DOT/FAA/AM-01/8 Data-Linked Pilot Reply Time

On Controller Workload and

Communication in a Simulated

Terminal Option

O. Veronika Prinzo

Civil Aeromedical Institute

Federal Aviation Administration

Oklahoma City, Oklahoma 73125

May 2001

Final Report

This document is available to the public

through the National Technical Information

Service, Springfield, Virginia 22161.

Office of Aviation Medicine

Washington, D.C. 20591

U.S. Department

of Transportation

Federal Aviation

Administration
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-01/8

4. Title and Subtitle 5. Report Date

Data-Linked Pilot Reply Time on Controller Workload and

Communication in a Simulated Terminal Option

May 2001

6. Performing Organization Code

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

Prinzo, O.V.

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

FAA Civil Aeromedical Institute

P.O. Box 25082

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

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

Office of Aviation Medicine

Federal Aviation Administration

800 Independence Ave., S.W.

Washington, D.C. 20591 14. Sponsoring Agency Code

15. Supplemental Notes

This work was performed under Task AM-B-98-HRR-516.

16. Abstract

This report describes an analysis of air traffic control communication and workload in a simulated terminal radar approach

control environment. The objective of this study was to investigate how pilot-to-controller data-link acknowledgment time

might affect controller-perceived workload and operational communication. Eight controllers provided air traffic services to

simulated arrival aircraft during a moderate-traffic density simulation in which voice radio and data link communications

were available. The effect of a delay in downlinked pilot acknowledgment time to controller uplinked messages was the

primary variable of interest. Each controller completed the same scenario twice, with the order of presentation

counterbalanced. That is, for the first simulation, half the controllers received immediate responses to their data link

messages while the other half received pilot acknowledgment time delayed by about 11 seconds. The primary finding was a

general, albeit not statistically significant, tendency among controllers towards an increase in subjective workload overall and

on individual dimensions of the NASA Task Load Index when the simulation environment supported mixed, rather than

immediate pilot responses. The analysis performed on communication measures revealed that controller workload was

affected by the communications capability onboard the aircraft but not ghost pilot response type (immediate, delayed).

Transmissions to data link-equipped aircraft took longer to complete and contained longer pauses than transmissions to

non-data linked aircraft. The mode of communications selected by controllers also influenced workload but ghost pilot

response type did not. When controllers used data link, their transmissions took nearly 2 seconds longer to complete than

comparable messages sent by voice. When compared with voice radio messages, controllers took longer to formulate and

transmit messages over a data link-communications system, but their messages were more accurate and contained fewer

message elements. This finding was replicated when messages with courtesies, salutations, and those involved in the initial

contact services were removed. Often, the data-link transmissions that contained longer pauses resulted from deliberate

actions on the part of the controllers. Longer latencies did not result in a loss of efficiency but reflected innovations on the

part of the controllers to optimize performance. For example, by placing data-link messages in a pending status, controllers

would communicate with a non-data-link-equipped aircraft. When the data link-equipped aircraft arrived at a transitionpoint along their filed flight plan (generally approaching a hand-off point or transition altitude or fix), controllers uplinked

their already composed messages to those aircraft.

17. Key Words 18. Distribution Statement

Air Traffic Control Communication, Data Link Document is available to the public through the

National Technical Information Service, Springfield,

Virginia 22161.

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

Unclassified Unclassified 21

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

iii

ACKNOWLEDGMENTS

I thank the air traffic controllers, supervisors, managers, and staff from the Southwest Region

for their help and participation in this study. Also, greatly appreciated was the assistance provided

by Mr. Al Hendrix (subject matter expert), the OMNI Corporation for providing the air traffic

control and pseudo pilots for this study, and Abacus Technologies for software and technical

support. Ms. Sandy Dooty, Mr. Scott Hughes, and Mr. James Robinson (subject matter expert)

receive a special acknowledgment for the many tedious hours of effort devoted to transcribing,

coding messages into speech acts, and preparing the databases for analysis.

TABLE OF CONTENTS

Section Page

INTRODUCTION .......................................................................................................................................... 1

Timing Parameters: Technology .................................................................................................................... 1

Timing Parameters: Human Performance .................................................................................................... 2

Workload ....................................................................................................................................................... 2

METHOD ........................................................................................................................................................ 4

Participants .................................................................................................................................................... 4

Apparatus ...................................................................................................................................................... 4

Simulation Support Staff .............................................................................................................................. 4

Materials ........................................................................................................................................................ 4

Simulated Prototypical Data Link Services .................................................................................................. 5

Procedure....................................................................................................................................................... 6

Experimental Design .................................................................................................................................... 7

Dependent Measures .................................................................................................................................... 7

RESULTS .......................................................................................................................................................... 9

The Relationship Between the Temporal Characteristics of the Simulation Environment and Subjective

Workload .............................................................................................................................................. 9

The Relationship Between the Temporal Characteristics of the Simulation Environment, Objective

Workload and Communication Performance....................................................................................... 9

The Relationship Between the Temporal Characteristics of Downlinked Ghost Pilot Responses and

Objective Measures of Initial Contact and Transfer of Control Performance ................................... 11

DISCUSSION ................................................................................................................................................. 12

REFERENCES ................................................................................................................................................ 14

v

1

DATA-LINKED PILOT REPLY TIME ON CONTROLLER WORKLOAD AND

COMMUNICATION IN A SIMULATED TERMINAL OPTION

“Men have become the tools of their tools.”

—Henry David Thoreau (1817-1862)

INTRODUCTION

The Aeronautical Data Link System (ADLS) is

being developed as part of the Federal Aviation

Administration’s (FAA, 1995, 1997) modernization

plan to improve safety, reduce delays, and increase

the efficiency of human and system resource utilization. As part of this effort, Controller/Pilot Data

Link Communication (CPDLC) is being developed

to re-direct routine air traffic services provided by an

overly taxed voice radio communications system.

CPDLC represents the first phase of the transition

from the analog voice system to an International

Civil Aviation Organization (ICAO) compliant system1

. Initially, CPDLC en route applications will

support limited services. However, as CPDLC matures, additional services will be added and it may

expand to include terminal applications. Ultimately,

CPDLC services will provide controllers with the

capability to initiate a data link uplink message, using

pre-defined message sets, to service eligible aircraft

and receive data link messages (FAA, 1998a, 1998b).

