DOT/FAA/AM-01/19

Linda M. Peterson

Larry L. Bailey

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

Ben F. Willems

William J. Hughes Technical Center

Federal Aviation Administration

Atlantic City International Airport, NJ 08405

December 2001

Final Report

This document is available to the public

through the National Technical Information

Service, Springfield, VA 22161.

Controller-to-Controller

Communication and

Coordination Taxonomy (C4

T) Office of Aerospace Medicine

Washington, DC 20591
NOTICE

This document is disseminated under the sponsorship of

the U.S. Department of Transportation in the interest of

information exchange. The United States Government

assumes no liability for the contents thereof.
i

Technical Report Documentation Page

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

DOT/FAA/AM-01/19

4. Title and Subtitle 5. Report Date

Controller-to-Controller Communication and Coordination Taxonomy December 2001

(C4

T)

6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.

Peterson, L.M.1, Bailey, L.L.1, and Willems, B.F.2

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)

1

FAA Civil Aerospace Medical

Institute

FAA William J. Hughes Technical

Center

P.O. Box 25082 Atlantic City International Airport,

Oklahoma City, OK 73125 NJ 08405 11. Contract or Grant No.

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered

Office of Aerospace Medicine

Federal Aviation Administration

800 Independence Ave., S.W.

Washington, DC 20591 14. Sponsoring Agency Code

15. Supplemental Notes

This report was supported by Approved Subtask # AM-B-00-HRR-518.

16. Abstract

While previous research in the air traffic control (ATC) communications area has generally concentrated on

controller-pilot communications, this program of research focuses on controller-to-controller communications. At

the Air Route Traffic Control Center (ARTCC), teams of two controllers, R-side and D-side, are required to

communicate on a continuing basis to coordinate the duties of their sector. As modernization of the ATC system

progresses, questions arise concerning the effects these changes will have on intra-enroute sector team (EST)

communications.

In anticipation of technology changes, the Federal Aviation Administration commissioned a series of studies

investigating intra-EST communication. This initial study details the design and subsequent field testing of the

Controller-to-Controller Communication and Coordination Taxonomy (C4

T). The taxonomy is designed to capture

the following general communication categories: Topic, Format (grammatical form), and Expression. The final

taxonomy resulting from this research contains 12 ATC topics (i.e., Traffic, Altitude, etc.). Communication

Grammatical Format contained 5 subcategories: Question, Answer, Statement, Command, and Command Answer.

Communication Expression consisted of 3 subcategories: Verbal, Nonverbal or a combination of Verbal and

Nonverbal, referred to as Both. A field study at an ARTCC was conducted with subject-matter experts coding intraEST communications using the taxonomy described. Field observations were made at 18 different sectors between

the hours of 07:00 and 019:00 based on moderate to high traffic levels. Descriptive statistics detail the results of the

taxonomy’s use in a field setting.

Testing and further refinement of the taxonomy allows its use in both field and controlled experimental settings,

provides a tool for training individuals to code C4

T communications, and enables the establishment of a C4T baseline

to investigate changes in communication patterns as modernization continues in the enroute ATC environment.

17. Key Words 18. Distribution Statement

Document is available to the public through the

National Technical Information Service;

Air Traffic Control, Communication, Controller Teamwork

Springfield, VA 22161

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price

Unclassified Unclassified 12

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

iii

ACKNOWLEDGMENTS

The Controller-to-Controller Communication and Coordination Taxonomy (C4T) is a joint project

of the Civil Aerospace Medical Institute, the William J. Hughes Technical Center, and the Mike

Monroney Aeronautical Center, in cooperation with the Miami Enroute Center. Special acknowledgments go to the following individuals for their contributions to the successful completion of this

research project: Mr. Paul DeBenedittis, Mr. Al Hendrix, Mr. Henry Mogilka, and Mr. Gary

Washburn. The authors also thank the management and personnel from the Miami Enroute Center for

their cooperation at the field study site and the National Air Traffic Control Association for their

support of the study.

