DOT/FAA/AM-05/19

Office of Aerospace Medicine

Washington, DC 20591

Terminal Radar Approach 

Control: Measures of 

Voice Communications 

System Performance

O. Veronika Prinzo

Mark McClellan

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

October 2005

Final Report
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-05/19 

4. Title and Subtitle 5. Report Date 

October 2005 Terminal Radar Approach Control: Measures of Voice Communications 

System Performance 6. Performing Organization Code 

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

Prinzo OV, McClellan M 

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

11. Contract or Grant No. 

FAA Civil Aerospace Medical Institute 

P.O. Box 25082 

Oklahoma City, OK 73125 

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

14. Sponsoring Agency Code 

Office of Aerospace Medicine 

Federal Aviation Administration 

800 Independence Ave., S.W. 

Washington, D.C. 20591 

15. Supplemental Notes 

This work was performed under Task AM-B-03-HRR-516. 

16. Abstract:

Effective communications in the National Airspace System (NAS) is an essential safety component of successful 

air travel. As the NAS migrates from its current ground infrastructure and voice communications system to one 

that encompasses both ground and airborne systems, digital data transmission may become the principal 

communication medium. As technological advances lead to innovations in communications system 

development, these emerging systems will be evaluated against the existing legacy system’s performance 

parameters such as setup delay, voice streaming, pause duration, and message propagation. The data presented 

here are but a first step in providing objective and quantifiable communications system performance metrics that 

may prove valuable to communication systems developers and personnel charged with evaluating, certifying, and 

deploying the next generation of communications systems. The authors analyzed nearly 8,000 transmissions that 

represented the busiest air-ground communications from the five terminal radar approach control facilities with 

the highest number of operations in the contiguous United States. Typically, setup delays lasted 81 ms, voice 

streaming 2568 ms, pause duration 127 ms, and message propagation 73 ms for a total of 2849 ms per 

transmission. On average, transmissions were separated by 1736 ms of silence. Disruptions to efficient 

information transfer can result from blocked, stepped-on, and clipped transmissions — but they are rare events 

and occurred in only 1.16% of the sampled transmissions. A comparison between aircraft with and without 

disruptions revealed that when a disruption was present, an average of 14.54 messages were transmitted, 

compared with an average of 9.90 messages when no disruption was present. Even so, there appears to be some 

type of a detection mechanism in place to alert the controller to the presence of blocked transmissions. The 

source is of this detection system is unclear; however, systems developers may want to exploit and expand this 

capability to include stepped-on and clipped transmissions. 

17. Key Words 18. Distribution Statement 

Communications, ATC Communication, 

Air Traffic Control 

Document is available to the public through the Defense 

Technical Information Center, Ft. Belvior, VA 22060; 

and the National Technical Information Service, 

Springfield, VA 22161

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

Unclassified Unclassified 23 

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



Terminal Radar Approach Control: Measur

Voice Communications System Perfor

Societies have always been shaped more by the nature of the media by which men 

communicate than by the content of the communication.

— Marshall McLuhan

Canadian communications and media theorist and Quentin Fiore 

The Medium Is the Massage, Random House (1967

According to a 2004 report issued by the Federal Aviation Administration (FAA) Office of Aviation Policy and 

Plans (FAA, 2004), there were more than 120 million

aircraft operations recorded in 2003. That report projected 

that the total civil aircraft activity will reach in excess of 

137 millions operations by 2015 and nearly 162 milli

by 2030. In response to the anticipated growth in aircraft 

activity, the FAA’s efforts have been directed at ensuring 

that sufficient system capacity will be available to support these traffic projections. In addition to increasing 

the number of runways at airports, the FAA will develop 

programs, procedures, and technologies to enable more 

efficient use of the airspace. Likewise, as the demand for 

air traffic services increase, there will be a corresponding 

increase in air traffic communications and voice radio 

frequency congestion. In anticipation, the FAA plans a 

transition from its current analog voice communications 

system towards a new, state-of-the-art, next generation 

voice- and data-based communications system. 

As the National Airspace System migrates from its 

current ground infrastructure and analog voice communications system to one that encompasses both ground 

and airborne systems, digital data transmissions will be 

the principal communication medium. Unlike the voicemode currently used in the analog voice domain, the 

future system may rely upon digital voice transmission 

techniques (National Airspace System Capital Investment Plan, 2005). The FAA envisions that data-linked 

communications (e.g., VHF Digital Link Mode 3) will 

provide route-of-flight clearances, airport information, 

aircraft position both on airport surfaces and in the air, 

weather conditions, and other information to data-linkequipped aircraft. However, voice communications will 

still be available to pilots flying unequipped aircraft and 

will operate as a back-up system in the event of an individual unit or system failure.

The FAA’s Mission Need Statement 137 (MNS 137

1995a) and the subsequent NEXCOM Investment

Analysis Report (1998) describe shortfalls in the frequency spectrum capacity of the current Air/Ground

(A/G) communications system. For example, demand 

is expected to grow at an annual rate of 4% for new 

A/G communication voice frequency assignments (especially for already congested terminal and surrounding 

airspace) and for frequencies to support a variety of new 

A/G communications services in the limited very high 

frequency (VHF) band. Therefore, this level of growth 

cannot be accommodated by the current analog system. 

Other needs called out in MNS 137 include a reduction

in logistical costs for maintaining radios, introduction of 

new data link capability, a reduction in radio frequency 

interference (RFI), and improved security against threats 

such as “phantom controllers.”

In response to these needs, the NEXCOM Investment 

Analysis identified a segmented program for upgrade and 

replacement of the present air traffic control (ATC) A/G 

communications string. The NEXCOM Requirements 

Document (RD) also identified a number of operational 

and technical constraints that must be accommodated 

while satisfying these requirements. Specifically, a fundamental requirement of NEXCOM Segment  was to

provide additional voice channels with no disruption of 

the present voice service. Furthermore, NEXCOM was 

to achieve this increased capacity with minimum disruption of the present VHF A/G communications physical 

system configuration. Finally, NEXCOM sought after a 

seamless evolution from the present analog double sideband-amplitude modulation (DSB-AM) A/G system to 

a new digital communications functional capability. 

Investigations performed by the NEXCOM Product 

Team of the voice communication literature revealed that 

the existing operational communications databases do not 

provide current information concerning the frequency 

of occurrence or the severity of stepped-on or blocked 

transmissions. This information is critical for an analysis 

of the operational and safety benefits that justify an investment in the development of any proposed communications systems. For instance, previous simulation studies 

demonstrated that the number of blocked transmissions 

increases both as the number of communications increases 

and with the amount of ground-air transmission delay 

(Nadler et al., 1993; Sollenberger, McAnulty, & Kerns,

2003). These delays consist of two components: setup 

and propagation. Optimally, before delivering a message, 
FAA (1995 b). NAS-SS-1000 Volume III states that a VHF/UHF communications outlet receiver/transceiver shall establish a carrier for radi

frequency signals within 35 ms after the transmitter enable signal enters the VHF/UHF communications outlet transmitter/transceiver.