Timing Parameters: Technology

The arrival of CPDLC messages to the intended

receiver is dependent, in part, upon the speed of endto-end information transfer among ADLS components. End-to-end transfer delay is defined as “the

period elapsed from the time at which the originating

user initiates the triggering event until the time the

transmitted information is available for display to the

intended recipient” (FAA, 1997, p. 39). Based on this

definition, the estimated end-to-end transfer delay

times and performance limits, as derived from the

initial requirements for CPDLC in the terminal domain, are presented in Table 1 (FAA, 1997).

A few studies of data link flight deck simulation

have reported throughput delays (this could include

the processing speed of a device in Mhz, the data

transfer rate, and other delays due to the limitations

of the technology). For instance, Waller and Lohr

(1989) incorporated a 4.1s end-to-end transfer delay,

and Knox and Scanlon (1991) included a 1.5s delay

(3s round-trip delay). Likewise, delays of 4-6s (mean

= 5s) have been used by the Data Link Benefits Study

Team (DLBST) to examine terminal data link (1996)

and delays of 6s multiples on downlink for an en

route Mode S radar system (1995).

Timing Parameters: Human Performance

Total transaction times will inevitably involve

human delays. Airborne systems, for example, include a 2s delay before a pilot could reply to an ATC

transmission (SAE ARP4791 Revision A, 7.7.11).

This parameter allows the non-flying pilot sufficient

time to read 10 words from a display with an average

silent reading rate of about 5 words per s (Stricht &

Hall, 1984). Therefore, under optimal conditions, a

pilot flying in terminal approach control airspace

could receive a message 5s after it was sent, read a 10-

TABLE 1. CPDLC End-to-End Transfer Delay Performance Presented by Domains.

Domain Mean End-to-End

Transfer Delay

95% End-to-End

Transfer Delay

99.996% End-to-End

Transfer Delay

Terminal 5 sec 8 sec 12.5 sec

En Route 10 sec 15 sec 22 sec

1

 The system shall comply with ICAO Standards and Recommended Practices (SARPs), Aeronautical Telecommunications,

Annex 10 to the Convention on International Civil Aviation, Volume III, Part I, Digital Communication Systems, Chapter 3,

Aeronautical Telecommunication Network (ATN).
2

word message in 2s and be ready to transmit a reply.

From the moment the message leaves the ground

until the pilot executes a response, 7s lapses.

On the airborne side, total transaction time has

been defined as “message receipt to pilot acknowledgement in data link” (Logsdon, et al., 1995, p.

327). It includes time for the pilot to access the

message, read it, and acknowledge it (McGann, et al.,

1998). However, in some instances, “total transaction time” and “response time” appear to be interchangeable. Waller and Lohr (1989), for example,

defined response time as “the time between the first

alerting signal indicating the arrival of a new message

in the cockpit and the copilot depressing the ‘ENT’

key to downlink a roger or unable response” (p. 8). In

contrast, Talotta et al (1990) decomposed total transaction time into Response Latency (RL) and Wilco/

UNABLE Response Time (WUT). RL was defined as

“the interval begins when ‘Touch for ATC Message’

appears on the display and ends when the pilot

touches the prompt ‘Touch for ATC Message’.” The

WUT was defined as “The interval begins with the

appearance of the ATC Instruction on the display

screen and ends with the touch of WILCO or UNABLE” (p. 35).

On the ground side, the results of the 1996 Data

Link Benefits Study Team (DLBST) study, performed

at the FAA’s William J. Hughes Technical Center for

terminal approach control data link, revealed controller transaction times that ranged from 8s to more

than 200s. In that study, total transaction time was

defined as “the period of elapsed time from the

controller’s input to send the message to the appearance of a downlinked response on the controller’s

display” (p.79). Furthermore, it “includes the technical system delays associated with uplink and downlink and the time required by the pilots to detect,

process, and respond to the message” (p. 77). A closer

examination of the frequency distribution of end-toend transfer delay times revealed that about 74% of

the transactions were completed between 10.1s and

16.0s. It was further noted that completing transactions in less than 10s occurred for fewer than 250 of

the 9,036 messages.

In a simulation study performed by Knox and

Scanlon (1991) “total message transaction time” was

defined as “the time from when a message is sent from

the ground until the ground receives the pilot’s roger

or unable response” (p. 9). This definition is comparable to the one provided in the DLBST Report

(1996). That is, total transaction time represents the

entire time span a controller would need to maintain

awareness of the status of a communication.

Clearly, technology- and human-based delays can

influence the efficacy of air-ground operational communication performance required for ATS communications supporting specific services, operations, or

procedures within defined homogeneous airspace.

For example, the Kerns (1991) review of the data link

literature indicated that, on the average, data link

total transaction times were twice as long as for voice.

Likewise, en route total transaction times varied from

19-21 seconds for data link and 8-10 seconds for

voice (Waller & Lohr, 1989; Talotta et al., 1990).

These added delays resulted, in part, from pilots

auto-loading information into their navigation computer Control Display Unit (CDU) before down

linking an acknowledgment (Waller & Lohr, 1989).

Workload

Kerns' (1991) review of the data link literature

states, “Virtually all of the studies report no significant effect on pilot or controller workload as a result

of using the data link” (pp191). In a later statement,

Kerns indicates that, with respect to pilots, "... the

research results do document a redistribution of

workload across the human’s information-processing resources: Visual and manual workload increase

whereas auditory and speech workload decrease ”

(Grouce & Boucek, 1987). Likewise, for controllers,

Talotta et al. (1990) reported a reduced speech

workload and an increased manual workload when

data link was used.

These outcomes suggest that the redistribution of

workload across input and response domains was

insufficient to influence overall workload. To address this peculiarity, Prinzo (1998) conducted a

simulation study. Following the completion of heavy

and light traffic simulations using voice radio only or

a combination of voice radio and a data link, controllers provided subjective ratings of workload, using

the NASA Task Load Index (TLX) (Hart & Staveland,

1988). An analysis was performed on the controllers’

subjective ratings along 6-dimensions of workload

and on a separate overall rating. In addition to their

ratings revealing a greater perceived overall workload

among controllers when providing approach control

services in a mixed compared with a voice only

environment, their ratings revealed a perceived increase on several of the individual dimensions.
3

For the individual ratings, controllers’ ratings were

significantly higher on the temporal and frustration

dimensions and approached significance on physical

demands. It may be that the perceived alterations in

workload resulted from controllers switching between voice radio and data link in a mixed-modality

environment, using a communications system that

was slower than voice radio, or both. Consequently,

their subjective ratings on the physical demand dimension may have increased as keyboard entries

multiplied. Likewise, their ratings on the temporal

demand dimension could have resulted from changes

in the temporal component of the communications

system (i.e., voice radio is virtually instantaneous,

whereas data link is not). These findings agree with

the perceptions reported by Auckland Oceanic controllers who estimated about a 40% increase in

workload with CPDLC, without a corresponding

increase in traffic (Ruitenbuerg 1998).