1

CONTROLLER-TO-CONTROLLER COMMUNICATION AND

COORDINATION TAXONOMY (C4T)

Air traffic has increased about 4% per year, although increases have reached 20% in certain highly

competitive locations (Garvey, 1998). The National

Civil Aviation Review Commission states that the

expected growth in aviation cannot be safely accommodated without significant breakthroughs in air

traffic modernization. Air traffic communications are

cited as critical components requiring modernization

in the Aviation system (Garvey, 1999). The important

role that communication plays in maintaining safety

and efficiency within the National Airspace is most

evident when communications fail and air traffic

control (ATC) operational errors, pilot deviations,

and runway incursions occur. The tragic accident at

Tenerife is perhaps one of the better examples of a

communications breakdown between controllers and

pilots under conditions of poor visibility, leading to

the loss of 583 lives (Stokes & Kite, 1994).

Communications at air traffic control facilities

occur at many levels both within and between facilities and between controllers and pilots. Traditionally,

analyses of air traffic control communications (ATCC)

have focused on the information exchange between

pilot and controller and the consequence of breakdowns in communications (Cardosi, 1993; Kanki &

Prinzo, 1996a; Prinzo & Maclin, 1996; Prinzo,

Lieberman, and Pickett, 1998a; Prinzo, 1998b).

Within enroute air traffic control centers

(ARTCCs), controllers often work as an enroute sector team (EST). The EST consists of a radar (R-side)

and data (D-side) team whose function is to handle

traffic within the sector. The D-side team member

assists the R-side when traffic reaches certain levels

within the sector. Duties of the R-side and D-side

controllers are defined in the ATC position standards

(FAA, 1992). Emphasis is placed on the importance of

team communications in the following areas: advising

other team members of situations requiring attention

or immediate action, initiating actions to resolve

problems, participating in planning activities, and

monitoring the air traffic environment. Thus, the

presence of a second controller at the sector facilitates

both the strategic planning and operational control of

air traffic. Information exchanges between the two

team members can either facilitate the safe and efficient flow of traffic across the sector or serve to delay

overall traffic flow. To date, little is known about the

intra-EST communication process. This study was

designed to (a) develop a taxonomy to capture and

categorize ATC R-side to D-side communications

and (b) to conduct an initial validation of the taxonomy at an en route traffic control center.

Gathering baseline communications data from controllers operating with existing equipment and procedures is critical to both determine the potential affects

of new technologies and procedures and to assist in

the development of those capabilities. We currently

have several emerging air traffic control technologies

and systems, including Data Link (DL), the 21st

Switching and Control System (VSCS), Display System Replacement (DSR), and the Standard Terminal

Automation Replacement System (STARS). In addition, the future will bring advanced decision aids and

the possible transfer of control functions between

pilots and controllers under certain free flight scenarios. A need to assess the effects of implementation

of these technologies and procedures on overall communications was a primary factor in this evaluation of

R-side and D-side communications.

METHOD

Taxonomy Development

Prior to observations at the FAA Academy’s Radar

Training Facility (RTF), subject matter experts (SMEs)

and Civil Aerospace Medical Institute (CAMI) researchers met to consider possible categories for the

taxonomy. All SMEs had prior ATC experience and

contributed to identifying communication content,

especially in regard to the topics of communication.

Common experience in ATC led to operational definitions of the category ATC Communication Topic.

The observations of intra-EST communication were

conducted at the RTF by SMEs, RTF instructors, and

CAMI researchers. Information was gathered during

four 15-minute air-traffic scenarios, at two levels of

traffic workload. The traffic workload chosen was

based on position requirements for different levels of

air traffic. ATC staffing at enroute centers is routinely

modified to accommodate variations in air traffic

volume that occur throughout a typical day. During

periods of increased volume, controllers work as Rside, D-side teams. This was defined as medium

workload. Air traffic may increase to a level where a
2

third position, called a tracker, is added to the team.

