O. Veronika Prinzo

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

Alfred M. Hendrix 

HCS Consulting Services

Roswell, NM 88201

October 2003

Final Report

This document is available to the public

through the National Technical Information

Service, Springfield, Virginia 22161.

 

Office of Aerospace Medicine

Washington, DC 20591

Automatic Dependent 

Surveillance - Broadcast / 

Cockpit Display of Traffic 

Information: Pilot Use of the 

Approach Spacing Application

DOT/FAA/AM-03/13
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

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1

AUTOMATIC DEPENDENT SURVEILLANCE - BROADCAST/COCKPIT DISPLAY OF

TRAFFIC INFORMATION: PILOT USE OF THE APPROACH SPACING APPLICATION

Celestial navigation is based on the premise that the Earth is the center of the universe. 

The premise is wrong, but the navigation works. An incorrect model can be a useful tool.

— Kelvin Throop, III 

Automatic Dependent Surveillance-Broadcast (ADSB)1

 is a data link application that will transmit from aircraft 

and vehicles equipped to emit surveillance data (such as 

position, type, speed, and identification) by means of a 

broadcast-mode to users who can receive these data. The 

Federal Aviation Administration (FAA) anticipates that 

ADS-B will provide many benefits — from extending 

the range and coverage of current ground-based secondary surveillance radar (especially in approach areas and 

on airport surfaces) to increasing air-to-air situational 

awareness. The FAA envisions that enhanced airborne and 

ground capabilities would provide for specific surveillance 

functions that would encourage co-operative air traffic 

management and collaborative decision-making among 

its users (Prinzo 2001).

Avionic devices that provide a cockpit display of traffic information (CDTI) capability will enable pilots to 

acquire, verify, and maintain prescribed spacing minima 

from other like-equipped ADS-B aircraft. This capability 

provides pilots with a visual depiction of the current traffic 

situation with its symbolic display of intent, geometry, and 

aircraft identity (i.e., traffic flight identifier) information. 

Instead of relying on communications from air traffic 

control (ATC) and monitoring the “party line,” a CDTI 

has the potential to enhance, supplement, and project 

the pilot’s mental representation of the traffic situation 

into the immediate future, using real-time continuous 

information.

It is of interest to the FAA to determine how the use 

of CDTI may influence pilot/controller operational communication and workload. While inclusion of the traffic 

flight identifier is not part of the standard phraseology, 

FAA Order 7110.65 does allow for appending additional 

information to traffic information messages. Using this 

option permits the evaluation of several communication 

procedures using the traffic flight identifier as additional 

information.

Anticipated Benefits from Pilot Use of a CDTI

Some of the anticipated near-term benefits from pilot 

use of a CDTI device include enhanced safety, optimized 

aircraft performance, and reduced radio frequency congestion. Each potential benefit is described below.

Enhanced Safety. When both the pilot and controller 

have access to the same information, they share a common 

understanding of the traffic patterns, obstructions, and 

situations. This shared mental model will allow them to 

participate more fully in collaborative decision-making 

so all aircraft can safely and efficiently traverse the National Airspace System. As pilots use CDTI to enhance 

their situational awareness, they will refine their out-thewindow scan to acquire and track traffic-to-avoid and 

traffic-to-follow using real-time information. 

Optimized Aircraft Performance. The use of CDTI 

is anticipated to enhance the techniques pilots use to 

aviate and navigate ownship to establish and maintain 

specified spacing intervals and speed restrictions. In 

theory then, pilots can improve upon the efficiency and 

economy of their flights by optimizing the performance 

of their aircraft. Ideally, improved flight operations and 

aircraft performance will translate into fuel savings, less 

wear and tear on engines, and increased profits for commercial aviation.

Reduced Radio Frequency Congestion. Another 

expected benefit from CDTI is greater accessibility to 

an often unavailable radio frequency — especially when 

talking to ATC is time critical. In today’s air traffic control 

system, certified professional controllers provide pilots 

with traffic position and sequence information (intermittently and when needed) over an assigned radio frequency. 

Once the CDTI becomes operational, that same information could be displayed in real-time as continuous graphic 

and text information to pilots who fly ADS-B equipped 

aircraft. When pilots and controllers have access to the 

same information they will be able to optimize communication by transmitting fewer messages.

Consider how CDTI might change air-ground 

communications by comparing the number and types 

of messages exchanged between a controller and a pilot of 

an aircraft not equipped with ADS-B/CDTI (Table 1) to 

those of the same aircraft equipped with ADS-B/CDTI 

1 In 1998, RTCA SC-186 completed Minimum Aviation 

System Performance Standards for ADS-B. In addition to 

describing ADS-B, this document also provides information 

on applications that may use ADS-B information. 
2 

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(Table 2). Message 1 is virtually the same in Table 1 and 

Table 2. In each message, the controller informs the pilot 

of ABC1 23 (the message recipient, Ownship) that a Boeing 737 will be the traffic to follow for the approach and 

to report when it is sighted. The pilot’s report (Message 

2) differs only in how the traffic is identified — in Table 

1 with the word “TRAFFIC” and in Table 2 by name 

and flight number spoken sequentially, “OTHERSHIP 

4 5 6.” 

By using the traffic flight identifier, any potential ambiguity as to the identity of the traffic is removed. The 

controller can eliminate messages 3 through 6 in Table 

1 by instructing the pilot to follow the sighted traffic at 

a pre-defined spacing interval, as is the case in Table 2 

(Message 7). The last message transfers communication 

to the tower and serves to close the transaction. Notably, more than half the messages presented in Table 1 

involve the aircraft-to-be-followed. It is anticipated that 

as more aircraft are equipped with ADS-B/CDTI, pilots 

and controllers will transmit fewer traffic- and routerelated messages.

