on runways 9L and 9R. All
runways had ILS capability.

Departure configurations were to the east off runways 9L and 9R, to the
west and northwest off runways 27L and 32R, and to the northeast off runway
4L and 4R.

Traffic flows were in accordance with procedures presently employed at
Chicago O'Hare International Airport as described in the Chicago ARTCC/O'Hare
Tower Letter of Agreement, dated July 28, 1976, and O'Hare Tower Order 7110. 6D,
entitled "Chicago Approach Control - Radar."

The field elevation for the airport was 700 feet mean sea level. The
video map depicted airport runways, boundaries, fixes, navigation aids
(NAVAID's), and descent areas.

Sigma V Computer. The Sigma V was used in conjunction with the graphic
digitizer to develop video maps and flight plans. It was utilized on a daily
basis for conduct of the simulation and data reduction and analysis.

PROCEDURES .

Controller Procedures. Standard ATC procedures as provided for in
FAA Handbook 7110.65 were applied. Detailed controller instructions on
Chicago O'Hare arrival and departure procedures were defined in a handout prior

to start of simulation. Those special controller procedures required when
VAS was present in the system (i.e., reduced aircraft separation standards)
were described for all controller personnel in briefings prior to simulation.

Simulator Pilot Procedure. Prior to the start of dynamic simulation, all
simulator pilots received a detailed briefing on the purpose and objectives of
the simulation. They were briefed on detailed pilot procedures and their
expected inputs.

TRAFFIC SAMPLES. The traffic samples, representative of O'Hare traffic, were
all controlled IFR aircraft. Traffic density was 150 aircraft per hour (9.0
arrival, 60 departure). The arrival aircraft were a mix of large (70. percent),
heavy (20 percent), and small (10 percent).

TEST MEASURES.

A complete history of each simulated flight was recorded on magnetic tape.
During each update cycle, elements of track position, track movement, flight
status, pilot keyboard messages, display tables, and communication line usage
were recorded. These data were analyzed utilizing offline data reduction
programs to provide quantitative measurements of system performance, capacity,
separation, and workload.

ATC SYSTEM PERFORMANCE MEASURES. Hourly operation rates were obtained for
(1) arrivals, (2) departures, and (3) combined arrivals/departures. Average
time in system for a completed arrival flight (in minutes) was obtained for
each 1-hour data period.

Total path-stretch delay (in minutes) was ascertained. This measure is the
difference between the nominal and actual flight times of completed arrivals in
the 1-hour data period.

Terminal approach path separation was measured every 20 seconds between aircraft
on the final approach for the following vortex clear zones: (1) threshold to
middle marker (MM), (2) threshold to outer marker (OM) , and (3) threshold to
20-nmi point on localizer.

The number of missed approaches under green and red conditions were determined.
The number of separation criteria violations were analyzed to determine (1)
total number of criteria violations, (2) aircraft in violation by controller
position, (3) time of violation, and (4) degree of separation criteria vio-
lation.

CONTROLLER WORKLOAD MEASURES. The following measures were taken:

1. Number of aircraft controlled per control position during 1-hour data
period.

2. Average number of radar vectors issued per control position per aircraft.

3. Average number of altitude changes issued per control position per aircraft
controlled during the data hour.

4. Average number of speed changes issued per control position per aircraft
controlled during the data hour.

5. Average number of control messages per control position per aircraft
controlled during the data hour.

6. Total aircraft time per control position of aircraft controlled during
the data hour.

7. Average duration of radio communications (controller to pilot) made per
aircraft controlled per control position.

VAS IMPACT MEASURES. Additional data reduction and analysis were required to
characterize quantitatively VAS activities in terms of the following:

a. Arrival rates

b. Operational procedures

c. Configuration changes

d. Separation standards

e. Flight deviations caused by VAS system changes and the effect on
traffic flow.

RESULTS

SERIES I.

Series I testing consisted of heavy-density (90 arrivals, 60 departures) IFR
traffic. These tests provided the baseline data on airport operations using
present-day (red condition) separation minima and control techniques (speed
and spacing criteria) as employed at the Chicago O'Hare Airport during the
final approach phase of a flight. Baseline data were obtained after 18 simu-
lation runs.

To insure validity, the baseline data were measured against a statistical
analysis of 0' Hare's Performance Measurement System (PMS) Summary Sheets for
the year 1977. The results are given in table 1. The baseline data show that
the simulation operations compared favorably with the PMS of present-day O'Hare
operations.