2

FAA (1995b). NAS-SS-1000 Volume III states that a VHF/UHF communications outlet receiver/transceiver shall remove the carrier for radi

frequency signals within 35 ms after the transmitter disable signal enters the VHF/UHF communications outlet.

3

Heterodyning is the production of a beat note at a frequency given by the difference in frequency of two interfering signals.

the sender depresses the push to talk (PTT) switch, waits 

until the setup delay elapses, and then begins speaking. 

The failure to wait until the setup time elapses may result in the initial portion of the voice stream not being 

transmitted, or “clipped.”

The propagation component of a transmission is the 

transit time for the voice signal to travel from its source 

to its destination. The propagation component that was 

measured consisted of the time that a transmitted signal 

was present in the sector of sufficient strength to block 

other similar transmissions. Once the PTT switch is 

released, the communications channel is open to either 

receive or send another message.2 If the intended recipient 

of the message happens to depress the PTT switch during 

the propagation component, the message-in-transit may 

be blocked. A blocked transmission is a radio transmission 

that has been distorted or interrupted due to the presence 

of multiple simultaneous radio transmissions such that 

they could cancel each other out. There is a widespread 

belief that such blocked transmissions are always detected 

(i.e., heterodyne heard);

3

 however, both messages could 

be completely lost (O’Neil, 2005).

“Stepped-on” transmissions, like blocked transmissions, occur when there are two simultaneous transmissions; however, rather than canceling each other out, two 

different outcomes exist. In the first case, the stronger 

signal in one transmission may override the weaker signal 

causing it (i.e., the weaker signal) not to be heard. Alternatively, the stronger signal from a part of one message 

may be appended to a message from a different speaker, 

thereby creating a complete, albeit erroneous, transmission. Consequently, the initiation of a transmission on 

an occupied frequency can result in vital information 

not being received by the intended air traffic controller 

or pilot. Both clipped and stepped-on messages often 

require additional transmissions – either in the form of 

a repeat, a request for a repeat, or to provide clarification 

to the intended recipient.

Unfortunately, the existing data on controller-pilot 

operational communications were collected 8-10 years

ago and may not constitute a valid basis for comparison 

with, or extrapolation to, the expected communications 

environment operating in 2010. Therefore, data on the

current operational communications system are needed to 

establish a baseline against which the future communications system’s performance can be compared. Operational 

voice communications from the busiest terminal radar 

approach control (TRACON) facilities were obtained, 

transcribed, and analyzed.

Met

This report presents compelling baseline analog voice 

communications system performance data that can be 

used to benchmark future systems. The data include 

duration parameters (e.g., setup delay, voice-streaming 

time, pause duration following voice offset, message 

propagation, frequency occupancy time, and lag time 

between successive transmissions) and the prevalence 

of disruptions to efficient information transfer (e.g., 

blocked, stepped-on, and clipped transmissions). The 

measures of voice communications system performance 

were derived from 10 hr of communications-intensive

operations — four 15 min samples of arrival and four 1

min samples of departure operations from each of the five 

busiest TRACON facilities in the United States. 

Materials

Audio Tapes. Five of America’s busiest Terminal Radar 

Approach Control (TRACON) facilities provided 5 hr 

of approach and 5 hr of departure control voice communications on digital audiotapes (DAT) for a total of 

10 hr of communications per facility. DAT recordings

were made using the NiceLoggerTM Digital Voice Recorder System (DVRS) to record and time-stamp each 

transmission. They included both voice communications 

data and PTT actions. Each DAT contained separate 

voice records of all communications transmitted on the 

radio frequency assigned to a particular sector position 

on the left channel. The right channel contained the 

Universal Time Coordinated (UTC) time code expressed 

in date, hour (hr), 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.

Audio Software. Adobe AuditionTM (1.5), a software

tool for audio editing, was used to extract voice and PTT 

onset and offset times. We used it to record, convert, and 

save the data as digital audio files for subsequent analysis. 

Audio data was displayed in either a waveform or spectral 

view. In the waveform view, the horizontal axis represented 

time, and the vertical axis represented the amplitude as a 
series of spikes. In the spectral view, the audio signal was 

displayed by its frequency components. The horizontal 

axis again represented time, but the vertical axis measured 

frequency and displayed the amplitude in an array of 

colors ranging from dark blue (low amplitude) to bright 

yellow (high amplitude). 

Selection of Data Sample

Audiocassette tapes were dubbed from each DAT for 

the transcribers to use to create verbatim transcripts. 

Each message was typed onto an electronic copy of the 

Aviation Topic Speech Act Taxonomy-Coding Form 

(Prinzo, Britton, & Hendrix, 1995), along with its onset

and offset time. Transmissions were divided into 15 min

samples according to sector and TRACON facility. For 

each TRACON facility, the results of a frequency analysis 

identified four 15 min approach and four 15 min departure samples that contained the most pilot-controlle

transmissions. They were selected for waveform analysis. 

Table  shows that the subsequent database consisted of

approximately 10 hr of pilot-controller transmissions

—  hr of approach and1 hr of departure transmission

for each facility.

Subject Matter Experts (SMEs)

The air traffic SME was an instrument-rated pilot and 

former controller who had worked as an FAA Academy 

instructor for 8 years and had worked for 12 years in

FAA supervision and management. He had an Airline 

Transport Pilot rating and was completing a Masters 

Degree in electrical engineering. The first author, serving 

as the third SME, had 12 years of experience analyzing

pilot controller communications.

Tape Analysis Procedures

Prior to transferring the audiocassette tape recording 

to the computer, the PC soundcard was adjusted using 

Record Control (In Line Volume) to record the bulk of 

data between ± 75% of scale, with no peaks outside ±

80%. This established the maximum dynamic range 

to retain small signal details. To maintain consistency 

in data recording, all dubbing took place at the same 

workstation and tape deck. The third SME dubbed all 

of the DVR recordings onto analog cassette tapes that 

were then recorded onto the computer-workstation using 

Adobe Audition for subsequent analysis.