In October 2000, a second operational evaluation 

(OpEval-2) of the ADS-B and CDTI occurred for which 

the purposes were to develop and evaluate specific ADSB applications, evaluate controller use of ADS-B, and 

demonstrate ADS-B technology.2 Under varying weather 

conditions, traffic patterns, and aircraft sequences (e.g., 

a Boeing 727 behind a Boeing 757) different spacing 

intervals might be desirable. The operational evaluation 

of ADS-B/CDTI provided an opportunity to examine 

some of these issues.

Specifically, this operational evaluation demonstrated 

new air- and ground-based capabilities and systems at an 

FAA-controlled airport tower and terminal radar approach 

control facility. With that in mind, it is important to note 

that it was not possible to apply true experimental and 

control conditions comparable to that of laboratory-based 

experiments. Likewise, the training that pilots and controllers received was unequal — making it impossible to 

exercise any statistical control over the data (e.g., treating 

the number of hours of training as a co-variant).

What the operational evaluation did provide was an 

opportunity to gather field data that could be used to 

guide the development of ADS-B applications (e.g., approach spacing, departure spacing/clearance, runway and 

final approach occupancy awareness, and airport surface 

situational awareness). During that 5-day event, objective 

and subjective data were collected from controllers and 

pilots, and a comprehensive report of the findings was 

prepared by the Operational Evaluation Coordination 

Group (OCG, 2001). Reported here is a summary of 

several analyses performed at the Civil Aerospace Medical Institute on the audiotaped communications between 

pilots flying aircraft equipped with a CDTI device and 

terminal radar approach controllers who provided them 

with air traffic services.

The objective of the voice tape analysis was to identify 

any change in workload, operational communications, 

or both that may have occurred during the operational 

evaluation of a variety of CDTI applications (e.g., Airport Surface Situation Awareness, Departure Spacing, 

Approach Spacing, Final Approach Runway Occupancy 

Awareness). This report provides a general description of 

the communication findings for the approach spacing 

application only.

METHOD

Participants

The pilots were paid volunteers that comprised 10 

flight crews who received briefings and participated in 

proficiency training exercises before the evaluation. The 

three controllers and two coordinators (also volunteers) 

were on a temporary detail during training and on a 

regular schedule during the evaluation.

Experimental Protocol

Before the evaluation, pilots and controllers participated in several simulations conducted at the Integration and Interaction Laboratory (I-Lab) of the MITRE 

Corporation’s Center for Advanced Aviation System Development (CAASD). During these simulations, pilots 

received instruction on how to respond to various types 

of ATC messages and listened to a combination of ATC 

and pseudo-pilot communications over a party line.

Experimental Flights. Pilots performed the approach 

spacing application with ADS-B information displayed on 

their CDTI. As shown in Figure 1, the approach profiles 

were designed to evaluate the ability of flight crews to 

manage pre-determined spacing intervals between arriving 

aircraft. The spacing interval decreased as pilots neared the 

airport. Typically, pilots flew their aircraft along the assigned routes in a pre-determined pattern, received a clearance to land the aircraft, taxied to the active runway and 

departed again. The sequence of aircraft in the approach 

pattern varied with each approach. ATC monitored both 

participating and non-participating aircraft and ensured 

preservation of standard separation minimums.

The evaluation included the initial (Figure 1, Point 4 to 

Point 2) and final approach (Figure 1, Point 2 to runway 

threshold). The initial approach began approximately 20 2

 Cited in Flight Crew Mission Guide (Operational 

Evaluation Coordination Group, OCG 2001).
4 

miles from the runway threshold. During descent, an 

aircraft on initial approach transitioned from a spacing 

interval of 6.5 nm to a final approach spacing interval 

of 5 nm (long spacing) and from 5.5 nm to 3 nm (short 

spacing). Final approach began about 10 miles from the 

runway threshold (Figure 1 Point 2). After visually acquiring the traffic to follow (i.e., out the window), the 

flight crew used the CDTI to reduce the final approach 

spacing interval to either the long or short spacing criteria 

by adjusting the aircraft airspeed. 

Procedure

The West portion of the TRACON airspace and the 

West runway were dedicated to the evaluation activities. 

In addition, a portion of the airfield was set apart from 

normal operations with limited access to the West runway 

for nonparticipating aircraft. This airspace was divided 

into two sectors using a normal tower position for the 

West runway. Furthermore, the observations occurred 

during periods of low traffic volume and when aircraft 

designated as “heavy” did not require that runway to 

depart or land. On alternating flight periods the role 

of pilot in command switched between the captain and 

first officer.

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During the pre-flight briefings that preceded each data 

collection activity, all of the participants reviewed the 

flight scenarios (see Figure 1) and scripts. The facilitators 

reminded them to follow established procedures and communication protocols (see Table 3). Afterwards, the pilots 

proceeded to their respective aircraft, and the controllers 

went to their air traffic control positions. Following each 

data-collection activity, pilots and controllers participated 

in post-flight debriefings. Each flight period started and 

ended at the appointed times.

Materials

The Terminal Radar Approach Control (TRACON) 

facility provided five, digital audiotapes (DAT), one for 

each test period. Each DAT contained separate voice 

records of all communication transmitted on the radio 

frequency assigned to the Ground East, Local West, or 

Approach West position on the left channel. The right 

channel contained the Universal Time Coordinated 

(UTC) time code expressed in date, hour (h), minute 

(min), and whole second (s). The NiceLoggerTM Digital 

Voice Reproducer System (DVRS) decoded and displayed 

time and correlated it with the voice stream in real time. 

