The Outcome of ATC 

Message Complexity on Pilot 

Readback Performance

O. Veronika Prinzo

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

Alfred M. Hendrix

Ruby Hendrix

Hendrix & Hendrix

Roswell, NM 88201

November 2006

Final Report

DOT/FAA/AM-06/25

Office of Aerospace Medicine

Washington, DC 20591
NOTICE

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

of information exchange. The United States Government 

assumes no liability for the contents thereof.

___________

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
i

Technical Report Documentation Page 

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

DOT/FAA/AM-06/25 

4. Title and Subtitle 5. Report Date 

The Outcome of ATC Message Complexity on Pilot Readback Performance November 2006 

6. Performing Organization Code 

 

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

Prinzo OV,1 Hendrix AM, Hendrix R2 

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 

2

Hendrix & Hendrix 

15 Circle Drive 

Roswell, NM 88201 

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

Office of Aerospace Medicine 

Federal Aviation Administration 

800 Independence Ave., S.W. 

Washington, DC 20591 14. Sponsoring Agency Code 

15. Supplemental Notes 

Work was accomplished under approved task AM-03-HRR-516. 

16. Abstract 

Field data and laboratory studies conducted in the 1990s reported that the rate of pilot readback errors and communication 

problems increased as controller transmissions became more complex. This resulted in the recommendation that controllers 

send shorter messages to reduce the memory load imposed on pilots by complex messages. More than 10 years have passed 

since a comprehensive analysis quantified the types and frequency of readback errors and communication problems that 

occur in the operational environment. Hence, a content analysis was performed on 50 hours of pilot and controller 

messages that were transmitted from 5 of the busiest terminal radar approach control facilities in the contiguous United 

States between October 2003 and February 2004. This report contains detailed and comprehensive descriptions of routine 

air traffic control (ATC) communication, pilot readback performance, call sign usage, miscommunications, and the effects 

of ATC message complexity and message length on pilot readback performance. Of importance was the finding that both 

the number of pilot requests and readback errors increased as the complexity and number of aviation topics in ATC 

messages increased — especially when pilots were performing approach tasks as compared with departure tasks. Also, 

nonstandard phraseology associated with a lack of English language proficiency and international communications were 

present in the data. In particular, pilot use of the word “point” as part of a radio frequency was included in the read back of

altitude (“three point five”) and speed (“two point seven on the speed”). To limit the occurrence of communication 

problems and misunderstandings, controllers should be encouraged to transmit shorter and less complex messages. With 

increases in international travel, areas of concern related to English language proficiency and language production need to 

be addressed.

 

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 36 

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

iii

EXECUTIVE SUMMARY

The results presented in this report provide a description and summary of the controller-pilot communication process that occurred during normal, day-to-day 

operations in the terminal radar approach control 

(TRACON) environment. On average, across the five 

sampled TRACON facilities, one aircraft requested 

and received air traffic services every 1 min 26 s in the 

approach sectors and 1 min 6 s in the departure sectors. Approximately 13 messages were exchanged (from 

initial contact until the aircraft was switched to the next 

controller in sequence) that involved an allocation of 

about 1 min 16 s of airtime per aircraft.

A comparison between the voice communications 

analyzed by Cardosi et al. (1996) with those analyzed 

here by Prinzo, Hendrix, and Hendrix revealed that 

more than 50% of controllers’ messages were fairly short 

but information rich. Pilots increased their production 

of full readbacks — up from 60% in 1996 to more 

than 82% in 2004. Most striking was the finding that 

10 years ago, pilots provided a full readback with a 

complete call sign about 37% of the time, and in 2004 

it accompanied a full readback in 61% of the pilots’ 

transmissions. Where Cardosi et al. (1996) reported 

that 24% of the full readbacks included a partial call 

sign, we found 18.8%, of which 13.4% excluded the 

prefix but included all the numbers/letters of the call 

sign. Likewise, pilot/controller call sign mismatch has 

decreased from 0.8% to 0.3%. 

Both the Cardosi et al. 1996 report and this report 

show that aircraft headings and radio frequency changes 

still are the most frequently occurring readback errors. 

Likewise, there is no change in the frequency with in 

which pilots request that controllers repeat all or some 

portions of their transmissions. 

The operational data analyzed in this report provide 

additional evidence that readback errors and pilot requests increased with increases in message complexity 

(amount of information in a communication element) 

and message length (when measured by number of aviation topics such as heading, altitude, speed instructions 

in a controller’s message). Importantly, pilots experienced the most difficulty reading back ATC messages 

with more than one aviation topic and ATC messages 

with a complexity value of 10 or greater when flying 

the approach segment of their flight. 

A new trend that is occurring in pilot communications is the tendency to round the numbers in the call 

sign and aviation topics. For example, Ownship67H 

became Ownship60H and Ownship528 became Ownship520. Some pilots truncated or otherwise abbreviated the numerical values in speed (“TWENTY FIVE 

KNOTS”), heading (e.g., “onefour” for a heading 

of one four zero), or altitude assignments (“DOWN 

TO FIVE HUNDRED).

Other forms of nonstandard phraseology were also 

associated with readback errors. It may be that some 

of the phraseology used (or heard) by pilots during 

international flights is making its way into the national 

airspace system (NAS). Some pilots used the “point” 

designation associated with radio frequencies when 

reading back altitudes (e.g., “THREE POINT FIVE” 

instead of “THREE THOUSAND FIVE HUNDRED”) and speeds (e.g., “TWO POINT SEVEN 

ON THE SPEED” for “two hundred and

seventy knots”). Likewise, several pilots flying 

for foreign air carriers displayed some problems in 

English proficiency and language production — for 

example, reading back a speed instruction as “two

zero hundred” instead of “two hundred knots,” 

or responding to “maintain visual from traffic” as 

“MAINTAIN VISUAL APPROACH.”

Communicating for safety is the primary objective 

of the phraseology developed for and provided in FAA 

Order 7110.65, The Handbook of Air Traffic Control for 

controllers and the Aeronautical Information Manual for 

pilots. With increased international travel and the gradual 

migration of other phraseologies into the NAS, pilots and 

controllers must remain vigilant in the accurate production 

and recitation of ATC clearances, instructions, advisories, 

reports, requests, and other communications.

1

The Outcome of ATC Message Complexity on

Pilot Readback Performance

“Speak properly, and in as few words as you can, but always plainly; for 

the end of speech is not ostentation, but to be understood.”

—William Penn, English religious leader and colonist (1644–1718)

As stated in the Federal Aviation Administration (FAA) 

Flight Plan 2006-2010 report (2006), the FAA’s mission 

is to provide the safest, most efficient aerospace system in 

the world. In the aftermath of 9/11, it is not surprising 

that the number of passengers and scheduled air carrier 

flights decreased. Since the implementation of changes 

in airport and aircraft security, consumer confidence has 

gradually returned, and the number of scheduled flights 

and passenger volume are at pre-9/11 levels. For example, 

in the year 2003, there were 120.5 million aircraft operations recorded. For the first time in several years, some 

of the busiest air traffic control (ATC) towers are again 

experiencing traffic delays and congestion.

The FAA has met with representatives of the airline 

industry and ATC facility personnel to resolve these 

problems. One solution was to reduce the number of 

departures per hour by developing new flight departure 

schedules with some of the larger airlines. A second solution was the construction of new runways at these busier 

airports in expectation of projected increases — the FAA 

has set a goal of adding an additional 500 flights per 

day — that is an increase of about 1% per year with an 

anticipated total civil aircraft activity of 137.4 million 

operations by the year 2015.

Increases in air travel go hand in hand with increases 

in the delivery of ATC services. The existing ground infrastructure and analog voice communications system is 

the medium by which services are delivered. They include 

the transmission of clearances and instructions as well as 

traffic and weather advisories. These transmissions are 

critical for the coordination of all vehicle movement to 

ensure safety while aircraft are on the ground and when 

they are in the air.

Unfortunately, at some of the busiest ATC facilities, 

air-ground and ground-ground communications are 

at their pre-9/11 saturation points during peak traffic 

periods. During these times, pilots often compete with 

one another for access to the same radio frequency to 

establish contact, receive clearances, make requests, etc. 

Too many pilots assigned to the same radio frequency 

can result in communication bottlenecks that can add to 

airport congestion, delays, and may increase the potential 

for communication problems.

Sometimes controllers adopt the strategy of sending longer, more complex transmissions in an attempt to reduce the 

number of times they need to be on frequency, while including 

all the information required by FAA policy/regulations. As 

well-intended as the strategy is, field studies (Cardosi, 1993; 

Cardosi, Brett, & Han, 1996; Prinzo, 1996) and laboratory 

experiments (Morrow & Prinzo, 1999) have documented 

that the rate of pilot readback errors and communication 

problems increased as controller transmissions became more 

complex. Often, the occurrence of pilot readback errors 

necessitates the exchange of additional messages to ensure 

that the intended meaning was received, understood, and 

confirmed. This process added to radio frequency congestion. The amount of information that pilots can actively 

read back is constrained by the inherent limitations of their 

verbal working memory.

Humans have limitations in the amount of information 

that they can successfully process, store, recognize and recall. 

At first, a person may form many groups or “chunks” with few 

bits of information per chunk. With learning and experience, 

the amount of information that a person can include in a chunk 

will vary — but the upper limit of verbal working memory is 

between five to seven chunks at a time. After that, successful 

recoding diminishes and forgetting occurs. Through experience, we learn to organize or recode sound into progressively 

larger groups by translating them into a verbal code (Miller, 

1956). He provides the following narrative to illustrate the 

concept of recoding into progressively larger chunks:

A man just beginning to learn radio-telegraphic code hears 

each dit and dah as a separate chunk. Soon he is able to organize 

these sounds into letters and then he can deal with the letters as 

chunks. Then the letters organize themselves as words, which are 

still larger chunks, and he begins to hear whole phrases. I do not 

mean that each step is a discrete process, or that plateaus must 

appear in his learning curve, for surely the levels of organization 

are achieved at different rates and overlap each other during 

the learning process. I am simply pointing to the obvious fact 

that the dits and dahs are organized by learning into patterns 

and that as these larger chunks emerge the amount of message 

that the operator can remember increases correspondingly. In 

the terms I am proposing to use, the operator learns to increase 

the bits per chunk.
2

For pilots, with the onset of an ATC message, the 

sounds at the beginning of the message stream enter 

into a pilot’s limited-capacity verbal working memory, 

where they are processed and temporally stored as phonological representations. That is, acoustically relevant 

sounds are extracted and encoded into phonemes (i.e., 

consonant-vowel-consonant clusters) that form syllables 

(e.g., stress patterns and intonation) that are assembled 

to create words, phrases, clauses, and other constituents. 