It is also possible that an environment supporting

both voice radio and data link unknowingly adds a

modality-monitoring task to the controller’s cognitive workload. This may inadvertently increase perceived ratings of workload. As shown in the upper

portion of Figure 1, in a unimodal communication

environment, controllers easily monitor their communication transactions with pilots and may hypothetically set an internal reply timer. If, after some

number of seconds, a reply is not received, the controller might verbally ask the pilot if the message was

received. The lower portion shows that, in a dualmode environment, controllers might set one implicit internal timer for voice radio (faster) and another

for data link (slower) and develop a strategy to monitor the status of each.

In a dual-mode environment, controllers could

create one internal timer based on a single mode, but

it could create problems. For example, using voice

radio as the default could add to controller workload

by increasing the number of queried data link transmissions (not letting the data link total transaction

time lapse before re-sending the message). Using data

link as the default could create delays or untimely

events for aircraft equipped for voice only communications. Consequently, inclusion of a second communications system into air traffic control operational

communications could alter communication performance and perceived workload.

Just as the controllers who participated in the

Prinzo study (1998) reported elevated subjective

workload, so did the assistant controllers who participated in DLBST Experiment 3 (1996). In that DLBST

experiment, the assistant controllers sent data link

messages to the aircraft, as did the controllers in the

Transmission + Event Readback + Event Hearback + Event Outcome

Send a voice radio message

Yes Correct

Set internal reply timer

(n sec delay: voice)

Receive a reply Readback Close transaction

No Incorrect Apply repair

procedures

Retransmit

Select modality type (voice radio or data link)

Send the message

Yes Correct

Set internal reply timer

(n sec delay: voice)

(n' sec delay: data link)

Receive a reply Readback Close transaction

No Incorrect Apply repair

procedures

Retransmit

Figure 1. Hypothetical cognitive workload for voice and data link messages.
4

Prinzo study. In both simulation studies, controllers

provided air traffic services during complex traffic

situations (a mix of aircraft arriving at the major and

satellite airports). The DLBST concluded that providing air traffic services during highly variable traffic situations (e.g., many different airport destinations,

an assortment of aircraft types and performance characteristics, multiple arrival/departure routes) can produce perceptions of greater workload when working

with a new set of communications procedures/tools.

It was unclear whether the changes in workload and

communication performance were attributable to the

inherent temporal delays associated with data link

transmissions, providing air traffic services in a dual

mode environment, or both.

Bower and Clapper (1989) pointed out that people

generally tend to pause longer while speaking, especially when heavily involved in planning the next

portion of their utterance (pp 281). This observation

also was noted by Prinzo, Lieberman, and Pickett

(1998) in one simulation study where 82% of the

controllers produced longer intra-sentential pauses

for light rather than heavy traffic simulations. Prinzo

et al. (1998) suggested that pause duration could

reflect differences in information processing demands

among controllers. That is, during a lighter workload

situation, controllers seemed to use a more flexible,

cognitive approach towards traffic management. In

effect, the lack of a traffic demand provided them

with more time to strategize and vector aircraft.

Then, as traffic increased, they seemed to switch to a

more automatic and repetitive approach towards

maneuvering traffic through their airspace. Thus,

hesitation pauses in the context of air traffic control

might reflect cognitive demands related to workload.

That is, reductions in workload could be reflected by

the presence of longer intra-sentential pauses, indicative of more deliberate thought.

The purpose of this simulation study was to disentangle the effects of delay from those due to the

communication environment on controller workload.

To accomplish this, temporal variations (all immediate responses or mixed) and ghost pilot response type

(voice immediate, data link immediate or delayed by

approximately 11s) were manipulated. Their independent and combined effect on controller workload,

communication performance, and the performance

of initial contact and transfer of control subtasks

were examined.

METHOD

Participants

Eight full-performance level air traffic control

specialists from a level 5 TRACON facility participated in the study. Collectively, they had 14.19 mean

years of terminal experience (SD = 5.9), with 9.1

mean years (SD =7.0) at the full-performance level.

Apparatus

The CAMI TRACON PC-based simulation laboratory includes 2 file servers, 2 controller workstations, 2 ghost pilot workstations, and 1 remote

position workstation. Airspace and radar data were

presented on 21-inch monitors. The laboratory is

designed to provide full audio and video recording

capability. A detailed description can be found in

Prinzo (1998).

Simulation Support Staff

The simulation support staff consisted of 2 ghost

pilots, 2 retired controllers serving as subject matter

experts (SMEs), and a software technician. The certified ghost pilots, formally trained and instructed in

ATC communications by a vendor at the FAA Academy in Oklahoma City, also provide support services

at the Academy. One SME reviewed and modified

the scenarios, developed briefing materials, trained

the ghost pilots on the TRACON system and scenarios, and provided them with on-line instruction

during each simulation. The other SME was the

receiving controller during simulations. The software technician developed front-end applications for

the TRACON software to reduce the amount of time

and level of effort required for database construction

and scenario development.

Materials

Scenario Development. The TRACON facility’s

“1997 Game Plan,” an internal document, was used

to select the time intervals for simulation. It provides

historical information pertaining to arrival rushes by

direction of arrival into the TRACON airspace. The

TRACON facility also provided computer data that

contained actual flight plans for a 24-hour (hr) time

interval. The flight plan data were reformatted and one

35-minute (min) time interval was selected for simulation. The flight plan data also were used to develop one

15-min practice scenario to instruct and train participants on the use of the TRACON simulator.
5

As shown in Table 2, the distribution of aircraft

approaching the sector during each 10-min time

interval was moderate. Table 2 also shows that approximately 60% of the arrival aircraft were from

American Airlines’ (AAL) fleet of MD80s, and all of

them were data link equipped.

The following constraint was imposed. Although

all of the airplanes in each of the scenarios were voice

radio equipped, only AAL’s MD80 aircraft were data

link equipped. The Feeder East arrival aircraft appeared approximately 5 miles (mi) from the Northeast and Southeast vortac. All arrival aircraft were on

a north flow. The last arrival appeared approximately

35min into the simulation. The simulation was

stopped once the last MD80 aircraft was handed-off

to, and accepted by, the receiving controller staffing

the Final sector. The fidelity and realism of the

scenario were evaluated by staff from the TRACON

facility to ensure that it was representative of day to

day traffic operations for the position.