Additionally, the baseline data were broken down by aircraft class, numbers of
pairs (heavy-small, heavy-large, etc.), and the median separations employed.

Aircraft were categorized into three classes based on weight criteria:

Heavy - over 300,000 pounds gross weight at takeoff

Large - between 12,500 pounds and 300,000 pounds gross weight at takeoff

Small - up to 12,500 pounds gross weight at takeoff

TABLE 1. AVERAGE ARRIVAL OPERATIONS PER HOUR IN RED CONDITION

Arrival Runway Configuration Simulation Baseline PMS Analysis

Runways 14L and 14R 71 70.1

Runways 27R and 32L 69.5 (peak periods of

Combined average, runways 14L-14R and 70.3 density, all

32L-27R configurations)

The mix of aircraft pairs studied was identified by separation minima required
between various classes:

Lead Aircraft - Trail Aircraft Separation Required

Heavy - small 6 nmi

Heavy - large 5 nmi

Heavy - heavy 4 nmi

Large - small 4 nmi

All others* 3 nmi

* (Large-large, small-large, small-small, small-heavy, and large-heavy)

The median separation distances were measured both inside and outside the outer
marker on the final approach defined as follows:

Inside outer marker: the distance between lead aircraft and trailing aircraft
based on lead aircraft last reported data position inside the outer marker
prior to touchdown.

Outside outer marker: the distance between lead aircraft and trailing aircraft
based on lead aircraft last reported data position prior to passing outer
marker inbound.

The baseline data descriptive statistics are given in table 2.

SERIES II.

Series II testing consisted of heavy-density (90 arrivals, 60 departures) IFR
traffic. These tests provided data on the procedural implications or impact
of VAS reduced separation on the ATC system and comparison data on arrival
operations gains or losses when weighed against baseline capacity data. The
tests were conducted using the VAS in the green mode with reduced separation
criteria employed within three distinct vortex clear zones on the final
approach (threshold to MM, threshold to OM, and threshold to 20-nmi fix on
localizer. The vortex clear zone is the portion of final approach corridor
over which the VAS predicts vortex dispersion within time parameters consonant
with a 3-nmi minimum aircraft separaton criterion. A total of 45 runs was
accomplished. Table 3 indicates the arrival rates that were achieved.

TABLE 2. BASELINE (RED-CONDITION) PERCENTAGE OF AIRCRAFT PAIRS AND MEDIAN
SEPARATION

Inside OM

Lead a/c - Trail a/c

Heavy-small
Heavy- large
Heavy-heavy
Large- small
All others

Percent of Total Pairs Median Separation (nmi)

5.6
5.0
4.1
4.2
3.7

Outside OM

Lead a/c - Trail a/c

Heavy-small
Heavy-large
Heavy-heavy
Large- small
All others

6.3
5.6

4.5
4.8
4.3

TABLE 3. AVERAGE NUMBER OF ARRIVALS PER HOUR IN GREEN CONDITION

Arrival

Runway

(Red)

Threshold-

Configuration

Baseline
71

Threshold-
72.6

-MM

Threshold-OM
71.7

20 nmi

Runways 14L-14R

72.7

Runways 27R-32L

69.5

73.0

74.6

80.0

Combined average,

70.3

72.8

73.2

76.4

runways 14L-14R and

27R-32L

From table 3 it can be seen that increases in hourly operations were effected
in every case for green verses red (baseline) conditions. This is supportive
of MITRE and TSC predictions in this area. An analysis of types and numbers
of pairs and the median separation employed inside and outside the OM under
the green condition are presented in table 4.

No procedural changes were required which would Impact the ATC system or deter
the implementation of VAS reduced separation at Chicago O'Hare Airport during
this test series.