These representations must be maintained in an active 

state (rehearsed); otherwise, they begin to decay in about 

2 seconds (Baddeley, Thomson, & Buchanan, 1975) or 

be overwritten by incoming information. Furthermore, 

Baddeley et al. proposed a linear relationship between 

the number of words correctly recalled and speech rate. 

Using mathematical modeling, Schweicker and Boruff 

(1986) found that 95% of the variance in memory span1

performance for words, digits, and colors was related to 

the number of items that were spoken in 2 seconds.

Baddeley’s (1987) phonological-loop model of verbal 

working memory has demonstrated that the ability to 

accurately recall information in the order in which it was 

originally heard is better for word sequences that have 

shorter as compared with longer articulatory durations 

(i.e., the amount of time taken to pronounce the word 

sequence). This effect holds true when two sets of words 

are matched in the numbers of phonemes and syllables 

in each word but differ in mean articulatory durations 

(Baddeley & Hitch, 1974; Mueller, Seymour, Kieras, & 

Meyer, 2003). 

An utterance’s complexity can be derived from its grammatical weight —the amount of information expressed 

in its constituents as measured by the number of words, 

syntactic nodes, or phrasal nodes in the constituent 

(Wasow, 1997). As pointed out by Miller (1956), to be 

successful at recoding sensory information into chunks 

that become progressively larger requires automatic recoding; otherwise, as new inputs are being transmitted, they 

will be sacrificed while attempting to retain the name of 

the last group.

These findings, classic to cognitive psychology and 

psycholinguistics, have been applied to aviation. In particular, field and simulation findings (see Prinzo & Britton, 

1993 for a review of the literature; Cardosi et al., 1996; 

and Morrow & Prinzo, 1999) led to the recommendation that controllers should transmit more messages that 

were less complex, rather than fewer but more complex 

messages. The rationale was that less complex messages 

(fewer topics and less information) should not tax pilot 

1 Memory span refers to the number of items (usually words or digits) 

that a person can hold in working memory. Tests of memory span are 

often used to measure working memory capacity. The average span 

for normal adults is 7.

memory to the same extent as longer, more complex ones 

(more topics and information). Their recommendation, 

if made policy and implemented, should lead to fewer 

readback errors and communication problems.

It has been 10 years since a comprehensive analysis has 

been conducted to quantify the types and frequencies of 

readback errors and communication problems that occur in the operational environment. It is important to 

determine whether the aforementioned findings remain 

representative. Therefore, the purpose of this report is 

to 1) provide current information regarding routine 

communication practices, 2) document the types of 

transmissions that are exchanged between pilots and the 

certified professional controllers who provide them with 

ATC services, and 3) record communication problems 

by type and frequency of occurrence.

Neither the aforementioned studies nor this study 

considered the impact of other information sources on 

communication. In particular, information presented on 

the controller’s situational display provides a rich context 

from which oral communications become meaningful. For 

example, alphanumeric information located in the data 

block provides indications of changes in an aircraft’s altitude, speed, track, transponder code, runway/approach, 

etc. as the pilot complies with an ATC transmission. 

Also spatial information on video map overlays provides 

airspace information, while primary and secondary radar track data indicate aircraft proximity and geometry. 

Together, these rich (and often redundant) information 

sources aid the controller in the decoding, comprehension, 

and decision-making processes. They can impact several 

elements of communication, including the decoding of 

otherwise unintelligible messages, hearback errors, repeated instructions/clearances (with slight modification), 

and possibly others. To include this visual reference in 

any study would require correlating the information on 

the controller’s situational display (video) with the voice 

communication (audio).

This report is similar to Cardosi et al.’s 1996 report in 

that both reports focus on clearance acknowledgments and 

miscommunications in response to ATC messages that 

differ in level of complexity. Both provide a comprehensive 

analysis of TRACON communications representative of 

actual operational communication exchanges between 

pilots and controllers. Where Cardosi et al. examined 

communications during periods of heavy and moderate 

workload (as determined by each facility), we examined 

communications during heavy workloads only (again, as 

determined by each facility). Both this report and Cardosi 

et al.’s 1996 report included communication samples 

obtained from the Dallas-Fort Worth, Los Angeles, 

and New York TRACON facilities. Cardosi et al. also 

obtained communication samples from Boston, Denver, 
3

Miami, Phoenix, and Seattle; we obtained samples from 

Chicago and Atlanta.

The two reports differ primarily in the tabulation of 

message complexity. That is, the definition of message 

complexity provided by Cardosi (1993) for the analysis 

of Air Route Traffic Control Center (ARTCC) communications changed for the analysis of TRACON communications (Cardosi et al., 1996). In an excerpt from 

the 1993 report, “Complexity level was computed by 

counting all elements containing information a pilot has 

to remember, such as taxiways, runways, who to follow, 

but not items such as aircraft and facility identification, 

‘Roger,’ or salutations. For example, the instruction ‘(Aircarrier) 3890, (Facility) Ground, give way to the second 

Dornier inbound, then taxi runway 32 left, intersection 

departure at Gulf, via outer, Charlie, Gulf’ was coded as 

containing the following eight elements: Give way, Traffic, 

Runway, Other, Location, Taxiway 1, Taxiway 2, Taxiway 

3. Although most of the instructions contained three or 

fewer pieces of information, over 35 percent contained 

four or more elements” (p. 5). That definition agreed 

with Prinzo, Britton, and Hendrix’s (1995) concept of 

the aviation topic.

Message complexity was defined in the 1996 Cardosi 

et al. report as the number of separate elements contained 

in a single transmission. “Each word, or set of words, the 

controller said that contained a new piece of information to the pilot, and was critical to the understanding 

of the message was considered to be an element. An element could be considered as an opportunity for error. 

For example, ‘Air carrier 123, heading two five zero’ was 

considered two elements (‘heading’ and ‘250’)” (p. 3). 

Cardosi et al. continued with “Numbers that constitute 

headings, speeds, runways, frequencies, etc., are each 

considered to be one element as are ‘left’, ‘right’, and the 

terms ‘heading’, ‘speed’, etc.” (p. 3).

As presented and used here, the level of complexity 

of a communication element is defined by each word or 

set of words transmitted by ATC to the flight deck that 

contains a new piece of information critical to the understanding of that communication element. As is often the 

case, a message transmitted by ATC may contain multiple communication elements, and message complexity 

would be the sum of the values assigned to each one. As 

noted in Prinzo (1996), communication elements are the 

fundamental unit of meaningful verbal language. Within 

aviation communications, communication elements are 

identified according to their functionality; that is, their 

purpose, operation, or action (Address/Addressee, Courtesy, Instruction/Clearance, Advisory/Remark, Request, 

and Non-Codable) and are restricted with regard to their 

aviation topic (altitude, heading, speed, traffic, route, 

etc.) (Prinzo et al., 1995).

What we attempted to do was remove as much of the 

subjective component as possible when counting the level 

of complexity present in communication elements. As 

noted in FAA Order 7110.65, The Handbook of Air Traffic Control (FAA, 2004), ATC prescribes that controllers 

use a rigid set of words/phrases. This phraseology tends 

to narrow the definition and meaning of communication elements. Some of these words and phrases serve as 

anchors that make the communication element more 

precise in its interpretation. 

Some anchors attach meaning to the numbers present 

in a controller’s message. For example, the significance of 

“3-5-0” is ambiguous until an anchor word appears with it 

in the transmission — “3-5-0” can easily be interpreted as 

a heading, altitude, or speed. Thus, degrees are associated 

with heading, knots with speed and descend/climb/maintain with altitude. When so used, anchors assist in the 

interpretation of communication elements and restrict the 

meaning assigned to aviation topics (ATs). Each anchor 

was assigned a complexity value = 1 as were numerical 

values, orientation (left, right, center), and the names of 

fixes, points, intersections, markers, etc. as determined 

by the phraseology usage by the controller according to 

the examples provided in FAA Order 7110.65.

Our scoring scheme attempts to reflect the added 

complexity imposed by communication elements that 

contain more information by assigning them larger values. 

This assumption holds, particularly for altitude instructions. For example, altitude instructions such as “three 

thousand five hundred,” “one-zero thousand” and “four 

thousand” are likely to impose quantitatively different 

loads on working memory. In particular, “three thousand 

five hundred” takes longer to pronounce and contains 

more words than “four thousand” (e.g., articulatory loop 

proposed by Baddeley, 2000) and utilizes more capacity 

(Miller, 1956). When serial reproduction is required, 

numerical content that utilizes more resources may be 

partially or completely omitted or lead the pilot to request 

a repetition (Morrow & Prinzo, 1999). 

To illustrate the difference between the two approaches, 

consider the ATC transmission presented in Cardosi et 

al. (1996), “Aircraft XX, change runway to two-five left, 

cross Santa Monica VOR at or above seven thousand, 

descend and maintain three thousand five hundred.” For 

Cardosi et al., the transmission contained five pieces of 

critical information (but they did not illustrate how this 

value was obtained). We suggest that the transmission 

contained four aviation topics: an address, an advisory 

to expect a change in route/position, an instruction 

involving an altitude restriction, and an instruction to 

change altitude. The altitude restriction had a complexity 

value = 5 (cross = 1, point = 1, at or above = 1, numerical 

value = 1, thousand = 1) and altitude had a complexity 
4

value = 6 (descend and maintain = 2, numerical value 

= 1, thousand = 1, numerical value = 1, hundred = 1). 

Therefore, for the present example, the transmission had 

a complexity value = 11. To be consistent with Cardosi 

et al., we did not include the address and advisory (other 

than for traffic or altimeter settings) in the computation 

of complexity values.

METHOD

Materials

Audiotapes. In this report 28 hr 13 min 23 s of 

approach and 23 hr 56 min 32 s of departure communications were provided by the five busiest TRACON 

facilities in the contiguous United States. The amount 

of voice communications varied from as little as 58 min 

55 s on one communication sample to as much as 5 hr 

13 min 49 s on another. However, each facility was asked 

to provide 5 hr of approach and 5 hr of departure voice 

communications for a total of at least 50 hr of recording. 

Digital Audiotape (DAT) recordings were made at each 

TRACON facility using the NiceLoggerTM Digital Voice 

Recorder System (DVRS) to record and time-stamp each 

transmission.

Each DAT contained separate voice records of all 

communication 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.