NASA Task Load Index. Hart and Staveland (1988)

developed the NASA Task Load Index (TLX). It is a

widely used and accepted multi-dimensional subjective rating procedure and is an index of perceived

workload. They conceptualized workload as a construct that involves 6 dimensions: Mental Demands,

Physical Demands, Temporal Demands, Performance,

Effort, and Frustration. Five of the scales are anchored with “low” (0) on the left and “high” (100) on

the right. The Performance scale is anchored by

“good” (100) on the left of the dimension and “poor”

(0) on the right.

Simulated Prototypical Data Link Services

Only the control instructions (CI), terminal information (TI), and transfer of communication (TOC)

data link services were simulated. The instructions

on how these services were executed are presented

below. The initial contact (IC) service was not examined in this study. Instead, current voice radio procedures for establishing radar contact were implemented.

The full data block would appear flashing on the

receiving controller’s terminal display and an asterisk

(*) would precede the call sign of a data-link-equipped

aircraft. The data link was active and available for the

controller to uplink the first message. When the

controller accepted the handoff, the ghost pilot would

state the aircraft call sign, altitude, and current Automatic Terminal Information Service (ATIS) code

(e.g., “American one twenty-three with you at one

two thousand with information Alpha”).

CI and TI Messages. Controllers could select any of

the CI and TI topics from the menu list presented in

Figure 2. The controller depressed [F16] and the

alphanumeric key associated with that entry (i.e., 1,

2, 3, A, B, etc.), slewed the trackball until the cursor

“E” was overlaid upon the position symbol “E” associated with the desired aircraft, and then depressed

the Enter key. During message construction, the

alphanumeric keys that were depressed were echoed

back in the preview area of the controller’s terminal

display (e.g., F16 1). After the Enter key on the

trackball was depressed, the information in the preview area was erased and replaced by the aircraft call

sign, message topic, and the status “SND” in the

TABLE 2. Distribution of Arrivals Presented by Aircraft Type and Time Interval in the

Scenario

Arrival Aircraft

Time Interval

(in Minutes) Props AAL (MD80) Other Jets Total

00-09 0 6 6 12

10-19 2 7 2 11

20-29 0 6 3 9

30-40 0 3 2 5

Total 2 22 13 37
6

Data Link Uplink Message List Uplink Message in the Full Data Block

CI TI

1. A050 A EXP RWY31R

2. A060 B EXP RWY35R

3. S210

*AAL123

110 250

A050

4. S230 E

5. H290

6. H280 A060

Figure 2. CI and TI Uplink Messages.

status list (e.g., AAL123 A050 SND). The CI message

also appeared in the third line of the full data block

with the numeric data and an appropriate preceding

letter of A, H, or S (e.g., A050).

As soon as the controller sent the uplinked message,

it appeared on the ghost pilot’s display in the third line

of the full data block for the receiving aircraft (e.g.,

A050). The ghost pilot selected “WILCO” to acknowledge receipt of the uplinked message and, either

immediately or after approximately 11s, sent the downlink to the controller. For the controller’s terminal

display, “WILCO” remained in the third line of the

full data block until the controller closed the transaction. The controller overlaid the cursor and datatag

position symbols and then depressed the Enter key on

the trackball. This procedure deviated from the original design presented in a MITRE document (1990). In

that document, the “WILCO” disappeared from the

controller’s display after a predetermined timeout parameter. No action on the part of the controller was

required. We chose to change this procedure because

the controller could be scanning a different area of the

controller’s terminal display and miss the downlink

acknowledgment.

Manual Mode TOC. The sending controller constructed and transmitted the TOC by depressing the

[F9] key which resulted in the appearance of the “TC”

in the preview area of the controller’s terminal display.

The controller then slewed the trackball until the

sending and receiving controller’s position symbols

were overlaid upon one another in the data block.

When the Enter key on the trackball was depressed, the

“TC” disappeared and no further action was required.

The ghost pilot did not “WILCO” the TOC, and there

was no change to the status list. When the receiving

controller accepted responsibility for an aircraft, the

sending controller’s display was replaced with a

radar secondary track, and the sending controller’s

position symbol was replaced by the receiving

controller’s position symbol.

The current CPDLC TOC design specification

requires a “WILCO” acknowledgment from the

pilot for a TOC. It includes the display of a message

sent in the data block. It also includes a change in the

sending controller’s display from a “data link eligibility (*)” to an “active session (+)” indicator meaning that the aircraft is capable of data link

communication but the sending sector is no longer

eligible to send a message. This feature was not

included as part of the transfer of communication in

the current study since the sending controller could

not communicate with the aircraft by data link once

radar transfer was accepted and the TOC was transmitted.

Although the data link services reported here

deviated from the current designs being evaluated

by FAA for data link services, the fundamental

concept is not unique to this simulation study. That

is, data link services will be provided by a communications system, that uses digitized signals, as was

done here. Likewise, input into that communications system will be performed by controllers who

will make entries using a keyboard-input module

and trackball, as was done here.

Procedure

After the required administrative paperwork was

completed, each controller received 4hrs of

familiarization training on the simulator. They also

were instructed on the procedures for transfer of
7

communications (TOC), constructing controller instructions (CI) and terminal information (TI), and

transmitting messages using the ARTS IIIA data link

interface. During that time, the trainer observed the

controllers demonstrating their skill and dexterity

with the ABC-style keyboard-input module. The

controller completed a 15-min practice scenario representative of arrival traffic on the sector. Any problems were noted and corrected through additional

training or practice.

Following familiarization training, controllers

completed a practice scenario to reinforce and increase their data link communication skills. Then

controllers completed the first 30-min scenario, provided TLX workload ratings, and received a 15-min

break, and so on until each of the scenarios was

completed twice. While providing air traffic services

to simulated traffic, controller voice communications were recorded onto digital analog tapes and

time stamped. Radar display and ARTS IIIA keyboard and trackball entries were recorded on VHS

tapes. At the end of the study, each controller completed a questionnaire and provided a written evaluation and comments related to using data link on the

simulated sector.

Experimental Design

All of the controllers provided air traffic services in

a simulation environment supporting dual modes of

communications. The temporal characteristics of the

ATC simulation environment were examined by

manipulating the delay interval of the downlinked

responses to ATC messages. Ghost pilots were instructed to provide immediate responses to both data

link and voice radio transmissions (immediate condition) or immediate responses to voice radio and

delayed responses to data link (mixed condition). All

non-data-link-equipped aircraft received immediate

responses to their voice radio messages.