TABLE 4. COMPARISON OF PERCENTAGE OF AIRCRAFT PAIRS AND MEDIAN PAIR SEPARATION
FOR VAS GREEN AND BASELINE RED CONDITIONS

Lead-Trail Aircraft

Inside OM
Percent of Total Pairs

Threshold to
Base-
line MM OM 20 nmi

Median Separation (nmi)

Threshold to
Base-
line MM OM 20 nmi

Heavy- small

5.6

4.4

3.9

3.3

Heavy-large

5.0

4.5

4.0

3.7

Heavy-heavy

4.1

3.6

3.7

3.3

Large-small

4.2

3.9

3.6

All others

70
Out

75
side OM

3.7

3.4

3.7

3.6

Heavy-small

6.3

5.4

4.8

4.0

Heavy-large

5.6

5.1

4.7

4.3

Heavy-heavy

4.5

4.4

4.3

4.1

Large-small

4.8

4.7

4.5

4.3

All others

4.3

4.1

4.5

4.3

Results show that the percentage of total pairs remained fairly constant under
all categories, indicating a consistency of traffic mix for comparison purposes
(appendix A). Median separations show that sufficient separation reductions
were obtained under green conditions to support predicted operational gains,
and that the VAS reduced separation operation does not affect separation
closure rates between OM and touchdown (appendix B) .

SERIES III.

Series III testing consisted of heavy-density (90 arrivals, 60 departures) IFR
traffic. This series of tests was designed to investigate operational gains
or losses and procedural implications on the ATC system when transitioning
from one VAS condition to another under IFR conditions (table 5). (A total of
29 runs was accomplished) . Tests were conducted with two vortex clear zones
on the final approach path (threshold to OM and threshold to 20-nmi fix) under
two specific operational parameters.

1. VAS transition from green to red instantaneously.

2. VAS transition from green to red in 5 minutes

As can be seen from table 5 for both the instantaneous and 5-minute transitions,
the traffic mix at O'Hare, which is predominently of the large aircraft class
(70 percent) with a relatively small mix of heavy-small pairs, precluded any
major difficulties in transition from a 3-nmi separation standard a 3-, 4-, 5-,
6-nmi separation standard.

TABLE 5. AVERAGE NUMBERS OF ARRIVALS PER HOUR IN GREEN/RED CONDITION

Arrival Rate - Threshold to OM

Arrival Runway Configuration Zero Transition 5-Minute Transition

Runways 14L and 14R 71.0 70.5

Runways 27R and 32L 72.7 73.3

Combined average, runway 71.8 71.9
14L-14R and 32L-27R

Arrival Rate - Threshold to 20-nmi Fix

Runways 14L and 14R 75.5 72.0

Runways 27R and 32L 75.5 73.6

Combined average, runway 14L-14R 75.5 72.8
and 32L-27R

During the 5-minute transition phase, no difficulty was experienced in tran-
sitioning from one mode of operation to another. The trend during this phase
was to employ the expanded separation early in the 5-minute transition period,
effecting a general decrease in operations rate.

During the zero or instantaneous transition phase in most cases, no difficulties
were experienced or special action required, due to the traffic mix at time of
transition (i.e., large-large, large-heavy, etc.) These pairs of traffic mix
require the same separation (3-nmi) under both red and green conditions. Where
a traffic mix of heavy-small or heavy-large aircraft (which requires 6- or
5-nmi separation under a red condition) existed during the transition phase,
the determining factor for controller action was the aircraft position on final
approach. Beyond the outer marker, separation usually could be achieved prior
to touchdown. Where it could not, a maximum of one missed approach per transi-
tion was required. Inside outer marker, depending upon separation being
employed, the trailing aircraft might be required to make a missed approach,
a condition considered operationally unsatisfactory. The simulation high-
lighted the fact that the design of VAS should be such that when an instan-
taneous transition occurs, aircraft within the OM should be permitted to land
safely.

During this series of tests, no major procedural changes were required which
would impact the ATC system as a result of transitioning from green to red
operation (i.e., transition from reduced separation to increased separation).

SERIES IV.

Series IV consisted of heavy-density (90 arrivals, 60 departures) IFR traffic.
This series of tests was designed to evaluate VAS as a factor in the runway
selection process. Assuming availability of a similar or higher density runway
configuration in a green condition, it investigated benefits or impacts of

rerouting arrivals from a runway in a red condition to a runway in a green
condition under heavy traffic densities. A total of 13 runs was accomplished,

Results indicate that, while there is a slight decrease in overall operations
rate (table 6) below the baseline figure (table 1) as a result of a runway
change during the hour of changeover operations, it can be assumed an overall
increase in operation's rate would occur over the baseline operation's rate
during the succeeding hours that the new runway configuration would be in a
green condition.