There were 12-arrival and 11-departure sectors represented on DATs from the 5 highest-level terminal facilities, 

and the traffic was typical for a level-5 terminal facility. 

The traffic was primarily air carrier, with some private 

jets, and a few general aviation pilots flying the Coastal 

VFR Corridor. All sectors had some foreign carriers. The 

recordings were made between October 2003 and February 2004. Each facility representative was instructed that 

DAT recordings were to reflect communications-intensive 

periods during peak traffic loads (as determined by that 

facility). For the outbound push, the sampled recordings 

represented morning (7:30 am), afternoon (12:30 pm), 

mid-day (4:30), and evening (5:54 pm) departures and 

early-morning (8:45 am), mid-morning (11:00 am), 

afternoon (12:00 pm), mid-day (3:00, 5:00 pm), and 

evening (7:15 pm) arrivals during the inbound rush.

In addition to maintaining separation, a departure 

controller’s duties include: establish radar contact, verify 

the Mode C, initiate a radar handoff to en route, and make 

a communication transfer once the handoff is accepted. 

Communications involve: establish communication with 

the pilot, establish radar contact, listen to the altitude 

report and verify the Mode C, vector, issue speed assignment, altitude assignment, route assignment, and 

communication transfer to the receiving controller (usually the en route controller). Arrival controllers sequence 

traffic to a single runway and transfer communication 

to the tower. Occasionally, traffic is routed to a parallel 

runway. Their communications include: initial contact, 

listen for altitude reported by pilot, altitude assignment, 

route assignment vectoring, speed assignment, approach 

clearance, and communication transfer to the tower. 

Subject-Matter Experts (SMEs)

The first author had 12 years of experience analyzing 

pilot-controller communications. The second author, 

an instrument-rated pilot and former controller, had 

worked as an FAA Academy instructor for 8 years and 

had 12 years experience in FAA supervision and management. The third author had assisted the second author 

in encoding pilot-controller communications for more 

than 10 years.

A Guide to the Computation of Level of Complexity. 

Presented in Tables 1 and 2 are excerpts taken from the 

Instruction Complexity Guide (Appendix A) and the 

Advisory Complexity Guide (Appendix B). The tables 

were developed to increase the reliability and consistency 

of tabulating complexity for typical ATC phraseology usage. The first column presents the aviation topic; column 

two presents the complexity value. The smaller the value 

is, the less complex the phrase. Column three presents 

the phraseology extracted from FAA Order 7110.65 to 

support the delivery of that service. In several cases, the 

phraseology used by the speaker did not appear in FAA 

Order 7110.65 (e.g., tight turn, go fast) but was used so 

frequently that they were assigned values. Capitalized 

words designate anchors, are fixed in their meaning, 

and designate the action that the pilot is to perform. 

The italicized words in parenthesis are qualifiers that 

vary according to the geographical location and aircraft 

position. 

To determine complexity value, anchors, qualifiers, 

and excessive verbiage are assigned a value indicative of 

new information or importance towards understanding an 

instruction, traffic advisory, and altimeter setting advisory. 

In most cases, each anchor is counted as one element of 

complexity. There are several exceptions, however. Some 

communication elements contain multiple anchors, as is 

the case “turnleft/right heading (degrees).” The anchor 

“Turn left/right” provides the direction of the turn, 

while “heading” indicates the aircraft’s bearing.

Also, qualifiers such as the numbers that comprise an 

altitude must be evaluated according to the phraseology 
5

Table 1. Excerpt from the Complexity Guide for Instruction/Clearance Communication Elements 

Aviation Topic Level of Complexity Phraseology 

Altitude

6

5

4

*4-8

*4-8

*3-7

*3-8

*2-6

*1-2

3=(altitude) two digits +THOUSAND 

2=(altitude) one digit + THOUSAND 

3=(altitude) two digits + HUNDRED 

2=(altitude) one digit + HUNDRED 

2=(altitude) two digits 

1=(altitude) one digit

DESCEND/CLIMB & MAINTAIN

(altitude) THOUSAND (altitude) HUNDRED

 Three five 

DESCEND/CLIMB & MAINTAIN (altitude) THOUSAND 

one zero 

DESCEND/CLIMB & MAINTAIN (altitude) THOUSAND 

four

CONTINUE CLIMB/DESCENT TO (altitude)

AMEND YOUR ALTITUDE DESCEND/CLIMB AND MAINTAIN (altitude) 

AMEND YOUR ALTITUDE MAINTAIN (altitude) 

DESCEND/CLIMB TO (altitude)

MAINTAIN (altitude)

(altitude, omitted “THOUSAND” “HUNDRED”)

Heading

4

4

3

3

3

2

2

1

TURN LEFT/RIGHT HEADING (degrees) 

TURN (degrees) DEGREES LEFT/RIGHT 

TURN LEFT/RIGHT (degrees)

DEPART (fix) HEADING (degrees)

FLY HEADING (degrees) 

FLY PRESENT HEADING 

HEADING (degrees)

(degrees)

Table 2. Complexity Guide for Advisory Communication Elements 

Aviation Topic Level of Complexity Phraseology 

Traffic

6

6

5

2

TFC (number) MILES (o’clock) ALT xxxx (type etc.) 

YOU’RE FOLLOWING (type) (o’clock) (number) MILES ALT xxxx 

TFC (number) MILES (o’clock) ALT xxxx 

YOU’RE FOLLOWING (type) 

Altimeter 3 ALTIMETER (4 digits)
6

used by the speaker. That is, the number “three thousand 

five hundred” was assigned a value of 4 (a value of one 

for each number and a value of one for each anchor) 

since it would be more demanding than either onezero thousand (value = 3) or four thousand (value = 2). 

Finally, one element of complexity should be added for 

communication elements that contain excessive verbiage. 

Excessive verbiage is determined by comparing the utterance of the speaker against the phraseology designated 

in FAA Order 7110.65. If a pilot attempted a verbatim 

readback of a controller’s transmission, then the coding 

procedures were applied that were used to evaluate the 

controllers’ transmissions. 

A Guide to the Classification of Pilot Readback 

Errors. As used here, a readback error is defined as an 

unsuccessful attempt by a pilot to read back correctly the 

information contained in the communication elements 

that comprise the original message transmitted by air 

traffic control. As seen in Table 3, the column to the 

left displays the types of readback errors according to a 

particular type of aviation topic. The aviation topics are 

heading (HDG), heading modification (HDG MOD), 

altitude (ALT), altitude restriction (ALT RSTRN), speed 

(SPD), approach/departure (APCH_DEPTR), radio 

frequency (FREQ), position/route (RTE), transponder 

(TRNSPNDR), and altimeter (ALTM).

Many of the readback error types are common to all 

aviation topics. The more typical ones include errors of 

substitution, transposition, and omission. Presented in 

the right column of Table 3 are examples of each type of 

readback error according to the aviation topic in which 

it was embedded. Preceding each example of a particular 

type of readback error is the original ATC message. For 

example, ATC might transmit the following message to 

AAL10, “American Ten turn left heading two one zero.” 

If the pilot reads back either “three one zero” or “six 

zero,” it would be coded as a substitution error since the 

numbers in the original heading instruction included 

neither a three nor a six.

Some types of readback errors may pose a greater risk 

to safety than others. For example, transposing a number 

in an aviation topic may be more of a threat in some situations than the omission of a number or the substitution 

of an anchor word with its synonym.

Procedure

Data Transcription. One set of audiocassette tapes were 

dubbed from each DAT and provided to the transcribers who used them to generate the verbatim transcripts. 

Each message was preceded by its onset and offset time 

represented in hour (hr) minute (min) and second (s) as it 

was typed onto an electronic copy of the Aviation Topics 

Speech Acts Taxonomy-Coding Form (ATSAT-CF; Prinzo 

et al., 1995). Once the transcribers finished a set of tapes 

for a TRACON facility, the second and third authors were 

provided with copies of the transcripts, audiocassette tapes, 

video maps, air carrier identifiers, and approach/departure 

routes for use during the encoding process. This process 

was followed for each of the TRACON facilities.

Message Encoding. The SMEs met on five separate 

occasions. The first two meetings were used to operationally define message complexity and develop the rules and 

procedures for encoding each message. This was done to 

limit the arbitrary and subjective determination of what 

constitutes information complexity for verbal information. 

For part of the remaining meetings, the consistency of data 

encoding was evaluated as the transcripts for each of the 

remaining TRACON facilities were encoded. This was 

achieved by having the first and second author randomly 

encode the same set of 25 messages (for each facility) and 

then computing the percentage and degree of agreement. 

In each case, it exceeded 95%.

A follow-on reliability analysis (using Krippendorff’s 

alpha) was performed on 125 different messages after all 

the data were encoded. Krippendorff’s alpha is a reliability 

coefficient that was originally developed for evaluating 

agreement between coders performing a content analysis. 

It is a statistic that is widely applicable wherever two or 

more methods of processing data are applied to the same 

set of objects, units of analysis, or items to determine 

how much they agree (Krippendorff, 1980). Treating the 

ratings as ordinal data produced an α = .98982

, indicating 

high inter-rater agreement.

Computation of Level of Complexity for Communication Elements. Each transmission was first parsed 

into communication elements, labeled by speech act category and aviation topic using the procedures developed 

by Prinzo et al. (1995). Then the appropriate guide for 

computing level of complexity (cf. Table 1 and Table 2) 

was used to look up the appropriate value according to the 

phraseology used by the controller for that communication element. The value assigned to each communication 

element was entered into the appropriate column of the 

encoding spreadsheet. 

Like Cardosi et al. (1996), aircraft call sign/facility 

identification, courtesies, requests, and advisories (except 

air traffic advisory and the altimeter portion of weather 

advisory) were excluded. The elements of complexity 

were counted for the a) instructions/clearances speech 

acts that involved heading, heading modifier, altitude, 

altitude restriction, speed, approach/departure, frequency, 

route, and transponder aviation topics, b) advisory speech 

act that involved traffic, and c) the altimeter portion of 

weather advisories. 