Extraction of Voice Communications Data Point

The voice communications system performance duration measures were computed from the data points that 

were identified by listening to and inspecting individual 

waveforms of pilot and controller transmissions. The 

analysis points represented the time displayed in Adobe 

Audition at a zero crossing. They included push-to-talk 

onset, voice onset, voice offset, push-to-talk release, and 

push-to-talk settle. They are presented in the logical 

Table 1. Number and Duration of Transmissions

Number of Transmissions 

Source Pilot/Controller Land-line Total Duration 

Approach 

Atlanta 0816 028 0844 01 hr 02 min 04 s 

Chicago 0908 046 0954 00 hr 59 min 49 s 

Dallas Ft Worth 0774 035 0809 01 hr 00 min 02 s 

New York 1355 032 1387 01 hr 00 min 01 s 

Southern California 0631 016 0647 01 hr 00 min 28 s 

Approach Total 4484 157 4641 05 hr 05 min 24 s 

Departure 

Atlanta 0719 065 0784 00 hr 59 min 28 s 

Chicago 0581 056 0637 00 hr 59 min 34 s 

Dallas Ft Worth 0742 060 0802 00 hr 58 min 47 s 

New York 0681 025 0706 00 hr 59 min 50 s 

Southern California 0782 020 0802 01 hr 00 min 20 s 

Departure Total 3505 226 3731 04 hr 57 min 59 s 

Grand Total 7989 383 8372 10 hr 03 min 23 s 
sequence that they occur in the transmission cycle. That 

is, push-to-talk onset precedes voice onset and voice offset 

follows voice onset, etc. Their definitions and the process 

of identification are expounded upon next.

Push-to-Talk Onset. PTT onset was defined as the 

first transition from static signaling that the mic had been 

keyed and the voice stream was imminent. As shown in 

Figure 1, PTT onset occurred at 2 min 13 s 226 ms int

the tape. The dashed vertical line identifies the point of 

PTT onset.

Voice Onset. Voice onset was defined as the start of the 

first word in a transmission. By listening to and advancing 

the transmission starting point along the timeline from 

PTT onset until the first word was affected aided us in 

determining the point of voice onset. The next example 

(Figure 2) shows that the audio signal began to resemble 

human speech at 2 min 13 s 357 ms, and that point was

recorded as voice onset. Figure 3 shows that voice onset 

usually corresponded to a clear step magnitude increase 

of frequencies up to 4000 Hz.

Voice Offset. Voice offset was defined as the end of the 

last word in a transmission. Once again, by listening to 

and delaying the transmission end point along the timeline 

until the final word was affected aided us in determining 

the point of voice offset. Figure 4 shows an example of 

the earliest point that the last word was unaffected (no 

cutoff detected). In this case it occurred at 2 min 18s 630

ms. Continuation of the signal beyond the point of voice 

offset was due to electronic effects from the transmitter, 

as evidenced by the spectral view. 

Figure 1. Magnification of PTT onset 

Figure 2. Magnification of voice onset 
3

Figure 3. Spectral display of voice onset 

Figure 4. Waveform and spectral view of voice offset 
PTT release was defined as the start of the microphone 

(mic) pop-back that occurred when the PTT button was 

released following the offset of the voice transmission. 

For most controller transmissions, like the one presented 

in Figure 5, this pop was very noticeable.

Push-to-Talk Settle. PTT settle was defined as the first 

return to a low signal condition following the release of 

the mic switch. The low signal condition starts when 

the transmission has decayed to less than 90% of its peak 

value. In this low signal condition, transmissions from 

other sources are not blocked. As shown in the example 

in Figure 6, PTT settle occurred at 2 min 18 s 990 ms.

Accuracy in the Extraction of the Voice Communications Data Point

To ensure the accuracy and consistency of the data, 

only the pilot-SME extracted the voice communications 

data points. To evaluate the accuracy of the communications data points, data generated by the pilot-SME was 

compared with a subset of the data coded by the third 

SME. Some variability was expected in estimating the 

point of voice onset and voice offset as well as the PTT 

measures for pilot as compared with controller transmissions. The most obvious sources of variability included 1)

differences in cockpit background noise, 2) ambient noise 

conditions at the TRACON facilities, and 3) differences 

in the air-ground voice communications systems.

 Figure 5. An example of PTT release with magnification 

Figure 6. Example of PTT settle with magnification 
Four samples of 25 transmissions were randomly 

selected for waveform analysis. The voice communications data points were independently extracted by each 

SME and used to compute an absolute difference score 

for each pair of data points. For example, if both SMEs 

encoded the PTT onset for transmission  as occurring at

937 ms into the tape, then PTT onset T = |0|. However, 

if one SME had encoded PTT onset T as occurring at 

937 ms and the other SME encoded it at 941 ms, then

PTT onset T = |4|. The absolute difference scores were 

used to determine the percent agreement. 

Presented in Table 2 is a frequency distribution of 

the absolute differences in values for each type of communications data point. For example, it shows that for 

the extraction of PTT onset, the SMEs had a 0 ms difference for 23.5% of the controller and 57.1% of the

pilot transmissions. There was a 1-5 ms difference for an

additional 60.8% of the controller and 40.8% of the pilot 

PTT onset times. Taken together, the data show that the 

SMEs were within 0 ms to 5 ms of each other in determining PTT onset time for 84.3% of the controller and 

97.9% of the pilot transmissions. The largest disparities 

in agreement occurred for voice onset and voice offset 

times – that is not surprising given the added factor of 

noise present on the flight deck and background noise 

at the air traffic control facility.

Of the 400 possible data points, seven were left blank 

(indicating that values could not be extracted from the 

waveform) and four had extreme absolute differences 

|127|, |153|, |165|, and |290| m

A closer re-evaluation of these differences revealed that 

one of the SMEs did not include the word “and” (127

ms) as part of the transmission, “and Delta twelve fifty 

heavy runway two seven left,” whereas the other SME 

did. Also, for that same transmission, one SME excluded 

the word “left” as part of the waveform (290 ms). The 

other two extreme absolute differences were attributed to 

the exclusion of the final phonemes in two transmissions 

by one of the SMEs (“en” in the word “seven” and “isk” 

from the word “six”). Because extreme values are known 

to skew results, they were excluded from the computation of the accuracy estimates summarized in Table 3. As 

expected, voice onset and voice offset times were more 

variable than estimates of PTT onset and PTT release. 

Even so, the largest absolute mean difference of |22.86| 

ms that occurred for determining the voice onset time 

Table 2. Percentage Agreement in the SMEs’ Absolute Difference Accuracy Estimates 

Pilot and Controller Voice Communications Time Extractions

Absolute PTT Onset Voice Onset Voice Offset PTT Release PTT Settle

Difference

(milliseconds) C* P* C P C P C P C P 

0 23.5% 57.1% 21.6% 06.1% 02.0% 06.1% 33.3% 16.3% 23.5% 14.3% 

1-5 60.8% 40.8% 43.1% 28.6% 43.1% 30.6% 62.7% 53.1% 45.1% 59.2% 

6-10 02.0% 07.8% 12.2% 07.8% 08.2% 02.0% 14.3% 13.7% 12.2% 

11-15 02.0% 02.0% 07.8% 06.1% 19.6% 12.2% 12.2% 09.8% 02.0%

16-20 02.0% 05.9% 04.1% 03.9% 12.2% 02.0% 05.9% 02.0%

21-25 04.1% 02.0% 04.1% 04.1%

26-30 08.2% 05.9% 06.1% 02.0%

31-35 04.1% 09.8% 06.1% 

36-40 05.9% 08.2% 02.0% 02.0%

41-45 03.9% 02.0% 02.0% 02.0%

46-50 02.0% 04.1% 02.0% 

51-55 02.0% 04.1% 

56-60 04.1% 04.1% 

61+ 02.0% 06.1% 03.9% 04.1% 

No data 09.8% 02.0% 02.0% 

* C indicates Controller Transmissions and P indicates Pilot Transmissions 
in pilot transmissions still reflects a high degree of accuracy in data point extraction. The pilot-SME had set 

an accuracy-encoding criterion of 25 ms and, based on 

the presented data, that criterion was achieved.