Each controller completed the same scenario twice,

once with immediate downlink responses and once

with downlink responses delayed by approximately

11s. The order of presentation was counterbalanced.

Type of aircraft equipage (data link equipped, nondata link equipped), mode of communications (voice

radio, data link) and ghost pilot response type (all

immediate, mixed) were the experimental variables

of interest. Since the scenario was representative of an

actual sample of traffic from the facility, it was

congruent with controller expectancies and should

not adversely influence workload.

Dependent Measures

Subjective Workload. Participant ratings, provided on each dimension of the NASA Task Load

Index, served as indices of perceived workload. Individual and composite ratings were obtained following each simulation.

Objective Workload. There were 3 measures of

objective workload. They were 1) aircraft activity

levels, 2) total time on frequency and, 3) hesitation

pauses (Eisler 1968). They are enumerated below.

Aircraft Activity Levels. Workload was measured

by the number of active aircraft still on frequency

when the controller initiated a transmission. An

aircraft was counted as being under positive control

once it established initial contact with the controller.

It was no longer under positive control after the

controller completed the 2-stage hand-off procedure:

1) an automated radar hand-off and 2) transfer of

communication to the next controller in the sequence. This approach was used to maintain continuity with current transfer of control procedures.

Total Time on Frequency. For messages transmitted by voice-radio, the total time on frequency was

measured by placing cursors at the onset and offset of

speech, as determined by visual examination of the

waveform and by listening to the speech sample. For

data link transmissions, total time on frequency was

measured as the amount of time that lapsed between

the onset of a hand movement to the [F15] or [F16]

key on the ARTS IIIA keyboard and when the trackball

[ENTER] key was depressed. This was accomplished by

viewing the time-stamped audio-video recordings.

Hesitation Pauses. For all participants, withinutterance pause duration and pause frequency were

extracted from the audio-videotapes. Some articulatory gestures of speech (e.g., stop consonants, breathing) necessarily result in brief periods of silence2

. For

motor production, both the within-message pause

duration and pause frequency were extracted by viewing the videotapes. Pause duration was defined as a

2

 Breathing is necessary for fluent speech production. Setting a pause duration of 25 milliseconds (ms) as a lower bound

eliminated it as a factor.
8

lack of a hand movement about the keyboard during

message construction. Only transmissions containing pauses exceeding .25s were included. Pause frequency was computed by counting the number of

pauses in a transmission.

Communication Performance. Measures of communication performance were associated with the

contents of controller messages and message production. Message contents involved the number of aviation topics in a message and non-standard

communications. Message production included the

adjusted time on frequency (total time on frequency

adjusted for hesitation pauses) and message restarts.

Message Production. The adjusted time on frequency and number of restarts were computed for

each utterance and data link transmission. The total

time that a controller was on frequency consisted of

three components: pause duration, message selection/construction, and transmission time. The adjusted time on frequency was derived by subtracting

pause duration from total time on frequency.

The number of restarts was defined as the reinitiation of a transmission to an aircraft. This was

determined by listening to voice-radio transmissions

and counting the number of times the controller told

the pilot to “disregard” or reinitiated a message. For

data link transmissions, the number of restarts was

determined by reviewing the audio-video tapes and

counting how often the controller depressed the clear

key on the keyboard during message construction.

Message Contents. By parsing messages into message elements, the number of aviation topics in voiceradio and data link messages was determined, tallied,

and recorded according to the methods and procedures outlined by Prinzo, Britton, and Hendrix

(1995). The number of non-standard communications per message was determined by comparing the

phraseology spoken to the required phraseology contained in FAA Order 7110.65 (FAA, 2000).

Initial Contact and Transfer of Control Services.

Time is a crucial component of message delivery in

maintaining safety. Temporal factors associated with

downlinked pilot responses could influence when

controllers accept or transfer responsibility for separation assurances between aircraft. An analysis was

performed on the controllers’ communication and

performance to provide initial contact and transfer of

control services and data were extracted from videotaped recordings made during each simulation. A

template of the aircraft arrival routes containing mile

markings from the airport was developed to calculate

the aircraft’s location along its filed flight plan. The

process used to extract temporal and location information pertaining to these services is enumerated below.

Initial Contact Services. For purposes of this study,

initial contact services included the acceptance of the

radar track and first transmission to a data-linkeligible aircraft. ACCEPT RADAR time was defined

as the time lapsed from the moment the controller

began to move the trackball onto a flashing target and

ended when the trackball Enter key was depressed.

ACCEPT RADAR fix was defined as the number of

miles the aircraft was from the airport along its arrival

route when radar contact occurred.

The INITIAL CALL-UP time to each aircraft was

extracted from the verbatim transcripts. It was recorded in minutes and seconds from the onset to

offset of the controller’s first transmission to a datalink-eligible aircraft. The INITIAL CALL-UP fix

was defined as the number of miles the aircraft was

from the airport along its arrival route when two-way

communications were established and generally occurred when the controller completed the first transmission to the aircraft. Only the amount of time that

it took the controller to state the aircraft call sign, call

sign plus “Roger,” “Contact,” or another acknowledgment was included to determine the initial call-up fix.

Transfer of Control Services. The INITIATE

TRANSFER OF RADAR TRACK time was measured in minutes: seconds. The moment the

controller’s hand extended to the keyboard-designation of the receiving controller’s position symbol, “J”

or “Z,” to when the hand moved to and depressed the

click-button on the slew ball was extracted from

videotape. The INITIATE TRANSFER RADAR

TRACK fix was measured in miles from aircraft to

airport when radar transfer was completed.

The TRANSFER OF COMMUNICATIONS was

measured in minutes and seconds from the onset to

offset of the final transmission made by the controller

transferring communications. Onset time was measured from the moment the sending controller’s hand

moved to and depressed the [F9] key on the keyboard. Offset time was measured from the moment

the sending controller’s hand moved to and then

depressed the Enter key on the trackball causing the

“TC” to disappear from the PVD. The transfer of

communication fix was the number of miles the

aircraft was from the airport when TOC occurred.
9

RESULTS

The Relationship Between the Temporal Characteristics of the Simulation Environment and Subjective

Workload

The temporal characteristics of the simulated ATC

environment were examined by manipulating the

response delays. Ghost pilots were instructed to provide immediate responses to data-link and voiceradio transmissions (immediate condition) or a

combination of immediate responses to voice radio

and delayed responses to data link (mixed condition).