2 We thank Andrew F. Hayes for not only developing the SPSS syntax 

for running Krippendorff’s alpha but also for computing it for us. 
7

Table 3. Readback Error Guide Presented by Aviation Topic 

Classification of Readback Errors Examples 

Readback Errors Type (HDG) 

1 = Substitution of message numbers 

2 = Transposition of message numbers 

3 = One type of information read back as another type 

4 = Incorrect direction of turn 

5 = Omission of one or more numbers 

6 = Not assigned 

7 = Omission of anchor word(s) 

8 = Substitution of anchor word(s)

ATC “AAL Ten turn left heading two one zero”

1-“three one zero,” or “six zero” 

2-“turn left heading one two zero” 

3-“ two one zero knots” 

4-“turn right two one zero,” 

5-“one zero,” “zero on the heading” 

7-“two one zero” 

8-“two hundred and ten degrees” 

Readback Errors Type (HDG MOD) 

1 = Substitution of rate of turn

ATC “AAL Ten increase rate of turn descend and maintain 

four thousand”

1-“decrease rate of turn” 

Readback Errors Type (ALT) 

1 = Substitution of message numbers 

2 = Transposition of message numbers 

3 = One type of information read back as another type 

4 = Not assigned 

5 = Omission of number element 

6 = Not assigned 

7 = Omission of anchor word(s) 

8 = Substitution of anchor word(s)

ATC “AAL Ten climb and maintain one two thousand”

1-“to one three thousand” 

2-“climb two one thousand” 

3-“one two zero knots” 

5-“two thousand” 

7-“twelve” 

8-“up to twelve thousand” 

Readback Errors Type (ALT RSTRN) 

1 = Substitution of message numbers 

2 = Transposition of message numbers 

3 = One type of information read back as another type 

4 = Omission of (point/fix) 

5 = Omission of number element 

6 = Transpose one (point/fix) with that of another 

7 = Omission of anchor word(s) 

8 = Substitution of anchor word(s)

ATC “AAL Ten maintain one thousand two hundred til 

DOOIN”

1-“cross DOOIN at one thousand four hundred” 

2-“cross DOOIN at two thousand one hundred” 

3-“slow to two one zero” 

4-“maintain one thousand two hundred” 

6-“cross LIMA at one thousand two hundred” 

7-“one twenty” 

8-“maintain one thousand two hundred til established,” 

“good rate up” 

Readback Errors Type (SPD) 

1 = Substitution of message numbers 

2 = Transposition of message numbers 

3 = One type of information read back as another type 

4 = Omission of (point/fix) 

5 = Omission of number element 

6 = Transpose one (point/fix) with that of another 

7 = Omission of anchor word(s) 

8 = Substitution of anchor word(s)

ATC “AAL Ten reduce speed two one zero knots til 

DEPOT”

1-“two five zero knots til DEPOT” 

2-“reduce one two zero knots til DEPOT” 

3-“left two one zero” 

4-“reduce two one zero knots” 

5-“ten knots til DEPOT” 

6-“reduce one two zero knots til RIDGE” 

7-“two ten til DEPOT” 

8-“we’ll go slow” 
8

Table 3 (continued). Readback Error Guide Presented by Aviation Topic 

Classification of Readback Errors Examples 

Readback Errors Type (APCH_DEPTR) 

1 = Substitution of message numbers 

2 = Transposition of message numbers 

3 = One type of information read back as another type 

4 = Substitution of one type of approach with another 

5 = Omission of number element 

6 = Transpose one (point/fix) with that of another 

7 = Omission of anchor word(s) 

8 = Substitution of anchor word(s)

ATC “AAL Ten cleared ILS runway two one right approach”

1-“cleared ILS runway two one left approach” 

2-“cleared ILS runway one two right approach” 

3-“right two one zero,” “cleared to land two one right” 

4-“cleared visual approach runway two one right” 

5-“cleared ILS approach” 

6-“cleared ILS at Ridge two one right approach” 

7-“cleared approach” 

8-“cleared for the final” 

Readback Errors Type (FREQ) 

1 = Substitution of frequency digits 

2 = Transposition of message numbers 

3 = One type of information read back as another type 

4 = Omission of contact location 

5 = Omission of number element(s) 

6 = Substitution of one contact location with another 

7 = Omission of anchor word(s) 

8 = Substitution of anchor word(s)

ATC “AAL Ten contact tower one one eight point three”

1-“contact tower one seven point three” 

2-“contact tower one eight one point three” 

3-“squawk one one eight three” 

4-“eighteen point three” 

5-“three to tower” 

6-“contact center eighteen point three” 

7-“tower eighteen three” 

8-“switching” 

Readback Errors Type (RTE) 

1 = Substitution of message numbers 

2 = Transposition of message numbers 

3 = One type of information read back as another type 

4 = Omission of (point/fix) 

5 = Omission of number element 

6 = Substitution of one (point/fix) with that of another 

7 = Omission of anchor word(s) 

8 = Substitution of anchor word(s)

ATC “AAL Ten via Victor nine J twenty eight ATL”

1-“via Victor five J twenty eight ATL” 

2-“via Victor nine J eighty two ATL” 

3-“speed two eighty” 

4-“Victor nine ATL” 

5-“Victor and J” 

6- “ATL nine J twenty eight” 

7- “nine and twenty eight” 

8-“to join the departure” 

Readback Errors Type (TRNSPNDR) 

1 = Substitution of message numbers 

2 = Transposition of message numbers 

3 = One type of information read back as another type 

4 = Not assigned 

5 = Omission of number element 

6 = Not assigned 

7 = Omission of anchor word(s) 

8 = Substitution of anchor word(s)

ATC “AAL Ten squawk two one two four”

1-“squawk four two one three” 

2-“squawk one two two four” 

3-“altimeter two one two four” 

5-“squawk one twenty four” 

7-“twenty four” 

8-

Readback Errors Type (ALTM) 

1 = Substitution of message numbers 

2 = Transposition of message numbers 

3 = One type of information read back as another type 

4 = Not assigned 

5 = Omission of number element 

6 = Not assigned 

7 = Omission of anchor word(s) 

8 = Substitution of anchor word(s)

ATC “AAL Ten Cleveland altimeter two nine nine two”

1-“altimeter nine two nine zero” 

2-“altimeter nine two two nine” 

3-“squawk two nine nine two” 
9

RESULTS

Routine ATC Communication

Presented in Table 4 are the number of transmissions, 

the duration of the communication samples, and the 

number of different aircraft for each TRACON facility 

and sector. A simple computation of the Approach total 

and Departure total values presented under the heading 

“Number of Aircraft” and “Duration of Communication 

Sample” revealed that, on average, one aircraft requested 

and received air traffic services every 1 min 26 s in the 

approach sectors and every 1 min 6 s in the departure sectors. The number of ground-to-air transmissions averaged 

7.25 messages per aircraft (Number of ATC Transmissions/Number of Aircraft) for approach control and 4.7 

messages per aircraft for departure control. From initial 

contact to the hand-off to the next controller in sequence, 

the entire transactional communication set involved the 

exchange of 13 messages, on average, between a controller 

and pilot (this includes all of the pilot transmissions to 

the controller) and an allocation of approximately 76 s 

of airtime (per aircraft).

Only controllers’ messages that contained instruction 

(e.g., heading, heading modification, altitude, altitude 

restriction, speed, approach, departure, radio frequency, 

route, position, or transponder aviation topics) or advisory 

(traffic, altimeter portion of a weather advisory) speech acts 

were selected for the computation of message complexity. 

Of the 14, 673 controller-to-pilot transmissions,12,148 

met the selection criteria — 89.8% instructions (10904 

messages), 5.8% advisories (704 messages), and 4.4% 

contained both (540 messages).

The 2,524 excluded transmissions involved aviation 

topics other than traffic and the altimeter portion of 

weather advisories (e.g., ATIS, general acknowledgment). 

Also excluded were requests (e.g., traffic, general sighting, 

type aircraft), courtesies (e.g., greeting, apology, thanks), 

and non-codable (e.g., delivery, equipment, other) transmissions. Neither the speaker nor receiver addresses were 

encoded. For a complete listing of aviation topics by 

speech act category, see Prinzo et al. (1995).

For approach control, Figure 1 shows that of the 10,957 

communication elements transmitted to pilots, the most 

frequently transmitted aviation topics involved headings (22%), speeds (21%), and altitudes (16%). Rarely 

transmitted were altimeter, heading modification, or 

transponder aviation topics (each were less than 1%). 

For departure control, controllers transmitted 6,665 

communication elements to pilots. The aviation topics most frequently transmitted were headings (31%), 

altitudes (28%), and radio frequency changes (20%). 

The most infrequent aviation topics involved altimeter 

(1%), altitude restriction (1%), and heading modification (less than 1%). Departure controllers would not 

Table 4. Number and Duration of Transmissions, Number of Aircraft, and Communication Duration 

Presented by ATC Sector and TRACON Facility 

Number of Transmissions 

Source ATC Flight 

Deck

Landline Total Number of 

Aircraft

Duration of 

Communication 

Sample

Approach 

Atlanta 1513 1580 104 3197 0219 05 hr 02 min 51 s 

Chicago 1730 1843 200 3773 0226 05 hr 03 min 58 s 

Dallas Ft Worth 1128 1231 168 2527 0247 05 hr 19 min 28 s 

New York 2860 2703 222 5785 0290 06 hr 47 min 55 s 

S. California 1350 1494 135 2979 0210 06 hr 01 min 11 s 

Approach Total 8581 8851 829 18261 1184 28 hr 13 min 23 s 

Departure 

Atlanta 1245 1249 281 2775 0239 04 hr 49 min 42 s 

Chicago 737 779 196 1712 0172 03 hr 12 min 37 s 

Dallas Ft Worth 1360 1374 272 3006 0253 05 hr 26 min 52 s 

New York 1190 1400 193 2783 0311 05 hr 13 min 32 s 

S. California 1560 1684 69 3313 0320 05 hr 13 min 49 s 

Departure Total 6092 6486 1011 13589 1295 23 hr 56 min 32 s 

Grand Total 14673 15337 1840 31850 2479 52 hr 09 min 55 s 
10

issue approach/departure clearances unless working a 

combined position, hence the absence of any of those 

aviation topics.

An examination of the frequency with which each 

type of aviation topic was transmitted shows interesting commonalities as well as differences. For example, 

regardless of the source of the transmission (i.e., ATC 

sector) altimeter, heading modification, and transponder 

information were transmitted infrequently. Approach and 

departure control messages involving traffic advisories 

and route/position were comparable in their frequency 

of occurrence. Departure control appeared to transmit 

more altitudes, headings, and radio frequencies; approach 

control transmitted more speeds. This finding is not surprising because there were more aircraft in the departure 

sample and more pilot requests for a repeat of the newly 

assigned radio frequency.

ATC Message Complexity

Table 5 shows the distribution of ATC messages 

by level of complexity. The majority of these messages (89.8%) contained instructions, 5.8% involved 

advisories, and 4.4% were a combination of instructions and advisories.