The data consisted of 15 hours of digitized voice communications, 9 hours of which were from the Approach 

West position.

Experimental Design

This study used a two-factor between-group design. 

The between group factors were Spacing Interval (Long 

— 6.5 to 5 nm, Short — 5.5 to 3 nm) and Time of Day 

(Day, Night). During day operations (Flight Periods 1, 

2, 4), the flight crews completed 24 long and 23 short 

approaches. For night operations (Flight Periods 3, 5), 

they completed 20 long and 19 short approaches. 

Whether or not an aircraft that was called-out as traffic 

appeared on the CDTI depended upon the sending and 

receiving ADS-B links among those aircraft. That is, if 

Ownship (the receiver of the transmission) only received 

1090 MHz and Othership (the aircraft identified as traffic) 

transmitted on 1090 MHz, then its symbol and related 

information were displayable on the CDTI. If Ownship 

received 1090 MHz and Othership transmitted on UAT, 

the data were not displayed. The type of ADS-B equipage 

(e.g., UAT or 1090 MHz) was denoted on flight strips 

that were provided to the controllers. Consequently, 

Traffic (displayable, not displayable) was included as a 

factor in some analyses.

Dependent Measures

The dependent measures included communication 

workload and operational communications. Measures of 

communication workload were grouped into efficiency 

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6 

and duration of communications. Measures of operational communication included usage of the traffic-flight 

identifier phraseology, communication problems, and 

operational concerns.

To measure changes in communication workload 

and operational communication (the primary variables 

of interest) for each approach, messages between the 

controller working approach control and the pilot of 

each aircraft were grouped into communication sets. A 

communication set is comprised of messages that share 

a common goal or purpose that are transmitted between 

a controller and pilot (Prinzo 1996). 

To illustrate, consider the partially encoded transcript 

presented in Figure 2. Approach #2 is comprised of messages 1, 2, 3, and messages 8 through 12 and Approach 

#1 includes messages 4 through 7. Four communication 

sets are associated with Approach #2. They involve messages transmitted to: (1) establish initial contact, (2) issue 

and complete a traffic advisory (VAT Comm. Set), (3) 

instruct the pilot to follow sighted traffic for an approach 

(IRP-F Comm. Set), and (4) transfer communications 

to the tower controller. 

Objective Measures of Communication Workload. 

Seven measures of communication workload were examined. The first three dependent measures involved 

efficiency of communications while the last four dependent measures comprised duration of communications. 

Efficiency of communications included (1) number of 

messages transmitted, (2) time on frequency (per message), and (3) frequency occupancy time. Duration of 

communications involved reply time latencies for: (4) 

Visual Acquisition Time (VAT), (5) Visual-Acquisition 

Time-Total (VAT-T), (6) Instruction Route-Position-Follow (IRP-F), and (7) Transaction Set Time (TST). They 

are described below.

Number of Messages: The number of pilot and controller 

messages was counted and aggregated according to the 

type of analysis that was performed. As seen in Figure 

2, Approach #2 is comprised of eight messages of which 

the first two involved initial contact, the next four were 

traffic related, and the last two focused on transfer of 

communications. The pilot transmitted two of the traffic-related messages, and the controller transmitted the 

remainder. 

Time on Frequency (TOF):TOF (a) was measured from 

the onset to offset of individual transmissions, and it 

represented the amount of time the radio frequency was 

in use per message. As shown in the example in Figure 

2, TOF for the first message was 2 s.

Frequency Occupancy Time (FOT): FOT was computed 

for each approach by summing the time on frequency for 

each message. For example, FOT for Approach 2 was 20 

s [FOT = Σ (M1+M2+M3+M8+M9+M10+M11+M12) 

= Σ (2+1+5+3+ 4+2+2+1)]. FOT for the Traffic Information communication set (TI comm. set) was 14 s [Σ

(M3+M8+M9+M10) = Σ(5+3+4+2)].

Latencies were computed for each communication set 

for approaches that began with initial contact and ended 

in transfer of communication to the tower controller. 

An example of how each was computed is presented in 

Figure 2. These measures were designed to provide a 

metric to help estimate the operational impact and the 

benefits of CDTI. 

Visual Acquisition Time (VAT): VAT (b) was measured 

from the offset of the controller’s first issuance of a message 

with traffic-related content to the onset of the pilot’s reply, 

indicating a positive outcome (e.g., traffic in sight, tally 

ho). The thicker dashed-line illustrates the computation 

of a VAT of 39 s. VAT began at the completion of message 1 at 788 s and ended at 827 s (message 6) when the 

pilot of ABC1 23 informed the controller that the target 

traffic (ABC4 5 6) was sighted out the window. 

Visual Acquisition Time-Total (VAT-T): VAT-T (c) was 

measured from the onset of the controller’s first message 

that contained traffic-related information (message 1) 

to the offset of the pilot’s final response to that traffic 

call (message 9). As shown by the thicker solid line, 47 s 

lapsed from the onset of the ATC traffic communication 

at 783 s to its offset at 830 s. 

Instruction Route/Position – Follow time (IRP-F): The 

thin dashed line shows that IRP-F time (d) was measured from the offset of a traffic advisory (message 1) 

to onset of the controller’s message instructing the pilot 

to follow traffic (message 10). Upon hearing the pilot’s 

sighting report (message 9) at 830 s, 1 s later the controller instructed the pilot to follow that traffic and issued 

an approach clearance (message 10). Consequently, 43 s 

lapsed from the offset of a traffic call-out to the onset of 

an instruction to follow traffic.