The traffic volume just prior to adding a tracker was

defined as high workload. RTF observations were

made during both medium and high workload scenarios with the EST teams consisting of active R-side

and D-side team members.

RTF observations of intra-EST communication

resulted in the identification of 12 Communication

Topics, five Communication Grammatical Formats,

and three types of Communication Expressions. The

12 ATC Communication Topics included: (1) Approval, (2) Handoff, (3) Point-out, (4) Traffic, (5)

Altitude, (6) Route, (7) Speed, (8) Weather, (9) Frequency, (10) Flow, (11) Equipment, and (12) Flight

Strips. The five Communication Grammatical Formats subcategories were adapted from Bales (1950)

and were identified as (1) Question, (2) Answer, (3)

Statement, (4) Command, and (5) Command Answer. The three types of Communication Expression

were documented as (1) Verbal, (2) Nonverbal, and

(3) Both (containing elements of both verbal and

nonverbal communication).

 Pre-testing Taxonomy Categories

Once the preliminary categories were formulated, a

beta test was conducted with the assistance of the

SME’s, RTF instructors, and CAMI researchers. ESTs

participated in simulations of the enroute radar environment during medium and high workloads for 15-

minute periods. During these simulations, intra-EST

communication was observed and recorded. Following comparison of the recorded observations, the

categories were further refined to develop the C4

T

taxonomy. With the taxonomy finalized, the selection

and training of field coders commenced.

Taxonomy Beta Testing in the Field

Field coders were selected based on their expertise

in ATC. All field coders were former air traffic controllers familiar with the intricacies of ATC communication. Three of the field coders were instructors at

the FAA Academy RTF. The remaining field coder

was an SME with extensive knowledge of ATC communication and experience with field studies.

Field coders were familiarized with the taxonomy

during simulated EST scenarios conducted at the

FAA Academy. A beta test in the actual enroute field

facility was conducted prior to implementation of the

taxonomy in the field study. The coders and the

principal investigator reached consensus, and the taxonomy was accepted for use in the field study.

Coding Devices: Hardware and Software

CAMI and the William J. Hughes Technical Center (WJHTC) combined resources to acquire four

Orasis hand-held computers from Dauphin Technology, Inc., for collection of observational data during

field research. These devices provided the coders the

mobility necessary to observe both verbal and nonverbal communications. The WJHTC personnel designed software to allow coding of the taxonomy

elements in a field setting.

Data Gathering

The Miami ARTCC was the designated field site

for taxonomy testing. This site was selected for its

variation in traffic density and configuration, and its

air space diversity, which includes military and transoceanic air space. The Miami ARTCC also experiences diverse weather conditions with seasonal rain

and hurricane conditions. This range of conditions

allowed observation of intra-EST communication

under a variety of different circumstances. The above

mentioned conditions existed during the time of the

study, with the exception of hurricane conditions.

Coding was conducted at 18 different sectors at

various times between 07:00 and 19:00 based on the

amount of air traffic present in a particular sector.

Observation of a sector was dependent on moderateto-high traffic, which allowed the observers to capture

the most intra-EST communication per observation

period. Initially, observation periods were scheduled

for 15-minute intervals. However, beta testing determined that a more optimal time segment for an entire

range of intra-EST communication was 30-45 minutes.

Coders observed the intra-EST communication in

teams of two observers per intra-EST. One of the

coders observed the radar position and coded only the

R-side communication of the intra-EST. The other

observed the data position and coded only D-side

communication. Communication coding was limited

to ATC issues. All participants were assured that

personal conversations were not included in this study

and would not be coded.

Table 1 includes the ATC Communication Topics, and provides an operational definition of each

topic, as well as examples of intra-EST communication for each topic. The examples for each topic are

derived from actual intra-EST communications and

are considered prototypical intra-EST conversations

for the topic.