Unlike the findings reported by Cardosi et al. (1996) 

where 59% of the ATC messages involved one or two 

pieces of information, only 3.3% of the controller messages reported here did. Instead, when ATC messages 

involved only instructions, the typical complexity level 

varied from 4 (23.1%) to 7 (10.2%). That is, 55.7% of 

the controllers’ messages that contained only instructions 

had a complexity level that ranged between 4 and 7 pieces 

of to-be-remembered information. There did not seem to 

be a pattern in the frequency of occurrence for advisories 

or messages that combined instructions with advisories 

as a function of complexity level.

Pilot Responses to ATC Messages

In response to the 12,148 ATC messages, there were 

10,042 full readbacks, 967 partial readbacks, 489 acknowledgment only (e.g., ‘Roger,’ ‘Wilco’), 149 other 

replies (e.g., in response to a traffic advisory, the pilot 

said, “SO HOW ABOUT IF WE CLIMB UP A LITTLE 

BIT SO WE CAN GET ABOVE HIS WAKE”), 42 

courtesies such as ‘Thank you,’ and 457 messages with 

no acknowledgment. 

In addition to these messages, pilots initiated 88 follow-up transmissions of which 43% were in response to 

traffic advisories. That is, pilots whose initial response 

was “Looking” updated their sighting reports with follow-up transmissions such as, “HE’S FIVE HUNDRED 

FEET ABOVE US RIGHT NOW.” Of the remaining 

57% follow-up responses, many involved uncertainty 

regarding previous ATC instructions. They included 

transmissions such as “CONFIRM THE HEADING,” 

“VERIFY ONE THREE THOUSAND,” and “SAY 

TOWER FREQUENCY AGAIN.”

As shown in Table 6, pilots provided either full (82.7%) 

or partial (7.9%) readbacks to controller instructions, 

advisories, or both. In Cardosi et al.’s 1996 report, full 

readbacks occurred for 60% of the previously issued 

ATC messages. The data presented here indicate a 22.7% 

1%

5%

4%

8%

20%

0%

31%

0%

1%

28%

1%

0%

6%

21%

7%

11%

0%

22%

8%

7%

16%

1%

Transponder

Traffic

Speed

Route/Position

Radio Frequency

Heading Modification

Heading

Approach/Departure

Altitude Restriction

Altitude

Altimeter

Approach

Departure

Figure 1. Percentages of ATC Aviation Topics Transmitted to Pilots 
11

increase in full readbacks with a corresponding decrease 

in partial readbacks — down from 26% in the Cardosi 

et al. report to 7.9%. We took the category ‘Other Replies’ that constituted another 7% of pilot responses in 

the Cardosi et al. report and split it into ‘Other Replies’ 

and ‘Courtesy.’ Together, they accounted for 1.6% of 

the pilot responses. Approximately 3.8% of the messages 

were not acknowledged.

These finding are particularly remarkable for lengthy 

controller transmissions. For example, in response to the 

ATC transmission, “OWNSHIP FIFTY SIX HEAVY 

TURN LEFT HEADING THREE ZERO ZERO 

YOU’RE NINE MILES FROM ANVAL MAINTAIN 

THREE THOUSAND FIVE HUNDRED ‘TIL ANVAL CLEARED FOR THE ILS TWO SEVEN LEFT 

APPROACH SPEED ONE EIGHT ZERO WILL BE 

FINE,” the pilot read back, “OWNSHIP FIFTY SIX 

HEAVY LEFT THREE HUNDRED CLEARED ILS 

TWO SEVEN LEFT THIRTY FIVE HUNDRED 

‘TIL ANVAL AND ONE EIGHTY SPEED.” The 

controller’s transmission had a complexity value = 20. 

Another example is the following pilot readback, “ONE 

EIGHTY TO THE MARKER TWO NINETY ON 

THE HEADING THIRTY FIVE HUNDRED 

CLEARED FOR THE APPROACH TWO FORTY 

EIGHT” in response to the controller’s transmission, 

“OWNSHIP TWO FORTY EIGHT TURN LEFT 

HEADING TWO NINER ZERO FOUR FROM 

ANVAL CROSS ANVAL AT THREE THOUSAND 

FIVE HUNDRED CLEARED ILS RUNWAY TWO 

SEVEN LEFT APPROACH MAINTAIN SPEED ONE 

EIGHT ZERO TO THE MARKER.” The controller’s 

transmission had a complexity value = 23.

Of the 457 ATC messages that received no pilot 

acknowledgment, 86.0% involved messages having 

one (67.2%), two (16.0%), or more than two (2.8%) 

instructions, while another 9.4% concerned single-topic 

advisories for traffic (7.2%) or altimeter (2.2%) settings. 

The remaining 4.6% unacknowledged messages were a 

combination of instructions and advisories that contained 

Table 5. Percentage of Controller Messages as a Function of Level of Complexity 

Types of ATC Messages 

Level of 

Complexity 

Instructions 

Only Advisories Only 

Instructions and 

Advisories

Percent of all 

Messages 

1 00.1% 000.1% 

2 03.0% .2% 003.2% 

3 07.7% .5% 008.1% 

4 23.1% .9% 024.0% 

5 11.5% .3% 0.1% 012.0% 

6 10.9% .2% 0.3% 011.4% 

7 10.2% .5% 0.5% 011.2% 

8 05.4% .9% 0.4% 006.7% 

9 04.7% .8% 0.3% 005.8% 

10 03.6% .9% 0.5% 005.0% 

11 03.5% .4% 0.5% 004.3% 

12 01.7% .1% 0.5% 002.3% 

13 01.2% .0% 0.3% 001.5% 

14 00.7% .1% 0.3% 001.1% 

15 00.6% .0% 0.3% 000.9% 

16 00.5% .0% 0.1% 000.6% 

17 00.7% .0% 0.1% 000.8% 

18 00.2% .0% 0.1% 000.3% 

19 00.2% .0% 0.1% 000.3% 

20 or more 00.2% .0% 0.0% 000.2% 

Table Total 89.8% 5.8% 4.4% 100.0% 
12

two (1.1%) or more than two topics (3.5%). Of the 

67.2% unacknowledged single-topic instructions, 29.5% 

involved changes in radio frequency, 15.3% pertained 

to heading, 9.4% to altitude, and 6.3% to speed assignments. Transponder (3.5%), route/position (2.2%), and 

altitude restriction (0.9%) comprised the remainder of 

unacknowledged single-topic instructions.

Use of Call Sign in Readbacks. The types of call signs 

used by pilots and their representative examples are shown 

in Figure 2. In Table 7, the frequency distributions of the 

usage of the various types of call signs are presented by 

their rate of occurrence as a function of pilot responses. 

There were 11,806 ATC messages in this sample. A more 

comprehensive analysis of call sign disparities is presented 

later in the report.

The data presented in Table 7 indicate that pilots 

provided either the full (69.9%) or partial (22.1%) call 

sign in 92% of their responses. Call signs were excluded 

in 7.6% of their responses, and 0.1% of the spoken call 

signs were unintelligible. Incorrect call signs constituted 

0.3% of their responses. 

There were 39 transmissions where pilots provided 

incorrect call signs (replacement of the assigned call sign 

with that of another). In 28 of these transmissions, the 

incorrect call signs resulted from importing numbers or 

letters not found in the actual call sign. For example, the 

pilot of Ownship 672 responded to an ATC transmission 

with, “Ownship sixseven zero.” In 7 other transmissions, pilots either omitted some numbers (Ownship 

719 was called ‘Ownship seven nine’), letters (‘H’ 

Table 6. Pilot Responses to ATC Messages 

Types of ATC Messages 

Types of Pilot Response 

Instructions 

Only

Advisories

Only

Instructions and 

Advisories

Percent of all 

Messages 

Full Readback 77.1% 4.4% 1.2% 082.7% 

Partial Readback 05.2% 0.0% 2.7% 007.9% 

Acknowledgment Only 02.8% 0.9% 0.3% 004.0% 

Other Replies 01.1% 0.1% 0.0% 001.2% 

Courtesy 00.3% 0.1% 0.0% 000.4% 

No Acknowledgment 03.3% 0.3% 0.2% 003.8% 

Table Total 89.8% 5.8% 4.4% 100.0% 

Call Sign Usage ACID Example 

 Complete UAL56H UNITED FIFTY SIX HEAVY LEFT THREE SIX ZERO 

 Partial 

Prefix w/ some 

numbers/letters 

DAL884 DELTA EIGHTY FOUR THREE SIXTY HEADING WE'RE 

SLOWING

Inc. prefix w/ all 

numbers/letters 

ACA1017 TWO SIX TO JOIN TWENTY TWO RIGHT LOCALIZER CANADA 

TEN UH SEVENTEEN 

No prefix w/ all 

numbers/letters 

TRS467 NINETEEN ONE FOUR SIXTY SEVEN 

No prefix w/ some 

numbers/letters 

GWY256 FIFTY SIX LOOKING 

 Incorrect call sign N21828CG EIGHTEEN SEVENTEEN TWO CHARLIE GOLF 

 Unintelligible AAL538 DOWN TO SIX AMER (UNINTELLIGIBLE) 

 No call sign HEADING TWO NINER ZERO ONE SIXTY KNOTS FOLLOW THE 

ATR CLEARED FOR APPROACH TWO SEVEN LEFT 

Figure 2. Examples of Various Types of Pilot Call Sign Usage 
13

for heavy as in ‘Ownship four twenty five

heavy’), or both (Ownship1401AL was called ‘ONE 

FOUR ONE ALPHA’). There were three transmissions 

where the pilot transposed some of the numbers in the 

call sign (e.g., N8453G was referred to as ‘Five Gulf’). 

Finally, in one transmission the pilot used the wrong 

company name with the correct flight number.

Miscommunications

Radio frequency congestion (especially during periods 

of heavy traffic) is a well-documented problem affecting 

communication efficiency (FAA 1995). Following the 

delivery of an ATC transmission, the controller listens 

for the pilot to accurately read back the original message. 

The presence of a mistake is called a readback error. Pilot 

readbacks that contain the correct information but are 

not phrased properly are not readback errors.

The results presented here examined the prevalence of 

pilot readback errors and requests for ATC to repeat all 

or part of a previous transmission as a function of ATC 

message complexity and message length (as determined 

by counting the number of aviation topics in the transmission) — excluding Address/Addressee and Courtesies. 