Transaction Set Time (TST): TST (e) was measured from 

the onset of the first ATC message in the transactional 

communication set that contained traffic information to 

the offset of the final message in that set. It included all 

messages involved in a Traffic Information communication set (TI comm. set). TI comm. sets typically included 

a traffic advisory and an instruction to follow traffic, 

coupled with an approach clearance. The thin solid line 

shows the values used to compute a transaction set time of 

54 s. It begins at 783 s (message 3) with the controller’s 

first message indicating traffic and ends at 837 s (message 

10) with the final message from the pilot that closes the 

transaction.

Changes in Operational Communications. Changes 

in operational communications were grouped according 
7

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8 

to Traffic Flight Identifier Phraseology Usage, Communication Problems, and Operational Concerns. Each one 

is described below.

Traffic Flight Identifier Phraseology Usage: The participants in the evaluation had not previously used the 

traffic flight identifier (TFID) as a routine part of their 

traffic information messages. Although inclusion of the 

TFID should clarify communications, its presence might 

unknowingly change the normal exchange of information flow. Consequently, participants were encouraged 

to include the TFID if the intruder could be displayed 

on Ownship’s CDTI.

To distinguish between an aircraft being spoken to

(Ownship) from an aircraft being spoken about (Othership) in a traffic call, the ICAO phonetic pronunciation 

of the English alphabet was retained (e.g., Ownship 

— NOVEMBER ONE NINER ALFA BRAVO). When 

an aircraft was designated as traffic (i.e., spoken about)

and it could be symbolically represented on the CDTI, 

its flight identifier was to be spoken using conventionally 

spoken English alphabet letters (e.g., Othership — ‘N’ 

One Niner ‘A’ ‘B’). Presented in Figure 3 is a list of TFID 

usage categories and examples of each.

Communication Problems: Given the newness of using 

the traffic-flight identifier as a message element in a trafficrelated message, new communication problems may have 

emerged. Specifically, a communication problem refers 

to “any disturbance of routine communication, where 

controllers and pilots do not follow standard procedures, 

and/or where they must interrupt information transfer 

in order to clarify the communication” (Morrow, Lee, & 

Rodvold, 1990, pp. 36). Communication problems involved the misidentification of target aircraft, confusions, 

uncertainty, and other disruptions to normal information 

flow as noted by the SMEs.

Operational Concerns: Operational concerns were 

identified and categorized according to Traffic Advisory, 

Position Operation, or both. For example, when pilots 

reported traffic on the CDTI but not out the window, 

the occurrence was reported as a concern since pilots 

were not maintaining compliance with traffic advisory 

procedures. Also, when pilots lost visual contact with 

traffic while maintaining self-separation from that traffic, the occurrence was encoded as a concern in flight 

deck operations. 

In summary, message counts, contents, duration, 

rates, and reply latencies were the objectively derived 

measures of workload and communication extracted from 

the time-stamped voice tapes. They were used to compute 

descriptive statistics expressed as means (M) and standard 

deviations (SD) that summarized the use of the CDTI by 

the flight crews who participated in the 5-day evaluation 

of ADS-B/CDTI applications. Operational communication in the form of traffic flight identifier phraseology, 

communication problems, and operational concerns 

provided some insights and implications for future air 

traffic operations and communications procedures.

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9

Data Extraction, Training, and Data Encoding 

Procedures

This section begins with a description of the qualifications of the Subject Matter Experts (SMEs), continues 

with the data extraction procedures and is followed by 

the procedures used by the lead SME to train the other 

SMEs. The section ends with an explanation of the data 

encoding process.

Qualifications of the Subject Matter Experts. The lead 

air traffic SME was an instrument-rated pilot, as well as 

a former controller, FAA supervisor/manager (12 years), 

and FAA Academy instructor (8 years). Two additional 

SMEs were air traffic instructors (Terminal Option) at 

the FAA Academy in Oklahoma City. The pilot SME 

was a recently retired airline pilot with 31 years of experience. Prior to serving as an SME, the pilot’s duties 

included serving as an instructor on the B-727 and DC-8 

aircraft; check airman on the DC-9 aircraft; amd pilot 

of the CV-880, DC-8, B-727, DC-9, L-1011, B-757, 

and B-767 aircraft.

Data Extraction Procedures. Five sets of audiocassette 

tapes were dubbed from each digital audio tape (DAT). 

The transcribers used one copy to generate five sets of 

verbatim transcripts, and each message was typed onto 

an electronic copy of the Aviation Topics Speech Acts 

Taxonomy-Coding Form (ATSAT-CF; Prinzo, Britton, 

& Hendrix, 1995). Each message was preceded by its 

onset and offset time represented in hour (HH) minute 

(MM) and seconds (SS) and was followed by a column 

to record comments and a column to code operational 

concerns. 

Training Subject Matter Experts on the ATSAT. The 

Lead SME provided the other SMEs with 16 hours of 

training on the data encoding process to achieve consistency and conformity in identifying communication 

transactions and evaluating the accuracy of content. They 

evaluated controller/pilot communications effectiveness, 

identified pilot misunderstandings that pertained to the 

presence of the TFID, and any repeat of the TFID by the 

controller. Furthermore, they were to comment on any 

situation of separation, awareness, and misunderstandings that could be derived from the transcriptions, (e.g., 

missed readback of the identifiers, routes, and altitudes). 