Communication Grammatical Format was coded

according to the grammatical presentation of the

communications as a Question, Answer, Statement,
3

Table 1. Controller-to-Controller Coordination Communications Taxonomy (C4T)

ATC

Communication

Topic Definitions and Examples

Approval Communications about inter-sector control/approval requests. (“Get me control for

descent on that aircraft.” “APREQ N1234 climbing to FL330.”)

Handoff Communications relating to the transfer of radar identification of a particular aircraft.

(“Handoff N1234.” “Did you handoff N1234?”)

Point Out Communications relating to the transfer of radar identification of a particular aircraft when

radio communications will be retained. (“Point out N1234 to 22.”)

Traffic Communications about a traffic situation involving a specific aircraft. Includes conflict,

spacing, other protected air space or terrain and the resolution of that situation. (“Are you

watching that aircraft?”)

Altitude Communications about altitude not in relation to traffic. (“N1234 is requesting flight level

220.”)

Route Communications regarding headings and/or amendments to route, not in relation to traffic

situations. (“N1234 is on a 330 heading.” “Next sector, 27, wants N1234 over WEVER.”)

Speed Communications about speed not in relation to traffic situations. (“These three aircraft are

slowed to 250 knots.”)

Weather Communications about weather display or weather updates. Often communicated

nonverbally by passing written information. (“Sector 22 says continuous moderate

turbulence above FL290.”)

Frequency Communications about an aircraft’s radio communications transfer or frequency

assignment. (“Have you switched N1234 yet?” “Tell them to switch to N1234.”)

Flow Messages Communications about traffic flow restrictions not referring to a specific aircraft. (“The

next sector is requesting 25 miles in trail.”) (due to radar outage)

Flight Strips Communications about flight progress strips. (“Where is that strip?”) Often communicated

nonverbally.

Equipment Communications about any ATC hardware. (“The radar is out of service.”)
4

Command, or Command Answer. Communication

Expression included Verbal, Nonverbal and Both.

Communication Expression required observation of

nonverbal gestures such as head nodding, pointing to

the screen, and gesturing thumbs up, among other

nonverbal gestures. Communications consisting of

only a nonverbal gesture were coded as Nonverbal.

Intra-EST communications with verbal and nonverbal elements were coded as Both. A communication

with only a voice element was coded as Verbal.

RESULTS

Eighteen different sectors were observed over a

three-day period at the Miami ARTCC. Data extraction was completed through coordination with the

WJHTC. Following data extraction, the data were

merged to create a composite R-side and D-side

profile. The profiles are represented in Figure 1, for

Communication Topic, Figure 2, for Communication Grammatical Format, and Figure 3, for Communication Expression. More extensive analysis was not

conducted on the data from this field study due to the

number of confounds associated with the field study

environment. Confounds included: sector complexity,

workload differential, individual differences among

team members, variability across EST teams, and

team composition changes during coding.

A primary goal of this study was to determine the

practical utility and validity of the communication

taxonomy. The coders reported few problems associated with the coding and classifying of R-side and Dside intra-EST communications. Most disparities in

coding had been resolved during the beta-testing

phase of the study. Coders reported that mobility was

essential to viewing nonverbal communication. They

also stated that mobility assisted with interpretation

of verbal coding by enabling them to view the contextual environment.