They were derived from 11,159 ATC transmissions. Each 

ATC transmission that met the selection criterion (i.e., 

it contained an instruction, advisory, or a combination 

of instruction and advisory speech acts) was paired with 

the pilot’s response to that message. Each pilot readback 

was evaluated for accuracy, and the number of errors 

present was recorded (e.g., a zero indicated no error 

while a value of 3 indicated 3 errors). There were 723 

individual readback errors present in 688 pilot transmissions — approximately 6% of the pilots’ readbacks 

contained a readback error. Pearson correlations revealed 

that readback errors increased significantly as the complexity, r(11159)=.196 and message length (i.e., number of 

aviation topics), r(11159)=.180 in a controller’s message 

increased, p<.05. Likewise, albeit to a lesser degree, the 

number of pilot requests increased significantly with 

message complexity, r(11159)=.020, and message length, 

r(11159)=.054, p<.05.

Message Complexity. Table 8 shows that 10,471 

messages resulted in no readback errors — 93.8% of 

the pilots’ readbacks were correct. For the 6.2% faulty 

pilot readbacks, 654 contained 1 error and another 34 

contained 2 or more errors.3

ATC messages with complexity values of 10 or greater 

were more difficult for pilots to read back correctly, as 

evidenced by the presence of 2 or more errors per readback. In fact, the percentage of readback errors reached 

double-digit status once the threshold of 10 was crossed. 

Prior to reaching a complexity value of 10, the percentage 

of readback errors was fairly stable — ranging from as 

little as 2.28% (62/2718) to 6.14% (41/668). Message 

complexity values between 11 and 13 resulted in an 

increase in readback errors from 10.84% to 19.16%, 

while complexity values that exceeded a value of 16 had 

an error rate that approached 38%.

3 Applying a liberal scoring criterion (i.e., partial readback of some 

numbers in a heading, speed, altitude, or radio frequency and excluding 

some anchor words such as fixes or points not counted) resulted in 

1.3% readback errors. 

Table 7. Pilot call sign usage as a function of the type of pilot response 

Type of Pilot Response 

Pilot Call Sign 

Usage 

Full

Readback 

Partial 

Readback 

Ackn. 

Only

Other 

Replies Courtesy 

Followup Percent

 Complete 61.1% 5.8% 1.9% .6% .0% .5% 69.9%

 Partial 

Prefix w/ some 

numbers/letters .3% .1% .0% .0% .0% .0% .4%

Inc. prefix w/ all 

numbers/letters 3.7% .2% .1% .0% .0% .0% 4.0%

No prefix w/ all 

numbers/letters 13.4% 1.3% .9% .2% .0% .2% 16.0%

No prefix w/ some 

numbers/letters 1.4% .1% .2% .0% .0% .0% 1.7%

 Incorrect call sign .3% .0% .0% .0% .0% .0% .3%

 Unintelligible .1% .0% .0% .0% .0% .0% .1%

 No call sign 5.0% .7% 1.0% .5% .3% .1% 7.6%

Table Total 85.3% 8.2% 4.1% 1.3% .3% .8% 100.0%
14

Each ATC message was classified as either low (< 09) 

or high (≥10) complexity. Each pilot transmission had 

a readback value, and the average of those values was 

computed for each aircraft. An ATC Sector (Approach, 

Departure) by Message Complexity (Low, High) Analysis 

of Variance (ANOVA) was conducted on pilot readback 

performance. The results, evaluated using a criterion 

level set to p< .05, revealed that pilots produced more 

errors while in an approach (Mean = .126 SD = .304) 

compared with a departure (Mean = .038 SD = .153) 

sector [F(1,3700) = 129.00]. Also, more complex ATC 

messages had a higher incidence of being read back incorrectly (Mean = .172 SD = .375) than messages that 

were less complex (Mean = .038 SD = .117), [F(1,3700) 

= 154.39]. However, these statistically significant main 

effects must be qualified by the presence of a statistically 

significant ATC Sector by Message Complexity interaction 

[F(1,3700) = 97.18], which is presented in Figure 3.

The Tukey Honestly Significant Difference (HSD) 

statistic revealed that pilots experienced more difficulty 

reading back approach control high-complexity messages 

than reading back departure control high-complexity messages or low-complexity messages from either approach 

or departure control.

Message Length. As shown in Table 9, very short 

messages containing only one aviation topic occurred for 

54.2% of the transmissions, and they resulted in 3.84% 

readback errors (232/6049). Messages with 4 aviation 

topics appeared in 5.2% of the transmissions, producing 

25.69% readback errors. Once again, pilot mean readback 

performance scores were computed for each aircraft call 

sign. The results of the ATC Sector (Approach, Departure) by Message Length (1AT, 2AT, 3AT, 4AT) ANOVA 

revealed that more readback errors occurred when pilots 

were in the approach (Mean = .113 SD = .307), as compared with the departure (Mean = .0343 SD = .157) 

Table 8. Distribution of pilot readback errors as a function of ATC message type and complexity 

Type of Message 

Instructions Advisories Combination 

Number of Readback Errors 

ATC

Message 

Complexity 0 1

2 or 

more 0 1

2 or 

more 0 1

2 or 

more

Percentage of 

Readback 

Errors

1 0006 000 000 0 000 00 00.00% 

2 0279 012 003 0 000 00 04.08% 

3 0773 049 034 1 000 00 05.83% 

4 2583 061 070 1 003 00 02.28% 

5 1260 049 026 0 014 00 03.63% 

6 1158 054 024 0 027 02 04.43% 

7 1074 063 049 0 043 04 05.43% 

8 0590 033 093 0 039 05 05.00% 

9 0505 039 088 0 034 02 06.14% 

10 0384 034 04 101 0 0 052 02 0 06.93% 

11 0357 047 01 038 0 0 049 06 0 010.84% 

12 0158 024 04 014 0 0 054 03 0 012.06% 

13 0103 028 03 003 0 0 029 01 0 019.16% 

14 0059 022 02 003 0 0 031 04 0 023.77% 

15 0054 020 02 002 0 0 026 02 0 022.64% 

16 0030 019 04 000 0 0 012 02 0 037.31% 

17 0054 027 03 001 0 0 012 02 0 032.32% 

18 0014 010 03 001 0 0 009 01 0 036.84% 

19 0015 009 01 002 0 0 006 02 0 034.29% 

20 or more 0020 013 06 002 0 0 001 01 0 050.00% 

Total 9476 613 34 554 2 0 441 39 0
15

0

0.05

0.1

0.15

0.2

0.25

High Low

Level of Complexity

Mean Readback Errors

APPROACH

DEPARTURE

Figure 3. Mean Pilot Readback Errors Presented by ATC Sector and 

Message Complexity 

Table 9. Distribution of pilot readback errors as a function of ATC message type and length 

Type of Message 

Instructions Advisories Combination 

Number of Readback Errors 

ATC

Message 

Length 0 1

2 or 

more 0 1

2 or 

more 0 1

2 or 

more

Percentage of 

Readback 

Errors

1 5379 230 438 2 0 0 03.84% 

2 2755 177 10 99 0 199 12 06.12% 

3 996 86 4 17 0 160 19 08.50% 

4 346 120 20 0 0 82 8 25.69% 

Total 9476 613 34 554 2 441 39

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

1 2 3 4

Message Length

Mean Readback Errors

APPROACH

DEPARTURE

Figure 4. Mean Pilot Readback Errors Presented by ATC Sector 

and Message Length 
16

sectors [F(1,5599) = 78.48]. As expected, the number 

of readback errors varied with the number of aviation 

topics [F(3,5599) = 21.62]. Tukey HSD comparisons 

revealed that the fewest readback errors occurred when 

ATC messages contained one aviation topic (Mean = .036 

SD = .139). There was no reliable difference between 

messages with 2 or 3 aviation topics (2AT = .062 SD = 

.214; 3AT = .082 SD = .258). However, messages with 

4 aviation topics contained the most readback errors 

(Mean = .30 SD = .513). These main effects are qualified by a statistically significant ATC sector by message 

length interaction.

Figure 4 shows that as approach control messages 

increased from one aviation topic to between 2 and 3 

topics and 4 aviation topics, that the mean number of 

pilot readback errors increased accordingly. The effect 

of message length is apparent only for approach control. 

There was no discernible difference between readback 

performance for approach and departure sectors for one 

aviation topic.

Readback Errors and Aviation Topic. Table 10 presents 

the distribution of readback errors according to the types 

of aviation topics read back incorrectly. Column (c) shows 

that 33% of the 723 identified readback errors involved 

speed instructions. Like the Cardosi et al. findings, there 

were proportionally more heading errors than radio frequency errors and proportionally fewer readback errors 

that involved altitude instructions. Route/position, approach/departure, altimeter, and transponder instructions 

captured the remaining 6.77% readback errors. 

The results presented in Column (c) of Table 10, 

although interesting in demonstrating the overall composition of readback errors, fail to take into account the 

frequency of delivery of those instructions by controllers. 

There may be more opportunities to incorrectly read back 

a speed instruction simply because controllers issue them 

more often. Therefore, another analysis was performed 

that compared the number of readback errors of a particular aviation topic (e.g., speed) to the total number 

readbacks of that aviation topic. Column (d) shows that, 

when the number of readback errors is examined in 

conjunction with the number of actual pilot readbacks 

produced in Column (a), then reading back the content 

of an altitude restriction seems to posit greater difficultly 

than reading back the elements comprising a heading 

instruction, as well as any of the other aviation topics. In 

fact, there were 7.68 times more attempts at reading back 

headings than altitude restrictions (4176/544). 

Presented in Table 11 is the distribution of type of 

readback errors categorized by aviation topic. Readback 

errors fall within three major classifications — omission 

(63.76%), substitution (33.61%), and transposition 

(2.63%). The distribution of error classes differed across 

aviation topic. For instance, of the 18.95% omission of 

anchor word(s), 12.45% involved heading (e.g., “eight 

zero”); almost half (11.20% of the 24.62%) of omission 

of number element(s) concerned speed (e.g., “eighty on 

the speed,” “eighty knots”); and over two-thirds of the 

omission of point/fix related to speed (e.g., in response 

to “... maintain speed one eight zero to depot,” the pilot 

readback “I’ll keep one eighty speed”).

Substitution of anchor word(s) and substitution of 

number element(s) represented nearly three-fourths of 

the 7 types of substitution errors. Substitution of anchor 

word(s) was more likely to involve altitude restrictions and 

speed assignments than headings or approach clearances. 