TABLE 6. AVERAGE NUMBER OF ARRIVALS PER HOUR FOR RUNWAY CHANGEOVER HOUR

Arrival
Configurations

Combined average,
runways 14L-14R
and 32L-27R

Baseline

70.3

Threshold to
OM

67.2

Threshold to
20-nmi Fix

69.6

Runway changes from the basic configuration to a configuration of parallel
approaches to runways 9R and 9L were implemented in this series under the same
zones and operational parameters established in series III.

While VAS, if adopted, is expected to be a factor in the runway selection
process, it should be realized that many other factors would also contribute
to the process of changeover to alternative runways, such as a noise abatement
procedures and ground environment conditions. However, using VAS, there
were no procedural implications found in simulation which would impact the ATC
system to any greater degree than a runway change now impacts ATC operations.

Due to the limitation of the DSF, this simulation dealt solely with the air
operations of the O'Hare terminal environment, and many factors in the ground
control operation from the runway exits to terminal aircraft gates could affect
predicted VAS reduced separation gains, such as the scheduling interrelation-
ship between arrivals and departures.

While a departure position was employed and departure aircraft were incorporated
into the simulation environment, no data were taken in relation to departure
operations. Departures were employed only to insure that arrival aircraft
followed prescribed inbound routing and altitude restriction similar to those
used at- O'Hare and to provide validity of simulation results of airside opera-
tions.

SUMMARY OF RESULTS

Based on comparison of the baseline red condition and the VAS reduced separa-
tion green condition data, it is clear that increased hourly operational
arrival gains of from 2.5 (MM to threshold), 2.9 (OM to threshold), and 5.1
(20 nmi to threshold) for combined runway configuration can be achieved util-
izing the VAS reduced separation or no-vortex condition. (These increases are
compatible with Landrum and Brown and Mitre Corporation capacity gain studies.)
(references 1 and 2) (appendix C) . Similarly, the presence of VAS reduced
separation within the terminal environment does not create any procedural
implications or major impacts on the ATC system.

From all test series data, any increases prevalent in the controller communi-
cation workload (table 7) are a direct result of increased arrival rates
gains and not as a result of any VAS operating characteristic. • Control instruc-
tions (table 8) for all test series showed no significant incresses with VAS
reduced separation in the areas of vectors, altitude changes, speed adjustments,
and flightpath patterns. Separation criteria violations (table 9) showed no
increasing trends with VAS reduced separation.

TABLE 7. CONTROLLER COMMUNICATIONS

Configuration

Avg. Talk Time
Per Push-to-talk (Sec)

Baseline (red)
Threshold to MM (green)
Threshold to OM (green)
Threshold to 20 nmi (green)

Green-Red,
Green-Red,
Green-Red,
Green-Red,
Rwy Change,

OM, 0 min
20 nmi, 0 min
OM, 5 min
20 nmi, 5 min
OM

Rwy Change, 20 nmi

3.3
2.9
2.9
3.0
3.0
3.1
2.6
2.8
3.0
3.0

Avg. Contacts

Avg.

Talk Time

Per A/C

Per

A/C (Sec)

6.6

19.9

6.3

19.2

4.9

15.5

5.9

18.3

5.9

17.7

5.7

18.0

5.6

17.5

5.5 .

17.5

6.4

17.5

6.2

19.6

VAS indicated a potential application in the runway selection processes
through its ability to identify other high-capacity runway configurations in
the green condition which might exist when an in-use configuration goes red.

Transition from one separation condition to another on the final approach
(green to red) did not present any problems to controller personnel and, in
the worst case, resulted in a single aircraft missed-approach action.

In summary, it may be stated that, although the ultimate answers to procedural
questions can only be obtained through on-site operational test and evaluation
due to ground operations and other factors not considered in a controlled
simulation environment, no procedural implications emerged from the ATC

simulation of VAS reduced separation that would deter the operational Imple-
mentation of the system at Chicago's O'Hare International Airport.

TABLE 8. CONTROLLER ATC ACTIVITIES

Avg . No ,

Configuration

Baseline (red)
Threshold to MM (green)
Threshold to OM (green)
Threshold to 20 nmi (gre<
Green-Red, OM, 0 min
Green-Red, 20 nmi, 0 min
Green-Red, OM, 5 min
Green-Red, 20 nmi, 5 min
Rwy Change, OM
Rwy Change, 20 nmi

No. A/C

Avg . No .