Operational relevance was demonstrated by the use

of the entire range of potential ATC Communication

Topics. The topic of Approval comprised the smallest

percentages (R-side 1%, D-side 0.9%) of total intraEST communication, whereas the topic of Traffic

comprised the largest percentages (R-side 41%, Dside 37.9%). Further investigation and discussion

concerning operational issues revealed that the majority of inter-sector coordination is handled through

memoranda of understanding. The topic of Approval

is one example of inter-sector coordination using

Communication Topic

0

5

10

15

20

25

30

35

40

45

Percent

R-Side 1 3.6 5 41 7.1 14.2 2.6 5.5 5.9 5.2 3.3 5.6

D-Side 0.9 3.1 6.1 37.9 8 15.6 2.8 6.8 4.7 5.6 4 4.5

Approval Handoff Point-out Traffic Altitude Route Speed Weather Frequency Flow Equipment Flight Strips 

Figure 1: C4T Communication Topic
5

Communication Format

0

10

20

30

40

50

60

Percent

R-Side 12.2 42.8 29.7 0.5 5.8

D-Side 16.4 25.1 55.9 2.4 0.3

Question Answer Statement Command Command Answer

Figure 2: C4T Communication Format

Communication Expression 

0

10

20

30

40

50

60

70

80

Percent

R-Side 71.5 13.9 14.7

D-Side 69.3 13.9 16.8

Verbal Nonverbal Both 

Figure 3: C4T Communication Expression
6

standard operating procedures established through

memoranda of understanding; hence, less verbal communication is necessary.

The topic of Traffic, which showed the largest

percentage of intra-EST communication, was revised

following discussion and a consensus between coders,

researchers, and SMEs. The field study definition for

Traffic contained a considerable number of intraEST communications pertaining to the identification

of aircraft (i.e., Aircraft ID). Therefore, Aircraft ID

was separated from the topic of Traffic and added as

an ATC Communication Topic to the C4

T Taxonomy for use in future research (Table 2).

Communication Grammatical Format results revealed that the categories Statement (observations)

and Answer comprised the largest percentage of intraEST communication, accounting for 72.5% of R-side

communication and 81.0% of D-side communication.

Statements consisted of observations that sometimes

contained implied questions. The use of implied

questions raises issues as to why intra-ESTs embedded

implied questions in their communications. The usage of implied questions embedded in the category,

Statement, could be related to task orientation, or it

could be a function of individual differences. Further

research is necessary to investigate this phenomenon.

Subsequent taxonomy studies will limit the categories of Grammatical Format to three from the original

five. Command and Command Answer, as operationally defined, were insufficient to prove useful.

The intra-EST Communication Format differed as

a function of EST member. These differences were

most apparent in the categories of statements (observations) and answers. The D-side had a larger percentage of statements (D-side 55.9%, R-side 29.7%).

Correspondingly, the R-side had a larger percentage

of answers (R-side 42.8%, D-side 25.1%).

Table 2. Controller to Controller Coordination Communications Taxonomy (C4T) Revised.

ATC

Communication

Topic Definitions and Examples

Approval Communications about intersector control/approval requests. (“Get me control for descent

on that aircraft.” “APREQ N1234 climbing to FL330.”)

Handoff Communications relating to the transfer of radar identification of a particular aircraft.

(“Handoff N1234.” “Did you handoff N1234?”)

Point Out Communications relating to the transfer of radar identification of a particular aircraft when

radio communications will be retained. (“Point out N1234 to 22.”)

Traffic Communications about a traffic situation involving a specific aircraft. Includes conflict,

spacing, other protected air space or terrain and the resolution of that situation. (“Are you

watching that aircraft?”)

Altitude Communications about altitude not in relation to traffic. (“N1234 is requesting flight level

220.”)

Route Communications regarding headings and/or amendments to route, not in relation to traffic

situations. (“N1234 is on a 330 heading.” “Next sector, 27, wants N1234 over WEVER.”)

Speed Communications about speed not in relation to traffic situations. (“These three aircraft are

slowed to 250 knots.”)

Weather Communications about weather display or weather updates. Often communicated

nonverbally by passing written information. (“Sector 22 says continuous moderate

turbulence above FL290.”)

Frequency Communications about an aircraft’s radio communications transfer or frequency

assignment. (“Have you switched N1234 yet?” “Tell them to switch to N1234.”)