Similarly, substitution of number element(s) was more 

likely to involve radio frequency, followed by heading and 

Table 10. Distribution of pilot readback errors by type of information 

Type of Aviation Topic 

Number of 

Readbacks

(a)

Number of 

Readback

Errors

(b)

Proportion of 

Readback

Errors

(c)

Percentage of 

Readbacks in 

Error

(d)

Altimeter 0092 003 00.41 % 03.26 % 

Altitude 3390 040 05.53 % 01.18 % 

Altitude restriction 0544 101 13.97 % 18.57 % 

Approach/Departure 00843 022 03.04 % 02.61 % 

Heading 4176 164 22.68 % 03.93 % 

Radio frequency 2115 130 17.98 % 06.15 % 

Route/Position 1082 023 03.18 % 02.13 % 

Speed 2264 239 33.06 % 10.56 % 

Transponder 0040 001 00.14 % 02.50 % 

Total 14546 723 100.00% 03.26 % 
17

Table 11. Distribution of the types of pilot readback errors according to the affected aviation topic 

Type of Aviation Topic 

Type of Readback Error Altm Alt 

Alt

Rstr

App/

Dpt Freq Hdg 

Rte/

Pos Spd Sqwk Percent 

Omission of anchor word(s) n = 137 1.38% 1.24% 00.41% 12.45% 0.14% 03.32% 0018.95%Omission of contact location n = 46 06.36% 006.36%Omission of number element(s) n = 178 0.28% 3.60% 0.14% 0.55% 04.98% 03.73% 11.20% 0.14% 024.62%Omission of (point/fix) n = 100 2.35% 11.48% 013.83%Substitution of anchor word(s) n = 50 3.04% 0.14% 00.69% 1.38% 01.66% 006.92%Substitution of number element(s) n = 128 0.14% 1.80% 1.38% 05.81% 04.43% 1.24% 02.90% 017.70%Substitution of one aviation topic with another type n = 56 0.14% 5.12% 0.28% 00.28% 00.97% 0.28% 00.69% 007.75%Substitution of one contact location with another n = 1 00.14% 000.14%Substitution of one direction with that of another (left/right) n = 3 00.41% 000.41%Substitution of one type of approach with another type n = 3 0.41% 000.41%Substitution of runway numbers, left/right/center n = 2 0.28% 000.28%Transposition of number element(s) n = 1 00.14% 000.14%Transposition of one (point/fix) with another n = 18 0.55% 0.14% 0.14% 01.66% 002.49%Percent n = 723 0.41% 5.53% 13.97% 3.04% 17.98% 22.68% 3.18% 33.06% 0.14% 100.00% 
18

speed instructions. The combination of altitude instructions with altitude restrictions accounted for about 18% 

of the readback errors involving substitution of number 

element(s).

Transposition readback errors involved reordering 

the number element(s) or point/fix. About 95% of the 

transposition errors involved reversing the order of one 

point/fix with another.

Hearback Errors. While a pilot’s inaccurate readback 

of a message is called a readback error, a controller’s failure 

to notify a pilot of a readback error is called a hearback 

error. As noted previously, readback errors are rare events. 

Of the 12,148 pilot transmissions that comprised this 

database, 688 contained faulty read backs —about 1 in 

every 18 pilot transmissions. Table 12 shows that the 

majority of these faulty readback errors were not corrected by ATC.

ATC Corrected Readback Errors. Table 13 displays the 

corrected readback errors according to error classification 

and aviation topic. Of the corrected readbacks, 13.80 % 

involved omission, 79.31% involved substitution, and 

6.90% involved transposition errors. It may be that some 

types of readback errors are more critical than others. A 

reexamination of the corrected readback errors was performed to compare the opportunity to correct an error 

with the actual number of corrections made. The findings 

show that only 1.74% of all the omission errors (8/61), 

18.83% of the substitution errors (46/243), and 21.05% 

of the transposition errors (4/19) were corrected.

Pilot Requests for Repeat of Part or all of the Transmission. There are times when pilots are busy setting-up 

for the approach, completing checklists, or performing 

other station-keepings tasks, they hear, or think they hear, 

their aircraft’s call sign on the communications system. 

Uncertain of the accuracy of an attempted readback, they 

may request a repeat of all (say again) or part (what was 

that heading again?) of the message. In other instances, 

they may request confirmation of the aviation topics that 

they thought they heard (confirm we’re cleared down to 

five thousand). 

An examination of the data revealed 133 messages 

where pilots asked controllers to repeat earlier information 

in either the form of a request (45.1%) or confirmation 

(54.9%). Of the 60 requests made, 18.3% were for a 

full repeat, 78.4% a partial repeat, and 3.3% asked the 

controller to identify the recipient of the message (who 

was that for?). As shown in Figure 5, radio frequency 

(38%) and heading (17%) assignments were more frequent partial “say agains” than altitude (5%) and route 

(5%) assignments.

There were 73 pilot requests for confirmation — 4.1% 

for a full transmission, 65.8% for a specific aviation topic, 

and 30.1% for the recipient of the message (was that for 

me?). Figure 6 shows that 23.0% of the confirmations were 

for headings and 16.0% were for altitude assignments.

Radio Communications Phraseology and Techniques

Presented in this section of the report are the results 

from the voice tapes for pilot report of altitude information, call sign discrepancies, wrong aircraft accepting a 

clearance, and coincident factors.

Pilot Report of Altitude Information During Initial 

Contact.There were 1,980 pilot reports of altitude information upon initial contact made by domestic and foreign 

air carrier and cargo pilots (87.5%), of which 24.8% of 

the pilots reported their assigned attitude only, 64.9% 

reported both the altitude leaving and altitude assigned, 

5.0% reported only the altitude leaving, and 5.3% did 

not include any altitude report. Of the 282 pilot reports 

of altitude information made by general aviation pilots 

(12.5%), 51.7% reported only their assigned altitude, 

29.8% included both the altitude leaving and altitude 

Table 12. Percentage of hearback errors by aviation topic 

Type of Aviation Topic Number of 

Readback Errors 

Number of 

Hearback Errors 

Percentage of 

Hearback Error 

Altimeter 003 003 100.00% 

Altitude 040 034 85.00% 

Altitude restriction 101 097 96.04% 

Approach/Departure 022 018 81.82% 

Heading 164 153 93.29% 

Radio frequency 130 112 86.15% 

Route/Position 023 018 78.26% 

Speed 239 229 95.82% 

Transponder 001 001 100.00% 

Total 723 665 
19

Table 13. Distribution of the types of controller corrected readback errors according to the affected aviation topic 

Type of Aviation Topic 

Type of Corrected Readback Error (corrected/total readback errors) Alt

Alt

Rstr Altm

App/

Dpt Freq Hdg 

Rte/

Pos Spd Sqwk Percent 

Omission of anchor word(s) n = 2/137 0.00% 0.00% 0.00% 03.45% 0.00% 0.00% 003.45%Omission of contact location n = 1/46 01.72% 001.72%Omission of number element(s) n =4/178 1.72% 0.00% 0.00% 1.72% 0.00% 0.00% 03.45% 0.00% 006.90%Omission of (point/fix) n = 1/100 0.00% 01.72% 001.72%Substitution of anchor word(s) n = 2/50 1.72% 0.00% 01.72% 0.00% 0.00% 003.45%Substitution of number element(s) n = 38/128 6.90% 3.45% 0.00% 29.31% 10.34% 06.90% 08.62% 065.52%Substitution of one aviation topic with another type n = 3/56 1.72% 0.00% 1.72% 0.00% 01.72% 0.00% 0.00% 005.17%Substitution of one contact location with another n = 0/1 0.00% 000.00%Substitution of one direction with that of another (left/right) n = 1/3 01.72% 001.72%Substitution of one type of approach with another type n = 0/3 0.00% 000.00%Substitution of runway numbers, left/right/center n = 2/2 3.45% 003.45%Transposition of number element(s) n = 1/1 01.72% 001.72%Transposition of one (point/fix) with another n = 3/18 1.72% 01.72% 01.72% 005.17%Percent n = 58/723 10.34% 6.90% 0.00% 6.90% 31.03% 18.97% 8.62% 17.24% 0.00% 100.00% 
20

Figure 5. Requests for repetition

Figure 6. Requests for clarification 
21

assigned, 4.3% provided the altitude leaving and 14.2% 

reported no altitude information.

Pilot Responses to Altitude Clearances. Once initial 

contact is established, controllers instruct pilots to climb 

and maintain, descend and maintain, or maintain the 

aircraft’s current altitude. There were 1,911 pilot readbacks 

of their first altitude assignment following radar contact 

and 1,320 readbacks of all subsequent transmissions 

with new altitude assignments. Among the domestic and 

foreign air carrier and cargo pilots, 93.5% reported their 

assigned altitude only, 5.9% reported both the altitude 

leaving and altitude assigned, 0.1% reported only the 

altitude leaving, and 0.6% did not include any altitude 

report. Likewise, 86.8% of the general aviation pilots 

reported only the assigned altitude, 10.1% included 

both the altitude leaving and altitude assigned, and 3.1% 

reported no altitude information. 

Pilot readback of the remaining ATC transmissions 

with new altitudes followed the same pattern as initial 

contact and first readbacks. Once again, 93.8% of the 

domestic and foreign air carrier and cargo pilots reported 

their assigned altitude only, 5.0% reported both the 

altitude leaving and altitude assigned, 0.1% reported 

only the altitude leaving, and 1.1% did not include any 

altitude report. Similarly, 90.2% of the general aviation 

pilots reported only their assigned altitude, 6.5% included 

both the altitude leaving and altitude assigned, and 3.3% 

reported no altitude information.

Altimeter Settings. There were 143 advisories issued 

by controllers that included the current altimeter setting, 

of which 90 of the readbacks contained 2 or more digits 

(13.3% 2 digits, 18.9% 3 digits, 67.8% four digits). Some 

pilots excluded the altimeter portion (14.7%) in their readbacks when ATC messages contained both the altimeter 

and instructions. Acknowledgments (10.5%), courtesies 

(1.4%), requests for repeat or a query (2.8%), incorrect 

readbacks (2.1%), and no response from the pilots (7.7%) 

made up the remainder of the transmissions. 

Call Sign Discrepancies. For this set of analyses, a 

detailed examination was performed of call sign usage 

for 28,671 of the 29,640 transmissions (969 transmissions were excluded since their contents were limited to 

courtesies or salutations). Unlike the analysis performed 

earlier that included only instructions and advisories, for 

this series all transmissions between pilots and controllers 

were examined. Of these transmissions, 76.7% contained 

the complete call sign, and 2.3% included an abbreviated 

call sign after communications were established.