Avg.

No.

Speed

Avg.

Distance

Controlled

Vectors

Alt.

Chg.

Flown (nmi)

53.6

140.2

45.4

73.8

67.8

54.4

146.0

44.4

73.8

69.5

54.5

144.2

44.9

68.2

66.0

m) 54.4

135.6

36.9

83.1

66.3

55.5

135.2

40.7

83.5

69.6

56.3

149.3

36.9

83.4

67.4

54.5

120.4

48.9

70.5

69.3

52.8

127.8

41.0

87.5

68.5

52.3

131.7

38.1

89.0

66.2

55.8

129.8

43.4

62.0

67.4

TABLE 9. SEPARATION CRITERIA VIOLATIONS FOR TOTAL AIRCRAFT PAIRS

Approach Zones

Inside OM

Total Pairs

Median Separation (nmi)

Percent Criteria Violations

Test Configurations
Baseline Green MM Green OM

541

4.5
* 9

603
4.0
1

510
3.8
3

Green 20 nmi

430
3.5
3

481

444

451

390

5.1

4.7

4.5

4.2

**7

Outside OM

Total Pairs

Median Separation (nmi)

Percent Criteria Violations

*Expanded separation (6.3- and 5.7-nmi median separation) (table 2) achieved
by controllers for heavy-small and heavy-large pairs was not sufficient outside
the OM to offset accordion effect at touchdown.

**Reduced separation (4.8- and 4.7-nmi median separation) (table 4) achieved
for heavy-small and heavy-large pairs outside OM to obtain 3-nmi separation at
touchdown is under separation criteria of 5.0 nmi.

CONCLUSIONS

From the results, it is concluded that:

1. No procedural implications emerged to deter implementation of VAS reduced
separation for aircraft arrivals.

2. Sufficient arrival rate increases were obtained to support cost/benefit
analysis.

3. An orderly transition of separation conditions was obtained in 5 minutes.

4. An instantaneous transition of separation conditions required, at worst,
one missed approach.

5. Arrival rates and ease of operation became more pronounced as the vortex
clear zone increased in size, i.e., was further expanded from the runway
threshold.

6. VAS could have application in the runway selection process.

REFERENCES

1. Landrum and Brown Aviation Consultants, Cost/Benefit Analysis of the
Proposed Vortex Avoidance System and O'Hare International Airport, February
14, 1977.

2. Avant, A. L., Procedural Feasibility of Reduced Spacings under VAS
Operation at O'Hare, May 1977.

APPENDIX A
COMPARISON TABLE OF ARRIVAL AIRCRAFT CLASSES AND PAIRING MIX

Chi-O'Hare Arrivals

Actual IFR (Sample size - 628 aircraft)

Class Breakdown Percent of Total

Small 63 10

Heavy 87 14

Large 478 76

Pairing Mix 566 Pairs Percent of Total

Heavy - small 7 1.5

Heavy - large 56 10
Heavy - heavy 13 2.5

Large - small 33 6

All others 451 80.

Actual VFR (Sample size - 852 aircraft)

Class Breakdown Percent of Total

Small 89
Heavy 132
Large 631

Pairing Mix 758 Pairs

Heavy - small 12

Heavy - large 85

Heavy - heavy 22

Large - small 50

All others 589

NAFEC Simulation Arrivals

Simulation (Sample size 90 aircraft per hour)

Class Breakdown Percent of Total

Small 9 10

Heavy 18 20

Large 63 70

Pairing Mix 4146 Pairs Percent of Total

Heavy - small 88 2

Heavy - large 653 16

Heavy - heavy 180 4

Large - small 267 7

All others 2958 71

Note:

All others = Small - small pairs
Small - large
Small - heavy
Large - large
Large - heavy

10.

15.

Percent of

Total

A-l

APPENDIX B

SPEED PROFILES (KNOTS) AT THRESHOLD AND OUTER MARKER FOR SMALL,
HEAVY, AND LARGE WEIGHT CLASSES

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APPENDIX C

PREDICTED GAINS (IN ARRIVALS PER HOUR)

Threshold to MM OM 20 nmi

Mitre Corp. Study - 1.7 3.1 4.1

Landrum-Brown Study - 2.4 3.8

NAFEC Simulation - 2.5 2.9 5.1

Avg. Predicted Gain 2.2 3.2 4.6

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