Dependent Measure

Measures of Voice Communications Systems Performance. The voice communications data points, like 

the ones presented in Table 4, were used to compute 

the duration measures of voice communications system 

performance. The duration measures use arithmetic 

duration (time stamp difference) to derive their values. 

The duration measures for each pilot and controller 

transmission included a) setup delay, b) voice-streaming 

time, c) pause duration following voice offset, d) message 

propagation, e) frequency occupancy time, f) lag time, 

and g) number of messages per aircraft. 

a) Setup delay was the momentary pause preceding 

voice onset. It was computed as the difference 

between the point of voice onset and point of 

PTT onset. For Transmission  in Table 4, the

amount of silence that preceded voice onset was 

12 ms (0:00.949 - 0:00.937).

b) Voice-streaming time was computed as the difference between voice offset and voice onset. Again, 

using Transmission  in Table 3, voice-streaming

time was 2 s 236 ms (0:03.185 – 0:00.949)

c) Pause duration was the amount of silence following voice offset. It was computed as the difference 

between PTT release and voice offset. For Transmission 1, 273 ms of silence preceded the release of

the push to talk switch (0:03.458 – 0:03.185).

d) Message propagation represents the minimum 

time for the switching mechanism to return to a 

resting state following release of the mic switch. 

During this “settling down time,” the opportunity 

for blocking can occur. For Transmission 1, PTT

settle was computed as the difference between 

PTT settle and PTT release, and it was 127 ms

(0:03.585 – 0:03.458).

e) Frequency occupancy time was computed as the 

difference between PTT release and PTT onset, 

and it represented how long the radio frequency 

was in use per transmission. For Transmission 1,

frequency occupancy time was 2s 521 ms (0:03.458

– 0:00.937).

f) Lag time between Transmission  and Transmission 2

was computed as the difference between PTT onset 

for Transmission 2 and PTT release for Transmission 1. Again, using the data presented in Table 1

lag time was 396 ms (0:03.854 – 0:03.458).

g) Number of transmissions per aircraft was tallied 

as the number of pilot and controller messages 

exchanged to and from a particular aircraft flight 

identifier.

Disruptions to Efficient Information Transfer. While 

listening to the recorded transmissions, all of the SMEs 

were instructed to pay particular attention to the detection of any disruption in information transfer between 

the controller and pilot. Specifically, they were to note 

the presence of any interference such as stepped-on, 

blocked, or clipped transmissions and record the type 

of interference next to the transcribed message on their 

coding forms. Messages that were unintelligible, as well 

as transmissions that required a repetition due to a lack 

of response or a request by the receiver (e.g., say again, 

who was that calling in), were also encoded as a disruption to efficient information transfer.

In most cases, blocked transmissions were easy to identify 

because they were announced to the controller by the spoken 

word “blocked.” At other times, the controller told the pilot 

that the transmission was blocked or stepped-on, as was the 

case in the following examples, “I BLOCKED YOU OWNSHIP TWENTY NINE SIXTY FIVE TEN DEGREES RIGHT 

CONTACT APPROACH ONE ONE NINER POINT FOUR” 

and “OWNSHIP SIX NINETEEN YOU WERE STEPPED ON 

SIR DESCEND AND MAINTAIN THREE THOUSAND.” In 

all instances, stepped-on, blocked and clipped transmissions 

were identified by careful examination of each waveform while 

listening to the message and reading from the transcript.

Table 3. Estimated Accuracy of the Voice Communications Data Points 

Accuracy Estimates (in milliseconds) 

Controller Pilot 

Source M SD Min Max N M SD Min Max N 

PTT Onset 02.30 03.09 0 17 46 00.73 01.80 0 12 49 

Voice Onset 08.78 13.45 0 49 50 21.86 22.10 0 79 49 

Voice Offset 13.22 15.60 0 87 50 15.89 16.18 0 59 47 

PTT Release 00.82 00.94 0 06 51 04.17 04.39 0 17 48 

PTT Settle 05.16 07.89 0 48 51 66.35 09.51 0 43 49 
Table 4. Examples of Voice Communications Data Points 

 TIME STAMP (minute:second.millisecond)

Trans 

# Speaker Message 

PTT

Onset

Voice

Onset

Voice

Offset

PTT

Release

PTT

Settle

 

1 ATC OWNSHIP SIX THIRTY FOUR 

DESCEND AND MAINTAIN 

THREE THOUSAND FIVE 

HUNDRED 

0:00.937 0:00.949 0:03.185 0:03.458 0:03.585 

2 OWN634 THREE THOUSAND FIVE 

HUNDRED OWNSHIP SIX 

THIRTY FOUR 

0:03.854 0:03.900 0:05.370 0:05.380 0:05.417 

3 ATC OWNSHIP FOURTEEN SIXTY 

FOUR CONTACT TOWER 

ONE ONE NINER POINT ONE 

0:07.095 0:07.198 0:09.760 0:09.986 0:10.105 

4 OWN464 NINETEEN ONE FOR 

OWNSHIP FOUR SIXTY 

FOUR GOOD DAY 

0:10.628 0:10.900 0:13.430 0:13.500 0:13.535 

Table 5. System Performance Parameters for Control Voice Radio Communications (Time in ms) 

 Percentile 

Source Mean SD 50 95 N 

Across Facilities and Positions 

Setup delay (ms) 0081 0111 0037 00297 7500 

Voice streaming time (ms) 2568 1507 2266 05418 7955 

Pause duration (ms) 0127 0110 0108 00308 7299 

Frequency occupation time (ms) 2820 1547 2512 05719 6950 

Message propagation (ms) 0073 0052 0063 00168 7262 

Lag time (ms) 1736 4183 0531 08415 7972 

N transmissions per aircraft 0011 0007 0010 00026 0725 
1

RESULTS

Measures of Voice Communications Systems 

Performance

Pilot and controller transmissions were aggregated 

for each aircraft according to TRACON Facility and air 

traffic control sector. Separate analyses were performed 

for approach and departure control. Presented in Table 

5 are the overall communications system performance 

parameters. The values presented beneath the column 

labeled “Mean” are representative of an average transmission and will be used along with the information 

presented in Figure 7 to illustrate the time progression 

of a generic ATC transmission. 