The mean and standard deviation (SD) of the

subjective ratings for each of the 6-dimensions of the

TLX are presented in the main body of Table 3 and

the overall rating is presented under the heading

“Mean.” As shown in Table 3, the average, overall

rating was higher for the mixed (49.83) compared to

the immediate condition (45.25). Likewise, their

ratings on each dimension of the TLX under the

mixed condition were higher than under the immediate condition. However, non of these differences

were statistically significant.

The Relationship Between the Temporal Characteristics

of the Simulation Environment, Objective Workload and

Communication Performance

All of the participants communicated with 14

data-link-equipped and 10 non-data-link-equipped

aircraft. Incomplete data led to the exclusion of 8

aircraft, of which 3 were data link equipped. Most of

these aircraft were in a handoff mode or on final

approach at the very beginning or end of the simulation and they served the purpose of filling time as the

aircraft of interest entered and departed the scenario.

This left 1,579 of the original 1,952 controllergenerated transmissions in the database. Two separate sets of analyses were performed on each of the

dependent measures.

For the first set, the data were arranged according

to the type of aircraft equipage and ghost pilotresponse type, while ignoring the mode of communications classification used for information transfer.

This allowed for a comparison between ATC operational communications for aircraft with and without

a data link capability. During one simulation, all of

the aircraft responded immediately to all transmissions (immediate condition). During the other simulation, controllers received immediate oral responses

from all of the aircraft to their voice radio communications. Acknowledgments to their data link transmissions from data-link-equipped aircraft were

delayed by about 11s (mixed condition).

For the second set of analyses, the data were

rearranged according to the mode of communications used by the controllers for message transfer and

ghost pilot-response type, while ignoring the type of

aircraft equipage. This allowed for a comparison

between mode of communications, independent of

aircraft equipage. During each simulation, the temporal characteristics were always immediate for voice

radio transmissions (immediate) and either immediate or delayed for data link (mixed).

Type of Aircraft Equipage and Ghost Pilot Response

Type (Collapsing on Mode of Communications)

Controller Workload. The results indicated that controller workload was affected by the communications

capability onboard the aircraft but not ghost pilot

response type. As shown in Table 4, transmissions made

TABLE 3. Mean NASA TLX Ratings Presented by the Temporal Characteristics of the

Simulation

NASA TLX Dimensions

Source Mental Physical Temporal Performance* Effort Frustration Mean

Temporal Characteristics

Immediate 45.75

(12.24)

45.62

(14.48)

45.75

(12.08)

42.12

(11.42)

43.38

(13.94)

48.88

(17.84)

45.25

(12.13)

Mixed 48.50

(15.35)

50.12

(13.66)

50.25

(11.07)

51.62

(12.11)

49.38

19.40)

49.12

(18.29)

49.83

(13.85)

* A lower value indicates a rating of improved performance
10

to data-link-equipped aircraft took longer to complete

(total time) than transmissions to non-data-linkequipped aircraft, F(1,7) = 60.06, p<.05. Likewise,

transmissions made to data-link-equipped aircraft contained longer pauses than those made to non-data-linkequipped aircraft, F(1,7) = 9.64, p<.05. The difference

observed between data link and non-data-link-equipped

aircraft for pause frequency was not significant, F(1,7)

= 2.31, p>.05.

Communication Performance. As in the previous

Prinzo study (1998), even when hesitation pauses are

removed from the controllers’ transmissions, they

still spent more time on frequency communicating

with data-link-equipped aircraft, F(1,7) = 29.17,

p<.05. In addition, messages transmitted to datalink-equipped aircraft were more accurate, F(1,7) =

69.51, p<.05, and contained fewer aviation topics,

F(1,7) = 20.27, p<.05. Controllers also transmitted

more aviation topics when ghost pilot responses were

immediate, F(1,7) = 9.19, p<.05. Finally, controllers

also restarted transmissions to data-link-equipped

aircraft more often than to non-data-link-equipped

aircraft, F(1,7) = 19.16, p<.05.

Mode of Communications and Pilot Response Type

(Collapsing on Aircraft Equipage)

Controller Workload. As shown in Table 5, the

mode of communications used by controllers influenced workload but ghost pilot response type did not.

That is, when controllers used data link, their transmissions took nearly 2s longer to complete than

messages sent by voice (mean total time DL= 4.39s,

V= 2.40s), F(1,7) = 35.93, p<.05. Also, data link

messages contained longer pauses (i.e., pause duration) than messages initiated by voice radio, F(1,7)

=7.89, p<.05.

TABLE 4. Measures of Workload and Communication Performance Presented by Aircraft Equipage

and Pilot Response Type (Collapsed over Mode of Communications)

Objective Measures of Workload and Communication Performance

Num

Aircraft

Pause

Duration

Pause

Freq

Total

Time

Adj

Time

Num

Restarts

Num

Topics

NSC per

Message

Num

Syllables

Aircraft Equipage

Data Link 6.22

(1.01)

0.39*

(.47)

.09

(.07)

3.49*

(.69)

3.10*

(.37)

.04*

(.03)

2.35*

(.14)

0.30*

(.30)

18.10

(1.03)

Non-Data Link 6.33

(1.04)

.03

(.03)

.05

(.05)

2.63

(.27)

2.60

(.27)

.02

(.03)

2.53

(.12)

.77

(.23)

19.02

(19.32)

Ghost Pilot Response Type

Immediate 6.19

(.72)

.14

(.20)

.08

(.06)

2.98

(.54)

2.84

(.40)

.02

(.02)

2.46*

(.18)

.58

(.38)

18.76

(1.38)

Mixed 6.36

(1.26)

.28

(.49)

.06

(.06)

3.14

(.80)

2.86

(.43)

.04

(.04)

2.42

(.13)

.49

(.33)

18.36

(1.13)

Aircraft Equipage by Pilot Response Type

Data Link - Immediate 6.15

(.66)

.24

(.24)

.08

(.07)

3.31

(.55)

3.07

(.40)

.03

(.02)

2.37

(.16)

.34

(.32)

18.36

(1.15)

Non-Data Link - Immediate 6.23

(.83)

.03

(.03)

.07

(.05)

2.64

(.27)

2.61

(.26)

.01

(.03)

2.56

(.14)

.82

(.26)

19.17

(1.55)

Data Link - Mixed 6.28

(1.32)

.54

(.61)

.09

(.07)

3.67

(.80)

3.13

(.37)

.05 9.04) 2.33

(.11)

.26

(.28)

17.84

(0.90)

Non-Data Link - Mixed 6.44

(1.28)

.02

(.03)

.02

(.03)