Finally, all SMEs were to record any benefits that may have 

occurred from the pilots having access to a CDTI.

Data Encoding. Each SME received a complete set of 

audiocassettes, transcripts, and the code and instructional 

manual. The audiotapes and transcripts aided the SMEs 

in the identification of communication sets. SMEs worked 

independently to identify and code the efficiency and 

accuracy of communications as they related to a traffic 

advisory, report traffic in sight, advise traffic to follow, 

report the traffic to follow, follow the correct traffic, and 

call sign usage. They met on a weekly basis to compare, 

discuss, and resolve any inconsistencies in data encoding. 

The lead SME chaired the review process. Less than 2% 

of the messages contained any encoding discrepancies, 

and all were resolved by consensus. The data entry clerks 

received the final copy of the coded data and entered the 

information into the database for analysis.

Database Construction

Each traffic-call that was initiated by the controller, 

including those that referenced ‘previously issued traffic,’ was assigned a unique communication set identifier. 

The communication sets were categorized according to 

whether the aircraft that was designated as traffic (i.e., 

Othership) could be displayed on the Ownship’s CDTI. 

The display of Othership on Ownship’s CDTI depended 

upon the sending and receiving ADS-B links among those 

aircraft. That is, if Ownship only received 1090 MHz 

and the Othership transmitted on 1090 MHz, the communication set was coded ‘displayed.’ Likewise, if Ownship only received UAT and the Othership transmitted 

on UAT, the communication set was coded ‘displayed.’ 

If Ownship only received 1090 MHz and Othership 

transmitted on UAT then the communication set was 

coded ‘not displayed.’

RESULTS

Changes in operational communication that may have 

resulted from ADS-B/CDTI were evaluated from verbatim transcripts and digitized voice recordings provided 

by the TRACON facility. Although requests were made 

during the planning of the event that baseline circuits be 

included during the evaluation, none were conducted. 

Consequently, comparisons between the routine and 

communication protocol developed for pilot use during 

the evaluation could not be performed since the baseline 

data necessary for comparison were not included as part 

of the operational evaluation.

The analyses were restricted to 67 approaches (9 Day 

Long, 23 Day Short, 19 Night Long, 16 Night Short) that 

included transactions containing traffic information (TI 

comm. sets). Each completed approach lasted between 

251 s and 618 s (M=425.8 s, SD=104.9 s) during the 

long spacing interval and from 187 s to 664 s (M=488.6 

s, SD=99.8 s) during the short spacing interval. The approach phase began with initial contact to approach 

control (see Figure 1 Point 6) and ended in transfer of 

communication to the tower. 

Nine hundred and twenty-nine (929) messages (pilots=536, controllers=549) were exchanged between pilots 

and controllers, of which 320 messages (pilots=158, controllers=162) comprised 169 TI communication sets.
10 

A frequency analysis, which included a chi-square 

test, was performed to determine if CDTI aided pilots 

in visually acquiring aircraft called-out by ATC as traffic. 

The results revealed that when traffic was displayable on 

the CDTI, 76% of the TI communication sets (99/131) 

resulted in positive sighting reports compared with 53% 

(20/38) when traffic was not displayable, (χ2(1)=7.44). As 

shown in Table 4, 60 of the 67 approaches were performed 

with traffic displayable on the CDTI. Following receipt 

of positive traffic sighting reports from the flight deck, 

ATC changed 11 previously issued approach clearances 

from Instrument Landing System (ILS) to visual approach 

clearances. This, in turn, resulted in a 48% increase in 

visual approach clearances — an increase from 23 to 34 

visual approach clearances.

Presented in Table 5 are pilots’ and controllers’ reply 

onset latencies associated with messages involving trafficinformation and approach clearances. As the mean latencies for visual approach show, 2.4 s following the offset 

of a traffic call-out message from ATC, pilots reported 

traffic in sight (a). Then, 1.5 s following the receipt of 

a positive, out-the-window traffic sighting report, the 

controllers’ transmissions included instructions for the 

pilots to follow that traffic and a visual approach clearance 

(b). Pilots acknowledged those instructions and clearances 

3.1 s later (c). On average, 12.7 s lapsed from the onset 

of traffic-calls until pilots reported that traffic in sight 

(d). Generally, 12.1 s lapsed from the onset of the traffic-calls to the onset of the approach clearances (e) with 

the entire transaction taking 46.4 s to complete (f). The 

1 s median reply-onset time indicated that controllers 

and pilots were very prompt in exchanging information 

with each other.

Communication Workload

Multivariate Analysis of Variances (MANOVAs) were 

performed on the measures of communication workload 

presented in Table 6 and Table 7. Univariate Analysis of 

Variance (ANOVA) was used to assess the statistically 

significant findings. The Tukey Honestly Significant Difference (HSD) statistic was performed on statistically 

significant interactions. An alpha level of .05 was set for 

all statistical tests.

Efficiency of Communications: A Spacing Interval by 

Time of Day by Speaker MANOVA was performed on 

the mean for each communication efficiency measure and 

is presented in Table 6 (standard deviations are enclosed 

in parentheses). It revealed statistically significant main 

effects of Spacing Interval [F (3,124)=6.31], Time of 

Day [F(3,124)=3.17], and Speaker [F(3,124)=73.36], 

in addition to a statistically significant Time of Day by 

Speaker interaction, [F(3,124)=3.10].