The setup delay (label 1) begins when the speaker

depresses the mic key and ends with voice onset. Voicestreaming time (label 2) represents the amount of time used 

to utter the message from voice onset to voice offset. Pause 

duration (label 3) represents the absence of voice following 

the end of the utterance to mic key release. Frequency 

occupancy time (label 4) is air-time measured from mic 

key depressed to mic key released. Message propagation 

(label 5) can be heard as a clacking sound that follows mic 

key release. It is followed by a resting state, indicating that 

the communications system is available to either transmit 

or receive another transmission. Lag time is the duration 

of this resting state. About 1 messages will be exchange

from pilot check-in to controller handed-off.

Approach Control

There were 334 aircraft that received air traffic control 

services (Atlanta TRACON = 60, Chicago TRACON= 61,

Dallas Ft Worth TRACON = 85, New York TRACON 

= 65, Southern California TRACON = 63). In light 

of the statistically significant main effect of TRACON 

Facility [F (28,1166.05) = 21.325], Univariate Analys

of Variance (ANOVA) and the Tukey Honestly Significant Difference (HSD) statistic were used to assess the 

statistically significant findings. An alpha level of .05 

was set for all statistical tests. Descriptive statistics are 

presented in Table 6.

Results from the ANOVAs revealed that pilots flying into New York TRACON’s airspace and controllers 

working there took less time to begin talking once the 

mic key was depressed than pilots and controllers at any 

of the other TRACON facilities. Pilots and controllers at Southern California TRACON had the longest 

setup delays. [Setup Delay F(4,329)=38.526]. Pilots and 

controllers at the Dallas Ft Worth TRACON facility 

released the mic key faster than their colleagues at the 

Figure 7. Spectral display of the derived measures of voice communications system performance 
Atlanta and New York TRACONs. Their colleagues at the 

Southern California TRACON facility were faster than 

their counterparts located at the New York TRACON 

facility [Pause Duration F(4,329)=8.190)].The New York

TRACON had the fastest time for the switching mechanism to return to a resting state following release of the 

mic switch, and Dallas Ft Worth had the slowest when 

compared with the other TRACON facilities. [Message 

Propagation F(4,329)=83.991]

The ANOVA results also revealed that not only did the 

pilots and controllers at the Southern California TRACON spend more time transmitting individual messages, 

they also spent more time overall on the radio frequency 

than their colleagues at the other TRACON facilities 

sampled [Voice-streaming Time F(4,329)=31.790], [Frequency Occupancy Time F(4,329)=26.758]. Controllers

and pilots at the New York TRACON spent significantly 

less time on frequency transmitting individual messages 

than pilots and controllers at the Dallas Ft Worth, Atlanta, and Southern California TRACON facilities, and 

they did so without a notable difference in frequency 

occupancy times. 

Notably, both the New York and Chicago TRACON 

facilities experienced the least silence between successive 

transmissions when compared with the Dallas Ft Worth 

or Southern California TRACON facilities. There was 

less “dead-air time” at the New York TRACON facility than at the Chicago TRACON facility [Lag Time 

F(4,329)=11.177]. An examination of the number 

messages exchanged between the pilots and controllers 

revealed that the New York TRACON facility transmitted 

more messages per aircraft than any of the other TRACON 

facilities, [N transmissions per aircraft F(4,329)=22.274]. 

The number of transmissions (per aircraft) at the Chicago 

and Atlanta TRACON facilities were statistically equivalent, yet greater than the number of transmissions made 

Table 6. Approach Control Voice Communications System Performance Parameters (Time in ms) 

Source Mean Median SD Min Max 95% 

Setup delay 

Atlanta TRACON 0079 007 04 001 033 014 

Chicago TRACON 0095 008 05 003 028 019 

Dallas Ft Forth TRACON 0104 010 05 001 027 020 

New York TRACON 0040 003 07 001 054 011 

Southern California TRACON 0148 014 07 000 036 028 

Pause duration 

Atlanta TRACON 0139 012 06 006 036 025 

Chicago TRACON 0134 012 07 003 051 022 

Dallas Ft Forth TRACON 0106 010 05 002 024 020 

New York TRACON 0160 015 05 008 032 027 

Southern California TRACON 0125 012 07 002 038 030 

Message propagation 

Atlanta TRACON 0068 007 02 002 019 010 

Chicago TRACON 0072 007 03 003 021 011 

Dallas Ft Forth TRACON 0103 010 02 007 014 014 

New York TRACON 0041 004 00 003 006 005

Southern California TRACON 0082 008 02 004 012 011 

Voice streaming time 

Atlanta TRACON 2512 254 61 061 377 360 

Chicago TRACON 2423 243 45 135 374 332 

Dallas Ft Forth TRACON 2491 246 57 160 421 382 

New York TRACON 2175 212 46 125 363 312 

Southern California TRACON 3229 317 68 209 579 462 
1

by pilots and controllers at the Southern California or 

Dallas Ft Worth TRACON facilities.

Presented in Table 7 are the frequency distributions for 

each of the time intervals aggregated across TRACON 

facilities. Column (a) shows that, for approximately 82% 

of the transmissions, less than 150 ms transpired from

the onset of PTT to the onset of the speaker’s voice (less 

than .025 ms = 32.8%, .025 ms - .049 ms = 16.6%, and

.050 ms - .074 ms = 9.8%). As can be seen from Column (b), about 38% of the transmissions used between 

1750 – 2999 ms for voice streaming. The exceedingly

low values (075 – 149 ms) represent the amount of time

used to produce the word “blocked.”

Following voice streaming (Column c) 88.2% of the 

transmissions sustained pause durations less than 250 ms. 

Pause duration represented the amount of silence prior 

to the release of the PTT switch. For 92% of the transmissions, less than 150 ms was needed for the switching

mechanism to return to a resting state following release 

of the PTT switch (Column d, Message Propagation 

62.9%+29.1%). As shown by the percentages in Column

(e) — 31.5% of the individually transmitted messages

(18.5%+13.0%) used between 2000 – 2999 ms (betwee

2 to 3 seconds). The amount of time that lapsed between 

consecutive controller and pilot messages (that is, no land 

line communications computed as intervening messages, 

Column (f) revealed only 250 – 499 ms of silence for 

25.5% and between 500 – 749 ms for 14.8% of the

transmissions. Rarely (3.0%) did more than 10,000 ms

transpire between successive transmissions.

Departure Contro

There were 386 aircraft that received air traffic control 

services (Atlanta TRACON = 82, Chicago TRACON= 

73, Dallas Ft Worth TRACON = 67, New York TRACON = 77, Southern California TRACON = 87). Once 

again, in light of the statistically significant main effect 

of TRACON Facility [F (28,1353.504) =18.566], bot

the Univariate Analysis of Variance (ANOVA) procedure 

and the Tukey Honestly Significant Difference (HSD) 

statistic were used to assess the statistically significant 

findings using an alpha level of .05 for all statistical tests. 

Descriptive statistics are presented in Table 8.