2.62

(.32)

2.60

(.30)

.02

(.03)

2.50

(.10)

.72

(.18)

18.87

(1.14)

*Statistically significant, p ≤ .05.
11

Communication Performance. As expected, controllers spent more time on frequency even when

pauses were removed from their data link transmissions (i.e., adjusted for pause duration), F(1,7) =

190.27, p<.05. Although data link, rather than voice

messages were restarted more often F(1,7) = 19.81,

p<.05, the data link messages were more accurate,

F(1,7) = 37.13, p<.05, and contained fewer aviation

topics than those transmitted by voice radio, F(1,7)

= 181.45, p<.05. This finding was replicated on

1,430 of the original 1,954 transmissions - transmissions devoid of common salutations, courtesies, or

both. Transmissions that involved initial contact also

were excluded since they were performed using voice

radio. Once again, more topics were transmitted by

voice radio (Mean =2.21 SD = .14) than data link

(Mean = 2.16 SD = .11), t(1,7) = -3.01, p<.05. When

aircraft, regardless of communications equipage,

responded immediately, controllers’ transmissions

were notably longer (number of syllables), F(1,7) =

5.93, p<.05, although not more informative (number

of topics) or requiring longer to transmit.

The Relationship Between the Temporal Characteristics of Downlinked Ghost Pilot Response Type and

Initial Contact and Transfer of Control Performance

Based on the previously reported studies, it was

anticipated that the amount of time controllers allotted for initial contact and transfer of control services

was a function of the temporal aspects of communications system. Therefore, directional, paired t-tests

were performed for only data-link-equipped aircraft

that completed initial call-up, initiate transfer radar

track, and transfer of communication subtasks. There

were 206 controller transmissions made to data-linkeligible aircraft (70 voice-radio and 136 data link). The

TABLE 5. Measures of Workload and Communication Performance Presented by Mode of

Communications and Ghost Pilot Response Type (Collapsed over Aircraft Equipage)

Measures of Workload and Communication Performance

Source Num

Aircraft

Pause

Duration

Pause

Freq

Total

Time

Adj

Time

Num

Restarts

Num

Topics

NSC per

Message

Num

Syllables

Mode of Communications

Data Link 6.23

(.98)

.63*

(.79)

.12

(.11)

4.39*

(1.01)

3.77*

(.46)

00.08*

(.06)

2.13*

(.01)

0.00*

(.00)

18.44

(.94)

Voice Radio 6.27

(1.10)

.02

(.02)

.04 9.04) 2.40

(.23)

2.38

(.22)

0.00

(.00)

2.60

(.10)

.74

(.34)

18.34

(1.13)

Ghost Pilot Response Type

Immediate 6.18

(.70)

.21

(.32)

.08

(.07)

3.30

(1.07)

3.09

(.82)

.03

(.040)

2.38

(.28)

.39

(.48)

18.64*

(1.09)

Mixed 6.31

(1.29)

.43

(.83)

.08

(.10)

3.49

(1.43)

3.05

(.79)

.05

(.07)

2.36

(.26)

.34

(.42)

18.19

(.94)

Mode of Communications by Ghost Pilot Response Type

Data Link - Immediate 6.20

(.66)

.40

(.38)

.09

(.10)

4.16

(.83)

3.78

(.58)

.06

(.04)

2.13

(.13)

0.00

(.00)

18.70

(1.08)

Voice Radio - Immediate 6.16

(.78)

.03

(.02)

.06

(.05)

2.43

(.19)

2.40

(.18)

0.00

(.00)

2.62

(.09)

.78

(.37)

18.59

(1.17)

Data Link - Mixed 6.25

(1.27)

.86

(1.03)

.14

(.12)

4.62

(1.17)

3.76

(.35)

.09

(.07)

2.13

(.11)

0.00

(.00)

18.17

(.76)

Voice Radio - Mixed 6.38

(1.40)

.01

(.01)

.02

(.02)

2.36

(.27)

2.35

(.27)

0.00

(.00)

2.59

(.11)

.69

(.32)

18.21

(1.14)

*Statistically significant, p ≤ .05.
12

mean and SD time (to perform each subtask and the

aircraft’s distance along its arrival route to the airport

are presented in Table 6.

As predicted for the Initial Contact Services, there

were no differences in where an aircraft was as it

approached the sector or how long controllers took to

accept radar contact or establish communications

since the procedure associated with each subtask was

retained. In addition for the Transfer of Control

Service, it is not surprising that no difference was

observed for the transfer of radar track subtask since

the receiving controller accepted the radar track as

soon as it appeared flashing on his radar display.

There was, however, a significant difference resulting

from the temporal characteristic of the downlinked

response - when responses were delayed, controllers

took an additional 1.2s to complete the transfer of

communications subtask, t(7) = -2.44, p < .05. A

final analysis revealed that the mode of communications that controllers selected did not influence the

distance or time measures of transfer of radar track

and communications subtask performance.

As mentioned previously, controllers could complete the transfer of control services transfer of communications subtask by voice radio or data link using

the manual mode TOC. When the analyses performed on the temporal characteristics of the ghost

pilot responses and mode of communications are

taken together, the data revealed that temporal demands — not mode of communications — significantly influenced controller performance of the

transfer of communications subtask.

DISCUSSION

Operational communication tasks, whether performed by pilots or by air traffic controllers, take time

to complete. As data link becomes fully integrated

into the air traffic control system and CPDLC is

transitioned into the operational environment, controllers will have the flexibility to determine which air

traffic services are provided by voice radio and by

data link. It stands to reason that the workload

experienced by controllers will change as communication become less verbally- (listen, speak) and more

visually- (eye, hand) driven. Along the same lines, it

would seem that changes in the temporal aspects of

the communications system would influence the performance of operational communication tasks by

controllers.

TABLE 6. Ghost Pilot Response Type, Air Traffic Service, and Mode of Communications

Presented by Distance from the Airport and Time to Complete Each Subtask

Distance and Time to Complete ATC Subtasks

Distance (in miles) Time (in seconds)

Source Radar Communications Radar Communications

Initial Contact Services

Ghost Pilot Response Type

Immediate 64.26 (1.23) 62.60 (1.01) 3.16 (0.45) 1.82 (0.31)

Delayed 64.46 (1.32) 62.73 (1.35) 3.98 (1.43) 1.82 (0.33)

Transfer of Control Services

Ghost Pilot Response Type

Immediate 43.63 (12.70) 33.50 (2.37) 2.72 (0.54) 2.64 (0.39)*

Delayed 44.15 (09.64) 33.42 (1.74) 2.59 (.30) 3.85 (1.14)*

Mode of Communications

Data Link 40.41 (11.87) 33.91 (2.60) 3.09 (0.72) 3.29 (0.38)

Voice Radio 42.91 (09.17) 34.32 (1.84) 2.68 (0.43) 2.65 (0.19)

*Statistically significant, p  .05.
13

In this simulation study, the temporal aspects of

the communications environment was manipulated

by having controllers experience either no pilot downlink delay or a delay of approximately 11s. Pilot

delays did not seem to unfavorably influence controllers’ perceptions of workload, although they generally felt that their performance was somewhat affected.