Subsequent ANOVAs revealed that when pilots flew 

the long, compared to the short, spacing interval, 1.17 

fewer messages were transmitted per approach [N Messages, F(1,126)=5.84]: however, those messages took .35 s 

longer to transmit [TOF, F(1,126)=13.74]. Furthermore, 

individual messages transmitted during the day took .12 s 

longer to transmit than comparable messages transmitted 

during the night [TOF, F(1,126)=5.25], and the radio 

frequency was occupied for five additional seconds (5 

s) per approach during the day as compared to night 

operations [FOT, F(1,126)=5.98].

The difference in radio frequency use is attributable to 

the controllers, who spent more time on the radio than 

pilots, both when transmitting individual messages [TOF, 

F(1,126)=223.11] and overall [FOT, F(1.126)=57.71]. 

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11

The statistically significant Time of Day by Speaker interaction, [TOF, F(1,126)=4.86] revealed that controllers 

spent more time on frequency (per message) during day 

as compared to night operations; however, it did not 

matter to the pilots whether operations were conducted 

during the day or night — their messages were equally 

short, especially when compared to those transmitted 

by the controllers.

Duration of Communication: A Spacing Interval (Long, 

Short) by Time of Day (Day, Night) by CDTI (traffic 

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displayable, traffic not displayable) MANOVA was performed on how quickly pilots reported traffic in sight 

(VAT) and the total amount of time spent on a traffic call 

– from the onset of the traffic call by ATC to the offset 

of the pilot’s sighting report (VAT-T). Presented in Table 

7 are the means and standard deviations (in parentheses) 

derived from the 119 positive sighting reports.

The results from the MANOVA revealed statistically 

significant effects for Time of Day [F(2,111)=6.50)] and 

for the Spacing Interval by Time of Day interaction 
12 

[F(2,111)=16.54]. Subsequent ANOVAs indicated that 

pilots were faster to report visually acquiring traffic during the night (5.41 s) as compared to the day (16.42 

s) operations [VAT F(1,112)=6.62], and this effect was 

found to be independent of the spacing interval they 

were to maintain. Although it took longer to complete 

traffic call-outs during the day when pilots were on the 

long spacing interval (54.33 s) than during any of the 

other flights (DS=24.82 s, NL=14.81 s, NS=15.50 s), 

[VAT-T F(1,112)=6.16], p<. 05, this effect was independent of whether traffic was or was not displayable 

on the CDTI.

Notably, 83% of the visual approach clearances that 

included instructions for the pilots to “follow that traffic” were transmitted in 1 s or less after pilots reported 

visual contact with that traffic. Likewise, pilots were very 

prompt in returning receipt — they transmitted 86% of 

their acknowledgements in 2 s or less. Typically, 35 s or 

less lapsed from the end of a controller-initiated traffic-call 

to the start of a visual approach clearance that included 

the instruction for the pilot to “follow that traffic.”

Changes in Operational Communications. There 

were 301 transmissions included in this set of analyses. 

The participating controllers and pilots received basic 

instructions on the call sign procedure prior to the evaluation. The number and types of approaches flown were as 

follows: 25 visual, 24 ILS, and 18 changed. The changed 

approaches included: 14 ILS approaches were changed 

to visual approaches following pilots reporting of ‘traffic 

in sight,’ and 4 visual approaches were changed to ILS 

approaches when pilots reported ‘visual contact lost.’

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13

Traffic Flight Identifier Phraseology Usage: Table 8 shows 

that when traffic was not displayable on the CDTI, as 

is the case in today’s operations, controllers typically 

referred to it by aircraft type (e.g., MD-80, Company 

traffic, Helicopter, VFR) or by the word ‘traffic.’ In fact, 

only 2.7% of their messages included the full traffic flight 

identifier when that aircraft was not displayable on the 

CDTI, compared to 87.7% when it was. This occurred 

because the controller relied upon flight strip markings 

for CDTI equipage information and used appropriate 

phraseology for that traffic pair until informed by the pilot 

that Ownship’s CDTI was not operating properly.

In contrast, pilot usage of the traffic flight identifier 

was more variable. As shown in the lower half of Table 

8, when traffic was not displayable on the CDTI pilots 

excluded the traffic flight identifier altogether — more 

often than not, they responded with the word “traffic.” 

However, when traffic was displayable on the CDTI, more 

than half of their reports directly referenced that traffic 

by inclusion of its traffic flight identifier (full=46.8%, 

partial=6.3%). 

A detailed examination of how the traffic flight 

identifier was spoken is presented in Table 9. It must be 

pointed out that one controller did not know that one 

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14 

of the participating aircraft was operating without the 

capability to display traffic information. The data in the 

upper portion of Table 9 indicate there was a tendency 

among the controllers to speak the traffic flight identifier 

using the conventional English alphabet with the flight 

numbers spoken sequentially (e.g., A-B-C-1-2-3, not 

A-B-C-1-23) based on the information provided to the 

controller. Consequently, controllers communicated with 

pilots as though their CDTIs were operational (based on 

flight strip markings) unless told otherwise by the pilots. 

Rarely did the controllers use the ICAO Phonetic Alphabet 

(e.g., Alfa-Bravo-Charlie-One-Two-Three or Alfa-BravoCharlie-One-Twenty-three) when referring to traffic. An 

examination of the transcripts revealed that the ICAO 

Phonetic Alphabet was spoken primarily during the first 

day when controllers were first applying the call sign 

procedure developed for the Operational Evaluation.