Table 6. System Performance Parameters for Approach Control Voice Radio Communications 

(con’t) 

Source Mean Median SD Min Max 95% 

Frequency occupation time 

Atlanta TRACON 2745 275 64 078 414 397 

Chicago TRACON 2641 263 48 173 402 350 

Dallas Ft Forth TRACON 2698 265 58 180 447 406 

New York TRACON 2515 239 71 099 616 371 

Southern California TRACON 3536 349 70 220 613 486 

Lag time 

Atlanta TRACON 01906 157 107 39 0482 409 

Chicago TRACON 01313 104 111 07 0668 401 

Dallas Ft Forth TRACON 02177 147 195 28 0987 655 

New York TRACON 00560 042 037 00 0193 137 

Southern California TRACON 02304 165 298 31 2075 608 

N transmissions per aircraft 

Atlanta TRACON 13.483 14 007 01 0027 026 

Chicago TRACON 14.852 14 008 01 0030 029 

Dallas Ft Forth TRACON 09.094 09 005 02 0022 017 

New York TRACON 20.692 20 013 01 0044 041 

Southern California TRACON 10.000 10 006 01 0034 021 
1

Table 7. Timing Distributions For Approach Control Voice Radio Communications 

Time Interval 

(milliseconds)

Set up 

Delay

(a)

Voice

Streaming

(b)

Pause

Duration

(c)

Message

Propagation 

(d)

Frequency

Occupation

(e)

Lag

(f)

000-074 064.5% 036.3% 062.9% 000.1% 010.1% 

075-149 17.6% 000.2% 026.1% 029.1% 000.1% 007.0% 

150-249 010.4% 001.7% 025.8% 007.6% 000.4% 011.8% 

250-499 06.2% 004.0% 010.5% 000.4% 002.3% 025.5% 

500-749 00.9% 002.8% 000.8% 000.0% 001.8% 014.8% 

750-999 00.3% 003.4% 000.4% 002.2% 0099.7% 

1000-1249 00.0% 005.7% 004.4% 003.5% 

1259-1499 008.0% 007.5% 002.2% 

1500-1749 008.6% 006.9% 001.4% 

1750-1999 010.2% 008.2% 001.4% 

2000-2499 016.4% 018.5% 002.1% 

2500-2999 011.5% 013.0% 001.4% 

3000-3499 007.7% 009.8% 001.2% 

3500-3999 006.0% 007.1% 000.8% 

4000-4499 004.3% 005.5% 000.6% 

4500-4999 002.6% 003.3% 000.7% 

5000-5499 001.9% 002.8% 000.5% 

5500-5999 001.3% 001.4% 000.4% 

6000-9999 003.6% 004.6% 002.0% 

10000-10000+ 000.1% 000.1% 003.0% 

Total 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 
1

Table 8. System Performance Parameters for Departure Control Voice Radio Communications 

(Time in milliseconds) 

Source Mean Median SD Min Max 95% 

Setup delay 

Atlanta TRACON 0055 005 04 001 031 012 

Chicago TRACON 0099 007 09 001 071 020 

Dallas Ft Forth TRACON 0094 009 06 001 024 020 

New York TRACON 0064 005 04 001 025 015 

Southern California TRACON 0067 005 05 001 023 016 

Pause duration 

Atlanta TRACON 00146 013 007 005 0047 028 

Chicago TRACON 00110 010 005 002 0029 024 

Dallas Ft Forth TRACON 00119 011 006 000 0031 023 

New York TRACON 00154 015 007 007 0066 023 

Southern California TRACON 00104 010 004 002 0029 017 

Message propagation 

Atlanta TRACON 0069 007 03 003 014 012 

Chicago TRACON 0053 005 02 003 013 009

Dallas Ft Forth TRACON 0055 005 01 002 012 007

New York TRACON 0120 012 02 009 022 014 

Southern California TRACON 0078 007 04 003 028 015 

Voice streaming time 

Atlanta TRACON 2477 248 44 160 451 328 

Chicago TRACON 2573 253 39 170 390 330 

Dallas Ft Forth TRACON 2667 263 58 124 425 353 

New York TRACON 2886 286 60 131 467 391 

Southern California TRACON 2849 285 55 095 401 383 

Frequency occupation time 

Atlanta TRACON 2512 250 46 154 390 325 

Chicago TRACON 2782 270 46 194 491 354 

Dallas Ft Forth TRACON 2931 292 65 093 492 417 

New York TRACON 3124 308 62 153 526 408 

Southern California TRACON 3005 297 59 102 437 398 
1

The ANOVAs revealed that pilots and controllers at the 

Atlanta, New York, and Southern California TRACON 

facilities took less time to begin talking after the mic key 

was depressed than pilots and controllers at either the 

Chicago or Dallas Ft Worth TRACON [Setup Delay 

F(4,381)=8.594]. Just as there was no reliable difference

in setup delay among the Atlanta, New York, and Southern California TRACON facilities, there was no reliable 

difference between Chicago and Dallas Ft Worth.

Interestingly, pilots and controllers at the Atlanta and 

New York TRACON facilities were slower to release 

the mic key when finished speaking than the pilots and 

controllers at either the Chicago or Southern California 

TRACONs. Pause durations were also longer at the New 

York TRACON when compared with those at the Dallas Ft 

Worth TRACON [Pause Duration F(4,381)=10.383

The Chicago TRACON had the fastest time for the 

switching mechanism to return to a resting state following 

release of the mic switch when compared with the other 

TRACON facilities. In contrast, the New YorkTRACON 

had the slowest switchover time when compared with the 

other TRACON facilities. The Dallas Ft Worth TRACON had a faster mean switchover time compared with 

either the Atlanta or Southern California TRACONs 

[Message Propagation F(4,381)=97.565].

The ANOVA also revealed that the pilots and controllers at the Atlanta and Chicago TRACON facilities 

spent less time transmitting individual messages than 

their colleagues at the Southern California or New York 

TRACON facilities. Pilots and controllers at the Dallas Ft Worth TRACON did not differ from any of the 

TRACONs [Voice-streaming Time F(4,381)=9.707].

Pilots and controllers at the Atlanta TRACON spent the 

least time overall on the radio frequency. The only other 

statistically significant difference involved the pilots and 

controllers at the Chicago TRACON facility — they 

spent less time on frequency overall when compared with 

their cohorts at the New York TRACON [Frequency 

Occupancy Time F(4,381)=14.307]

Notably, both the Southern California and Dallas 

Ft Worth TRACONs experienced the least amount of 

silence between successive transmissions and less than 

either the Atlanta or Chicago TRACONs. The New 

York TRACON also had less time between successive 

transmissions than the Chicago TRACON [Lag Time 

F(4,381)=11.222]. Interestingly, the Dallas Ft Wor

TRACON had the most transmissions exchanged per 

aircraft [N transmissions per aircraft F(4,381)=4.898].

The number of transmissions per aircraft was comparable 

at all of the remaining TRACON facilities.