Unlike the Prinzo study (1998) that reported a significant increase in the frequency of pauses in the

mixed modality environment, it was not a significant

factor in this study because all communications occurred in a mixed environment. Interestingly, the

same trend appeared for more frequent pauses

when communications involved aircraft equipped

with voice radio + data link than aircraft equipped

for voice radio.

The only evidence of changes in workload came

from the objective measures of total time on frequency and duration of pauses. Specifically, transmissions to data-link-equipped aircraft took .86s

longer to complete than those to non-data-linkequipped aircraft (DL aircraft = 3.49s, NDL aircraft

= 2.63s). The longer transmission times to data-linkequipped aircraft were the result of the mode of

communications selected by the controller. In particular, when controllers sent messages over a data

link, they took nearly 2s longer to complete those

transmissions than messages sent by voice radio (DL

message = 4.39s, V message = 2.40s).

Why did controllers’ data link transmissions take

longer? Part of the reason may lie in the observation

that, of the 114 transmissions containing pauses,

over 60% involved data link (DL = 73, V = 41). The

longer pauses in these data link messages suggest that

controllers had more time available for strategic processes and planning activities (Ericsson & Simon,

1980). Prinzo et al. (1998) suggest that the longer

pauses made by controllers in an earlier simulation

study could reflect cognitive activity associated with

strategizing during message construction. Just as

light traffic, coupled with expertise, may have afforded controllers in the earlier study more time to

think, providing controllers in this study with a

multi-modal communications system may have alleviated the communications bottleneck imposed by

one-user-at-a-time communications.

An examination of the controller data link messages revealed transmissions with pauses greater than

20s involved either a transfer of communications

(60%) or an instruction to descend to a lower altitude

(40%). Generally, controllers constructed these messages while an aircraft was approaching a sector

boundary or transition point. While waiting, they

had the opportunity to send a voice message to

another non-data-link-equipped aircraft. In fact, at

the controllers discretion, over 100 pairs of transmissions (i.e., 11% of the transmissions) involved parallel performance of operational communications tasks.

It was discovered that, if the first message gave

advance runway information (31%) or an instruction

(37%), the controller was simultaneously establishing radar contact (44%) or transferring communications (25%) with another aircraft. Temporal factors

did not seem to be a factor in the development of this

efficiency strategy. It appears, then, that the longer

pauses may have resulted from controllers selecting/

constructing messages best suited for the current

operational task, thereby providing communications

in parallel, or both. Using a multi-modal, simultaneous communications system provided these controllers with the capability for off-loading routine,

non-time critical, operational tasks to data link, while

simultaneously performing verbal communication tasks.

Communication performance also was influenced

by mode of communications. Controllers had the

option of sending single or multiple topic messages

either by voice (e.g., descend to and maintain five

thousand, turn right heading two eight zero, descend

and maintain six thousand) or data link (e.g., A050,

H280 A060). When messages were transmitted to

aircraft that were not data link equipped, they often

contained common salutations and courtesies typical

of social situations. Although such commonplace,

non-standard communications are not part of the

required phraseology contained in FAA Order 7110.65

nor are they included in the CPDLC message set3

.

The presence of these non-standard communications

also added to the length of voice radio transmissions.

Another analysis was performed on a restricted set of

messages devoid of common salutations, courtesies,

or involved in initial contact services. The results of

that analysis replicated the original finding and

3

 See US DOT FAA Specifications for Controller-Pilot Data Link Communications Build 1A, Appendix A.
14

supported the conclusion that mode of communications affected message length, but the temporal aspects of the mode of communications did not.

The absence of these non-standard communications in data link messages also helps to explain why

messages that were sent to aircraft equipped with data

link were more precise and shorter. First, data link

topics were preformatted, thus making them less

error-prone. Second, the data link message set does

not include any pleasantries or salutations. Third,

more of the messages that were transmitted to datalink-equipped aircraft were sent by data link than by

voice radio. Finally, controllers were not provided with

an option or opportunity to construct or append "freetext" as part of their uplink messages. Communication

performance also was influenced by the temporal

aspects of the communications environment. When

working dynamic air traffic, where all the aircraft

responded immediately, regardless of equipage, controllers’ messages were notably longer (number of

syllables) but not necessarily more informative (number of topics) nor did they take longer to transmit.

When aircraft response time varied, as they did in the

mixed condition, (immediate, delayed by 11s) controllers took an additional 1.2s to complete the transfer of communications subtask. And it is not surprising

that neither aircraft distance from the airport nor the

amount of time taken to complete the transfer of

radar subtask were affected by the 11s delay. Since the

delays associated with data link were independent of

radar subtasks, controllers were not compelled to

develop compensatory strategies because none were

needed. It is also possible that delay had no effect

because the receiving controller accepted the radar

handoff as soon as the data tag began flashing. Future

studies may be needed to explore the automatic mode

for the transfer of communications subtask. When

operating in the automatic mode, the CPDLC system

will send the TOC uplink message after hand-off

acceptance of the track4

.

In summary, the primary finding of this study was

that controllers took longer to formulate and transmit messages over a data link-communications system, but their messages were more accurate and

contained fewer message elements. It would seem

that controller access to and the availability of different modes of communication clearly improved the

efficiency of ATC/pilot communications. Prior to

data link, operational tasks were performed successively. When data link communications becomes

part of the air traffic control environment controllers

will have the option of performing tasks sequentially

or in parallel using either voice radio or CPDLC. As

indicated by the communications data, the longer

latencies associated with data link messages often

resulted from deliberate actions on the part of the

controllers. The longer latencies did not result in a

loss of efficiency but reflected innovations on the part

of the controllers to optimize their performance. By

placing data link messages in a pending status controllers could participate in concurrent communications with other non-data-link-equipped aircraft. This

benefit provided controllers with added flexibility and

clearly enabled them to decide which modality was best

suited for each operational communications task.

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4

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