The data in the lower half of Table 9 show that pilots 

did not use the traffic’s flight identifier if it was nondisplayable on the CDTI. However, if the traffic’s flight 

identifier could be displayed, there was a tendency to 

speak it using the conventional English alphabet with 

flight numbers spoken sequentially (e.g., L-L-L-3, RK-W-9). In some cases, pilots reverted to calling the 

traffic by its conventional name, spoken either with or 

without its flight numbers (e.g., Lanza Three, Rockwell, 

Five-oh-Five).

Communication Problems: Eight communication 

problems involved the misidentification of target aircraft, 

confusions, uncertainty, and other disruptions to normal 

information flow. Transmissions such as “Ownship, did 

you copy traffic?” and “Ownship, that was for an aircraft 

following you sir” are two examples of communication 

problems identified from the transcripts. Communication 

problems and their frequency of occurrence during the approach spacing application are presented in Table 10. 

Operational Concerns: Operational Concerns were 

identified and categorized as concerns related to Traffic 

Advisory, Position Operation, or both. For example, there 

were three transactions in which the controller wanted 

the pilot to verify the identity of the traffic — “Verify 

it’s one two three that Ownship has.” As shown in Table 

11, an operational concern for controllers involved an 

aircraft overtaking the aircraft that it was following for the 

approach — “Ownship, traffic you’re following now four 

miles ahead currently showing six zero knots faster than 

that traffic.” For pilots who were instructed to maintain 

visual separation from traffic while on final approach, 

controllers expressed concerns when pilots reported loss 

of visual contact with that traffic during the approach. 

The presence of an aircraft other than Ownship executing 

a clearance issued to another aircraft resulted only when 

traffic was displayable on the CDTI. This is best exemplified in the following ATC transmission: “Ownship, roger, 

that was not for you – follow Othership cleared visual 

approach runway three five left.” The controller action 

in these cases is no different than today’s traffic environment when one aircraft takes a clearance or information 

intended for another.

DISCUSSION

The analysis of voice communications from the approach spacing application suggests that neither the pilots 

who used the ADS-B/CDTI nor the controllers who 

applied current ATC procedures without the benefit of 

ADS-B displays experienced notable increases in communication workload. In particular, neither the number nor 

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15

duration of messages resulted in inefficient communications. The discovery of a general trade-off between the 

number of messages transmitted and duration is typical 

of day-to-day operational communications. In particular, 

there were fewer messages of longer duration that were 

transmitted during the long, compared with the short, 

spacing interval. Unfortunately, this evaluation did not 

include conditions that could provide direct comparisons; 

consequently, a need still exists for research to document 

the implications of the approach spacing procedure on 

safety and system efficiency, particularly during periods 

of heavy traffic capacity.

The results did reveal that, although the presence of a 

CDTI onboard the aircraft did not produce a statistically 

significant difference in how quickly pilots reported traffic 

in sight, it did change the number of actual sightings. That 

is, when traffic was displayable on the CDTI, 76% of the 

traffic calls ended with positive sighting reports compared 

to 53% when traffic was not displayable. Upon receipt 

of positive traffic sighting reports, controllers amended 

11 previously issued Instrument Landing System (ILS) 

approach clearances to visual approach clearances.3

A visual approach clearance authorizes the pilot to 

proceed visually and clear of clouds to the airport. To 

conduct a visual approach, the pilot must, at all times, 

have either the airport or the preceding aircraft in sight. 

This approach must be authorized and under the control 

of the appropriate air traffic control facility. Reported 

weather at the airport must be ceiling at or above 1,000 

feet and visibility of 3 m or greater. Once the controller 

issues the visual approach clearance and the pilot accepts 

it, separation and navigation to the airport become the 

responsibility of the pilot. 

Which type of approach is preferred by pilots — visual 

or ILS? Performing a visual approach is considered 

friendlier by pilots than an ILS approach because they 

can choose the most direct route to touchdown and the 

spacing behind preceding aircraft. However, many pilots 

prefer some type of guidance for the approach, rather than 

a strict visual approach — especially when entering the 

airspace of a heavily populated airport having a strong 

presence of commercial aircraft. That is because the more 

traditional ILS approach brings the aircraft to a specific 

point approximately 5 m from the end of the runway, at 

a specific altitude and speed. Although ILS approaches 

are less efficient than visual approaches, they serve the 

purpose of providing pilots exact information as to how 

they are managing their flight profiles to a specific and safe 

area on the runway surface. Consequently, pilots might 

prefer the flexibility of the visual approach, coupled with 

the precision that is afforded the ILS approach. 

The SMEs spent many hours listening to the voice 

tapes, reading transcripts, encoding communications, and 

meeting together to discuss their assessments. Their concerns centered on safety. In particular, current air traffic 

procedures do not allow visual separation by a following 

aircraft during instrument meteorological conditions. If 

CDTI electronic flight rules were developed for pilots to 

use CDTI in lieu of current visual, out-the-window traffic acquisition, separation responsibilities must be clearly 

understood by both the pilot and air traffic controller. 

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3

 The reference for visual approaches and the utilization 

of them for controllers in covered in FAA Order 7110.65 

Chapter 7, Section 4. Pilot’s responsibilities for flying visual 

approaches are best defined in the Aeronautical Information 

Manual Section 5-4-20.
16 

For example, during a portion of the operational evaluation, weather conditions became marginal. While the 

pilot was on approach, visual contact with Othership was 

lost, whereby the instruction to ‘maintain visual separation 

with that traffic’ could no longer be performed. However, 

the pilot reported, “We lost the visual on the traffic, but 

we still have them on the CDTI.” The example raises the 

question, “In the event that traffic is no longer visible 

out the window, but it is displayed on a CDTI, should 

the pilot be able to continue or relinquish responsibility 

to ATC?” 