To get a sense of the communications process typical at 

the departure control sectors sampled, presented in Table 

9 are the frequency distributions for each time interval 

aggregated across TRACON facilities. Column (a) shows 

that, for approximately 85% of the transmissions, less 

than 150 ms transpired from the onset of PTT to the

onset of the speaker’s voice (.024 ms or less = 42.9%, .025 

ms - .049 ms = 17.5%, and .050 ms - .074 ms = 8.4%).

Table 8. System Performance Parameters for Departure Control Voice Radio Communications 

(con’t) 

Source Mean Median SD Min Max 95% 

Lag time 

Atlanta TRACON 02784 215 204 036 1200 649 

Chicago TRACON 03393 250 046 194 0491 354 

Dallas Ft Forth TRACON 01755 116 181 007 1144 525 

New York TRACON 02139 170 218 032 1700 450 

Southern California TRACON 01462 096 124 014 0674 398 

N transmissions per aircraft 

Atlanta TRACON 08.732 009 004 001 0016 016 

Chicago TRACON 07.932 008 003 002 0017 013 

Dallas Ft Forth TRACON 11.045 011 006 002 0036 019 

New York TRACON 08.805 009 004 001 0022 018 

Southern California TRACON 08.954 009 004 001 0023 018 
1

Looking at Column (b), about 42% of the transmissions 

used between 2000 – 3499 ms for voice streaming. The 

exceptionally short values for voice streaming (000-149

ms) represent the amount of time used to produce the 

word “blocked.”

Following the voice stream measure, Column c shows 

that about 90% of the transmissions had pause durations 

under 250 ms. For 95.8% of the transmissions, less 

than 150 ms was needed for the switching mechanism

to return to a resting state [see Column (d), Message 

Propagation 51.8%+44.0%]. Column (e) shows that

43.6% of the individual pilot and controller messages 

lasted between 2000-3499 ms (approx 2 to 3.5 seconds). 

Finally, Column (f) shows that the time elapsed between 

their consecutive transmissions was 250 – 499 ms of 

silence for 20.7% and 500 – 749 ms for another 17.9%

of their transmissions.

Disruptions to Efficient Information Transfer

The final set of analyses examined the data for the 

presence of blocked, stepped-on, clipped, and other types 

of transmissions that could contribute to, or result in, 

disruptions to efficient information transfer. Table 10

shows that 178 out of the original 7989 transmissions

(2.2%) either created (1.16%) or had the potential to creat

(1.06%) a disruption to efficient information transfer. In

many cases, these disruptions led to the repetition of the 

original message by the speaker (29.21%) or a request for

a retransmission of that message by the receiver (23.03%). 

Consequently, an additional 140 messages were exchanged

between pilots and controllers to resolve problems in 

information transfer. A comparison between aircraft with 

and without disruptions revealed that when a disruption 

was present, 14.54 messages were transmitted compared

with an average of 9.90 messages when no disruption was 

present [t(735)=-7.257], p<.01

Table 9. Timing Distributions For Departure Control Voice Radio Communications 

Time Interval 

(milliseconds)

Set up 

Delay

(a)

Voice

Streaming

(b)

Pause

Duration

(c)

Message

Propagation 

(d)

Frequency

Occupation

(e)

Lag

(f)

000-074 071.0% 000.1% 038.0% 051.8% 000.0% 04.6%

075-149 014.1% 000.1% 031.0% 044.0% 000.2% 005.0% 

150-224 008.3% 001.2% 021.4% 003.3% 000.2% 008.8% 

250-499 005.5% 002.6% 008.6% 000.9% 002.1% 020.7% 

500-749 000.6% 001.2% 000.7% 000.1% 001.5% 017.9% 

750-999 000.3% 001.7% 000.3% 001.6% 011.6% 

1000-1249 000.1% 003.9% 000 002.8% 004.7% 

1259-1499 000.0% 007.1% 004.6% 003.4% 

1500-1749 007.7% 008.1% 002.6% 

1750-1999 008.8% 007.4% 002.0% 

2000-2499 018.1% 017.9% 002.3% 

2500-2999 013.3% 014.7% 002.0% 

3000-3499 010.3% 011.0% 001.2% 

3500-3999 008.3% 008.6% 001.4% 

4000-4499 005.1% 006.3% 001.1% 

4500-4999 003.3% 004.3% 000.9% 

5000-5499 002.9% 003.2% 000.6% 

5500-5999 001.9% 002.0% 000.5% 

6000-9999 002.4% 003.5% 003.3% 

10000-10000+ 000.1% 000.1% 005.5% 

Total 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 
1

Once again looking at Table 10, the most troubling

types of disruptions involved blocked transmissions 

(14.61%) and transmissions that were not acknowledge

(7.86%). When transmissions were completely blocked, 

an audible alarm (17 with the spoken word “blocked” and

2 heterodyning) typically alerted the controller (Atlanta 

5, Chicago 4, Dallas Ft Worth 1, New York 5, Southern

California 4). In 94.74% of those instances, the controller 

repeated the transmission (89.47%) or requested a repeat 

from the pilot (5.26%). Notably, even in the absence 

of an aural alert, the controller took similar action — in 

85.71% of those instances the controller either repeated

or requested a repeat of that transmission from the pilot 

(Atlanta 2, Dallas Ft Worth 3, New York 1, Southern

California 1). For example, in the following transmission, the pilot is attempting initial contact with approach

control, “THREE OH EIGHT SEVEN THOUSAND 

YANKEE.” The controller responds with, “I BLOCKED 

YOU, I KNOW WHO YOU ARE, I’LL BE RIGHT 

BACK.” 

Somewhat troublesome was the lack of a pilot acknowledgment to some controller instructions. Of the 14

transmissions that were not acknowledged, 1 originate

with the controller and all were rebroadcast (Atlanta 6, 

Chicago 1, Dallas Ft Worth 1, New York 1, Southe

California 2). It is unclear why the pilots did not respond 

to these controller-generated messages the first time they 

were transmitted. In contrast, pilot repetitions involved 

attempts to initiate radar contact with controllers.

No alerting system was detected for stepped-on 

(20.79%) or clipped (20.79%) transmissions through 

waveform analysis. What makes this finding interesting is 

that when controllers detected that a message was steppedon, that message was rebroadcast in all cases (Atlanta 1,

Dallas Ft Worth 3, New York 1, Southern California 1)

however, when either the beginning (Dallas Ft Worth 4, 

New York 12, Southern California 4) or end of a message

(Dallas Ft Worth 5, New York 6) was partially stepped-on 

only 16.13% of them were retransmitted (5 of the

partial messages). Not surprisingly, only 10.34% of the

clipped transmissions were retransmitted (3 out of 31;

Atlanta 6, Chicago 8, Dallas Ft Worth 4, New York 7, 

Southern California 6). 