Flow messages Communications about traffic flow restrictions not referring to a specific aircraft. (“The

next sector is requesting 25 miles in trail.”) (due to radar outage)

Flight Strips Communications about flight progress strips. (“Where is that strip?”) Often communicated

nonverbally.

Equipment Communications about any ATC hardware. (“The radar is out of service.”)

Aircraft ID

Identification of

Aircraft

Communications involving identifying a specific aircraft. (“Who was that calling?” “That

was N1234 calling.”)
7

Communication Expression was divided into the

categories of Verbal, Nonverbal, and Both (containing components of both nonverbal and verbal expression). The majority of communications by both R-side

and D-side were Verbal (71.5% and 69.3% respectively). Combining the categories of Nonverbal and

Both (which contains an element of Nonverbal), the

data showed that 28.5% of the R-side and 30.7% of

the D-side communication had a nonverbal component. This could have implications for configuration

of the workplace environment and line of vision for

the intra-EST members. Further investigation by researchers determined that written weather update

exchanges between the D-side and the R-side may

have accounted for some of the nonverbal exchanges

between EST members.

DISCUSSION

The results demonstrate that intra-EST communication is an integral part of job related coordination.

Controller communication is expressed in various

Communication Topics, Communication Grammatical Formats, and Communication Expressions (as

shown in Figures 1, 2, and 3, respectively). New

technologies designed to enhance the ATC environment may affect intra-EST communications. The

development and evaluation of new ATC technologies will require investigation of the technology’s

effects on the quantity and quality of intra-EST communication. Future studies should focus on the initial

impact of planned technological programs, as well as

the possible long-term consequences.

Ergonomic studies should take into consideration

the consequences of the view or lack of view of ATC

displays as well as line of vision and hearing obstructions for the members of the EST. Training and

development programs will require structure to assist

controllers in devising strategies to adapt intra-EST

communication to present and future technological

advances. Perhaps future advances in technology will

obviate the need for intra-EST communications; however, until such time, intra-EST communications

require adequate consideration.

Research in intra-EST communication should be

conducted as much as possible in advance of the

implementation of new technologies and accompanying procedures. When applicable, C4

T assessment

should be conducted prior to implementation, after

initial training, and periodically during the useful life of

the new technology to ensure there are no decrements in

intra-EST communication, and to record and verify

possible enhancements to intra-EST communication.

Future studies are scheduled in a controlled laboratory simulation setting using the new technologies

being proposed to enhance ATC functions within the

NAS. Videotaping allows for an in-depth review of

intra-EST communications. Additionally, laboratory

ATC simulations allow researchers to address the

issues of variability across both sectors and individual

team member differences by controlling workload

and sector complexity, and by using within-subjects

designs.

Future projects can include using the taxonomy to

assess the relationship between intra-EST communication and the electronic flight strip environment

(Vortac et al., 1996). The taxonomy may also be used

as a tool to explore safety-related issues involving

intra-EST communications including operational

deviations and errors. It also provides an assessment

tool for continued research on the relationship between intra-EST communication and ATC efficiency.

RECOMMENDATIONS

The following are recommendations for future

research concerning the C4

T Taxonomy:

(1) revise the taxonomy: redefine the Communication Topic, Traffic, and add Aircraft ID as a Communication Topic (Table 2); and eliminate Command

and Command Answer from the Communication

Grammatical Format; (2) until greater control of

potential confounds is achieved in the field, series

analysis are not appropriate using the C4

T and focus

should be limited to general trends across time and

sectors; (3) conduct controlled experiments to determine the affect that changes in workload and technology have on communication exchanges with intra-EST.

These experiments would test the sensitivity of the

C4

T to detect changes in communication exchanges

and allow time series analysis of communication exchanges using a repeated-measures design; (4) test the

ability to generalize laboratory experimental results to

field settings; and (5) use laboratory simulations and

field results to structure training recommendations

for intra-EST communications.
8

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