If the answer is to continue, then a reexamination 

will need to be conducted of current procedures and 

phraseologies that address separation standards, as well 

as the roles and responsibilities for pilots and controllers 

for final approach spacing. Specifically, unambiguous 

procedures and phraseologies must be established to 

handle normal traffic and occasions when the potential 

or threat of a loss of minimum separation may occur. 

This would include the expected/required flight path 

and separation responsibility during a missed approach 

or go around. The procedures, separation responsibility, 

and phraseology must apply during visual or instrument 

flight rules in visual, marginal visual, or instrument 

meteorological conditions.

Another important safety issue that was identified by the 

SMEs involved pilots maintaining specific approach spacing or separation distances and concerns about whether 

pilots should be permitted to perform CDTI self-spacing, 

and if so, under what conditions. During the operational 

evaluation, pilots were working to achieve an assigned 

spacing interval and often increased their aircraft speed 

to close the gap between Ownship and Othership. The 

communications analysis offered some objective support 

for this concern. There were a number of situations when 

controllers issued speed advisories from 20 knots up to 70 

knots of speed overtake. For example, “Ownship traffic 

twelve o’clock and three miles Othership you’re following 

is indicating thirty knots slower,” or “Othership, previously issued traffic now twelve o’clock and four miles 

indicating seven zero knots slower.” 

The voice tape analysis and SME comments further 

suggested a concern from controllers actively controlling 

and managing the airspace to only monitoring it when 

pilots were maintaining their own spacing and speed 

control — especially under reduced visibility. Should 

the loss in established minima occur, who would be 

cited with the infraction – the pilot or the controller? 

Once again, clearly defined roles and responsibilities 

for maintaining spacing minima using ADS-B/CDTI 

under changing visibility and weather conditions must 

be developed, understood, and agreed upon before the 

approach spacing procedure is approved.

Finally, an important issue for system developers and 

system maintainers that must be addressed is the need by 

ATC for timely updates of airborne CDTI operability. The 

participating controllers relied upon flight strip markings 

for CDTI equipage information and used appropriate 

phraseology for traffic pairs until informed by the pilot 

that Ownship’s CDTI was not operating properly. For 

example, in response to the ATC message, “Ownship 

one two three reduce your final approach speed, you are 

three and a half miles in trail Othership five-sixty doing a 

hundred and seventeen knots,” the flight deck responded 

with, “Roger, reduce to one one five and our unit is not 

working so we can’t space on him.” Will new situation 

displays automatically provide controllers with an aircraft’s 

ADS-B/CDTI equipage information and update it if the 

unit is no longer functioning, or will the pilot need to 

inform the controller upon initial call-up?

Not surprisingly, new research will be needed to determine the effect on the safe, orderly, and expeditious flow 

of traffic, with the use of pilot-managed approach spacing 

intervals and any developed CDTI-enhanced flight rules 

during marginal visibility. If the ADS-B/CDTI approach 

spacing application is implemented, then a clear understanding is needed of the roles, responsibilities, procedures, 

and phraseology for pilots and controllers who use it. A 

vital lesson has been learned from the implementation 

of a Traffic Collision Avoidance System (TCAS) that applies to ADS-B/CDTI. Any new or modified air traffic 

procedure that does not include a specific, unambiguous 

phrase to initiate that procedure contributes to an air 

traffic incident when not understood by both the pilot 

and controller.

REFERENCES

Federal Aviation Administration. (2000). Air Traffic Control (7110.65M). Washington, DC: U.S. Government Printing Office.

Federal Aviation Administration. (2001). Aeronautical 

Information Manual. Washington, DC: U.S. Government Printing Office.

Joseph, K.M., Domino, D. Battiste, V. and Bone, R. 

(2003). A Summary of Flightdeck Observer Data from 

SafeFlight 21 OpEval-2. Federal Aviation Administration, Office of Aerospace Medicine Technical 

Report DOT/FAA/AM-03/2, Washington, DC.

Minimum Aviation System Performance Standards for 

Automatic Dependent Surveillance Broadcast (ADSB). Washington DC: RTCA, Inc. (February 1998). 

(Prepared by SC-186). RTCA/DO-242.
17

Morrow, D.G., Lee, A., and Rodvold, M. (1990). Analysis 

of routine pilot-controller communication. In: Managing the Modern Cockpit: Third Human Error Avoidance Techniques Conference Proceedings. Warrendale 

PA: Society of Automotive Engineers, Inc.

Operational Evaluation Coordination Group (2001). 

CAA/FAA/Safeflight21 Operational Evaluation2 Final Report. www.faa.gov/safeflight21/orv/

opeval_2/index.html

Prinzo, O.V. (1996). An Analysis of Approach Control/Pilot 

Voice Communications. Federal Aviation Administration, Office of Aerospace Medicine Technical 

Report DOT/FAA/AM-96/26, Washington, DC.

Prinzo, O.V. (2001). Innovations in Pilot Visual Acquisition of Traffic: New Phraseology for Air Traffic Control Operational Communication. Federal Aviation 

Administration, Office of Aerospace Medicine 

Technical Report DOT/FAA/AM-01/19, Washington, DC.

Prinzo, O.V., Britton, T.W., and Hendrix, A.M. (1995). 

Development of a Coding Form for Approach Control/

Pilot Voice Communications. Federal Aviation Administration, Office of Aerospace Medicine Technical 

Report DOT/FAA/AM-95/15, Washington, DC.