The findings presented in Table 10 also revealed that

the largest contributor to information transfer problems 

involved message reception — 29.78% of the transmissions had static, background noise, or some other type 

of interference present (Atlanta 12, Chicago 9, Dallas Ft

Worth 5, New York 10, Southern California 17). Of thes

53 transmissions, 69.81% were rebroadcast. When the

intelligibility of the messages was a problem, as it was in 

18 transmissions, only two were rebroadcast. Given the

Table 10. Types of Disruptions Presented by Their Outcome on Communications 

Outcome on Controller and Pilot Communications 

Source

Speaker

Repeated the 

Transmission 

Receiver 

Requested a 

Repeat of the 

Transmission 

Continuation of 

Routine

Communications Total

Controller-detected blocked message 03 03 01 007

Voice saying “blocked” 15 01 01 017 

Heterodyne tone indicating blocked 02 00 00 002

No response to message 14 00 00 014 

Controller-detected stepped-on 03 03 00 006

First part of the message was stepped-on 03 01 16 020 

End of the message was stepped-on 00 01 10 011 

First part of the message was clipped 01 01 05 007

End of the message was clipped 01 00 23 024 

Message reception (static, interference) 07 30 16 053 

Mic click response to ATC messages 03 01 13 017 

Total 52 41 85 178 
1

habitual and repetitive nature of ATC communications 

and its constrained phraseology, controllers and pilots 

have the ability to understand distorted, and otherwise 

partially unintelligible transmissions. When other factors 

were involved, as was the case for 35 messages, those 

messages were always repeated. For example, pilots might 

repeat their initial contact message to departure control if 

not acknowledged promptly by the controller. The lack 

of an immediate response to the first pilot call by the 

controller could be due to many factors. Likewise, if a 

controller did not receive a timely read back to a control 

instruction or clearance acknowledgment, that message 

would be repeated. 

There were 17 occasions where pilots acknowledged

controllers’ messages with a mic click (Atlanta 4, Chicago 

5, Dallas Ft Worth 2, New York 3, Southern California 

3). Of these non-verbal acknowledgments, the controller 

retransmitted the original message 23.53% of the time, 

apparently expecting a verbal confirmation from the pilot. 

For example, the controller expected a pilot readback 

of the following clearance instruction, “OWNSHIP 

ELEVEN THIRTY SIX ONE SEVENTY AIRSPEED 

AND THEN MAINTAIN FOUR THOUSAND” and 

not a mic click. Accordingly, the controller reissued the 

instruction and the pilot read back, “ONE SEVENTY 

THEN FOUR THOUSAND OWNSHIP ELEVEN 

THIRTY SIX.” In some cases, a verbal acknowledgment 

was not deemed necessary — for example, a double click 

was given in response to the controller’s transmission, 

“OWNSHIP TWO NINETY SIX ATLANTA RADAR 

CONTACT.” 

Table 11. Type of Disruption Presented by Facility and System Performance Measures (in ms) 

Type of Disruption 

No Disruptions Clipped Transmissions 

 Start End 

Source Mean SD N Mean SD N Mean SD N 

Setup delay 

Atlanta 064 090 1494 032 001 2 115 120 4 

Chicago 092 125 1461 042 - 1 097 182 7 

Dallas Ft Worth 099 115 1487 009 - 1 102 093 3 

New York 049 075 1990 002 - 1 104 178 6 

Southern California 106 135 1375 009 011 2 030 033 4 

Pause duration 

Atlanta 137 114 1494 051 001 2 013 013 4 

Chicago 123 109 1461 027 - 1 154 228 7 

Dallas Ft Worth 109 109 1487 093 - 1 049 069 3 

New York 155 105 1990 - 1 162 247 6 

Southern California 111 105 1375 154 108 2 041 046 4 

Message Propagation 

Atlanta 067 048 1494 103 033 2 032 012 4 

Chicago 065 050 1461 003 - 1 081 034 7 

Dallas Ft Worth 082 055 1487 011 - 1 032 014 3 

New York 072 046 1990 - - 1 067 043 6 

S. California 081 061 1375 150 052 2 055 018 4 
1

DISCUSSIO

Effective communications in the National Airspace 

System is an essential component of safe and efficient 

air travel. Communicated information can be generated 

from radar track data, automatic dependent surveillance 

broadcast, voice radio, or a data link. It can be presented 

as visual or auditory information. Its source can be from 

either a ground-based or satellite communications systems. As technological advances lead to innovations in 

communications systems development, those emerging 

systems will need to be evaluated against the existing 

legacy system’s performance parameters. Some of those 

parameters include throughput measures — such as the 

ones presented in this report.

The major findings from the analysis of nearly 8000 

individual waveforms are that communications occur 

quickly and with little silence occurring between transmissions. On average, there were about 13 air-ground

transmissions generated for every minute sampled. 

Typically, once the push-to-talk switch was depressed, 

communications began 81 ms later. It took about 2.5 sec

to generate a message, and another 127 ms lapsed before

the push-to-talk switch was released. The communications 

system was ready to receive another transmission about 73 

ms later. From the moment that the push-to-talk switch 

was depressed and then released, nearly 3 sec lapsed. 

On average, transmissions were separated by 1.75 sec of

silence. In summary, approximately 70% of each minute 

was devoted to pilot and controller communications (39 

sec communicating and 2.5 sec for the communications 

systems to return to a steady state). Add in the number 

and duration of land-line transmissions, and it is easy to 

determine that the current communications system is 

approaching saturation levels.

Potential problems can result from blocked, steppedon, and clipped transmissions — but these are rare events. 

During the 603 minutes sampled, only 178 disruptions

were identified, of which 93 required a retransmittal of 

the original message. Even so, there seems to be some 

type of detection system in place to identify blocked 

transmissions and alert the controller to their presence. 

Future systems developers may want to exploit this detection system and expand it to include stepped-on and 

clipped transmissions.

Although it is unfortunate that there was insufficient 

data with which to perform the needed inferential statistics 

to identify whether a causal relationship exists between 

the duration measures of the communications system 

performance and the types of disruptions identified, some 

liberty was taken. Presented in Table 1 are the syste

performance measures (i.e., setup delay, pause duration, 

message propagation) for the transmissions that experienced either no disruption or were clipped. Intuitively, it 

would make sense to expect that clipped transmissions at 

the beginning of the voice stream to have shorter setup 

delays than transmissions that were not disrupted. An 

examination of the data presented in Table 1 support

that assertion. Also, it could be assumed that when the 

end of a transmission is clipped there would be shorter 

pause durations and, in fact, the data confirm that. 

The data presented in this report are but a first step 

in providing objective and quantifiable communications 

system performance metrics. Perhaps these metrics and 

parametric values will prove beneficial to communications systems developers and FAA personnel charged with 

the evaluation, certification, and deployment of the next 

generation of communications systems.

Re

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J. Hughes Technical Center.

This publication and all Office of Aerospace Medicine technical 

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

Institute’s publications Web site: www.faa.gov/library/reports/medical/

oamtechreports/index.cfm