 Taking 

into account rounding error, call sign exclusions (6%), 

unintelligible call signs (.3%), incomplete call signs (14%) 

and other types of call signs, such as substitutions (.7%) 

and transpositions (.1%), made up the remainder. 

The distribution of misspoken call signs according to 

type of aircraft (e.g., air carrier, general aviation), source 

(ATC, flight deck), and whether or not it was corrected 

is presented in Table 14. Approximately 80% (138/173) 

of the misspoken call signs involved communication 

exchanges between ATC and the air carrier flight deck 

with the remaining 20% (35/173) attributed to communications between controllers and general aviation 

pilots. Approximately 88% (121/138) of the air carrier 

and 94% (33/35) of the general aviation misspoken call 

signs were uncorrected.

Presented in Table 15 is a distribution of the outcome 

of misspoken call signs according to the speaker of the 

transmission. Roughly 87% (76/87) of the controllers’ 

disparities and 88% of the pilots’ (76/86) were substitution 

errors followed by transpositions errors — about 13% 

4 §4-2-4. Aircraft Call Signs. Pilots, therefore, must be certain that 

aircraft identification is complete and clearly identified before taking 

action on an ATC clearance. ATC specialists will not abbreviate 

call signs of air carrier or other civil aircraft having authorized call 

signs. ATC specialists may initiate abbreviated call signs of other 

aircraft by using the prefix and the last three digits/letters of the aircraft 

identification after communications are established. The pilot may 

use the abbreviated call sign in subsequent contacts with the ATC 

specialist. When aware of similar/identical call signs, ATC specialists 

will take action to minimize errors by emphasizing certain numbers/

letters, by repeating the entire call sign, by repeating the prefix, or 

by asking pilots to use a different call sign temporarily. Pilots should 

use the phrase “VERIFY CLEARANCE FOR (your complete call 

sign)” if doubt exists concerning proper identity. Also see §2-4-20. 

AIRCRAFT IDENTIFICATION for examples.

Table 14. Distribution of corrected and uncorrected misspoken call signs by source 

Misspoken Call signs 

Source Uncorrected Corrected Total 

Domestic, Foreign and Cargo Air Carrier 

ATC 51 12 63

Flight deck 70 5 75

Total 121 17 138 

General Aviation 

ATC 22 2 24

Flight deck 11 0 11

Total 33 2 35
22

(11/87) for controllers and 7% (6/86) for pilots. About 

5% (4/86) of the pilots’ messages involved releasing the 

mic key before the end of the transmission, resulting in 

the omission of the final portion of the aircraft’s call sign. 

(This was determined by visual and auditory examination 

of the waveform using Adobe AuditionTM software).

Approximately 10% (9/87) of the controllers’ misspoken call signs were detected by pilots and 6% by the 

controllers. Controllers either retransmitted the erroneous 

call sign when no readback followed their transmission 

(3%) or, upon self-discovery, they retransmitted the message with the correct call sign (2%).

A detailed analysis of the substitution errors revealed 

that for controllers, 74% (56/76) of their misspoken call 

signs involved replacing some numbers or letters with 

others (e.g., COMAIR855 replaced with COMAIR355), 

while another 21% (16/76) concerned exchanging one 

prefix for that belonging to another aircraft (e.g., AMERICAN for UNITED, DELTA for AMERICAN, JETLINK 

for EXECJET), and 5% (4/76) related to truncating 

of flight numbers (e.g., Ownship 422H was spoken as 

“OWNSHIP FOUR TWENTY HEAVY”).

Approximately 64% (49/76) of the substitution errors made by pilots involved numbers and letters. They 

also rounded the ending numbers in the call sign (28%; 

21/76), as well as the numbers at the beginning (5%; 

4/76) (e.g., Ownship1693H spoken as “OWNSHIP 

SIXTEEN HEAVY”). The smallest percentage of misspoken call signs involved the substitution of ownship’s 

prefix with that of another (3%, 2/76).

Approximately 48% of the controllers’ misspoken call 

signs involved transmissions with one instruction that 

included a heading (41%), altitude (19%), speed (12%), 

frequency (12%), route/position (7%), transponder setting (7%), or approach/departure (2%) aviation topics. 

About 25.3% had 2 instructions that included a combination of altitude (25%), heading (23%), speed (20%), 

frequency (14%), route/position (14%), approach/departure (2%), or transponder (2%) aviation topics. Both 

of the transmissions that contained 3 instructions 

(2.3%) involved approach clearances that provided 

a combination of route/position, speed, or heading 

aviation topics. In addition to providing an approach 

clearance, the transmission with 4 instructions (1.1%) 

also included a route/position, heading, and switch in 

tower frequency. No instructions were included in 23% 

of the controllers’ transmissions.

For pilots, 41% of their transmissions that contained a 

misspoken call sign involved the readback of one heading 

(37%), altitude (31%), frequency (20%), speed (9%), 

or transponder setting (3%). For transmissions with 2 

instructions (26%), their readbacks were a composition 

of heading (39%), route/position (23%), altitude (18%), 

approach/departure (9%), speed (9%), and frequency 

assignment (2%). The remaining six readbacks (7%) 

had 3 instructions that combined aviation topics such as 

heading (17%), altitude (17%), speed (17%), approach/

departure (17%), altitude restriction (11%), frequency 

(11%), route/position (6%), and transponder setting 

(6%). Approximately 3% of the transmissions did not 

include a readback to controller instructions, and 27% 

involved replies to transmissions that did not include 

instructions.

Wrong Aircraft Accepting a Clearance. As with the 

Cardosi et al. report that identified 7 instances of a stolen 

transmission, a thorough examination of the 50-plus hr 

of communication found four events that involved the 

wrong aircraft accepting a transmission meant for a different aircraft, and none of them involved anything less 

than the use of either the full or abbreviated call sign.

The first event involved the same airline but different 

flight identifiers. Both aircraft were on approach to the 

point where each was expecting a radio frequency assignment switching them over to the tower. The second 

event also involved the same airline but different flight 

identifiers. This time the aircraft were on departure and 

expecting a hand off to the next departure sector. When the 

radio frequency assignment was given, the wrong aircraft 

took the frequency. The controller could not correct the 

problem since the pilot had already switched to the radio 

frequency assignment intended for the other company 

aircraft. In the third event, two aircraft were establishing 

radar contact in rapid succession. Each aircraft was flying 

for a different airline, but they had the same beginning 

and ending numbers as part of their flight identification 

— two-eighty-three and two-fifty-three. The controller 

issued an altitude assignment upon radar contact with 

Ownship two-eighty-three but Othership two-fifty-three 

acknowledged it. The controller detected the problem and 

corrected it immediately. Unlike the other three events, 

the fourth one did not involve call sign similarities. Ownship was instructed to contact the center controller on a 

prescribed radio frequency. Before the pilot could reply, 

the departure controller issued an instruction to Othership but Ownship provided the readback. Once again, the 

controller immediately detected the error and informed 

Ownship, thereby preventing a potential problem.

Coincident Factors to Miscommunications. In this 

final analysis, transmissions that contained one or more 

faulty readbacks were examined for the presence of factors that might be correlated with, or have contributed 

to, its occurrence. Coincident factors included clipped/

abbreviated transmissions, nonstandard phraseology, pilot 

expectation, language barriers, and transmission overlap 

(stepped-on, blocked transmissions). 
23

Table 15. Distribution of outcomes of misspoken call signs according to source 

Misspoken Call Signs 

Source Uncorrected Corrected Total 

Controller 

Transpose numbers 

Response by intended aircraft 9 9

ATC retransmits when no response 2 2

Substitution 

Response by intended aircraft 49 0 49

ATC retransmits when no response 6 3 9

Pilot wants to know the intended receiver 0 6 6

No reply 7 0 7

ATC self-corrects on next message 0 2 2

Detected by Pilot 0 3 3

Total 73 14 87

Pilot 

Transpose numbers 

Response by intended aircraft 5 5

No reply 1 1

Substitution 

Response to intended aircraft 59 1 60

No reply 1 0 1

Detected by ATC 0 4 4

Initial call-up 11 0 11

Clipped 

Response by intended aircraft 4 4

Total 81 5 86

 
24

There were 207 pilot readbacks that began with an 

abbreviated speech act (e.g., “THIRT Y HEADING,” 

“EIGHTY SPEED,” “ONE ZERO FOUR THOUSAND”) that may have resulted from poor microphone 

technique, poor phraseology, or differences in aircraft 

radio transceivers. Also, once the pilot began a readback, 

nonstandard phraseology was another factor associated 

with 91 transmissions with readback errors. There was 

a tendency among some pilots to truncate or otherwise 

abbreviate the numerical values in speed, heading, or 

altitude assignments. In a similar way, aircraft call signs 

also were truncated. For example, Ownship67H became 

Ownship60 and Ownship528 became Ownship520. 

Some pilots used the “point” designation associated 

with radio frequencies when reading back altitudes and 

speeds or substituted “decimal” for the word “point” 

when reading back a radio frequency. Also, several pilots 

flying for foreign air carriers displayed some problems in 

English proficiency and language production. Finally, 

pilot expectation (n = 16) played a coincidental role in 

pilot readback errors and was associated with the pilot 

of one aircraft reading back the contents of a message 

meant for the pilot of a different aircraft.

DISCUSSION

Routine ATC Communication

The ideal controller-pilot communication process 

would show a direct 1:1 relationship between the 

production of an ATC message and its parroting back 

by the pilot-recipient. To avoid the occasion for faulty 

communications, general aviation pilots sometimes jot 

down the contents of an ATC message on a kneeboard 

or scratch pad clipped onto the yoke of the aircraft. In 

commercial aviation, pilots often change the dials on 

their mode control panel as they receive changes to their 

aircraft’s heading, altitude, or speed; radio frequencies are 

dialed into their second radio transceiver.

When the controller finishes the message, the pilot will 

read it back along with the aircraft’s call sign. In return, the 

controller actively listens to (i.e., hearback) the recitation 

of the message to verify that the contents of the original 

transmission were properly received and understood by 

the intended pilot. This process is commonly referred to 

as the ‘readback/hearback’ loop. 

In the unlikely case that the pilot erroneously reads 

back some of the contents of the original message, the 

controller has the opportunity to correct it by retransmitting either the entire message or only the portion that was 

read back incorrectly. A readback error is the incorrect 

recitation of an ATC transmission by the intended recipient of that transmission. Likewise, a hearback error is the 

failure of the originator of that transmission to correct 

the faulty readback.

The results presented in this report provide a description and summary of the controller-pilot communication 

process that occurred during normal, day-to-day operations in the terminal approach control environment. On 

average, across the five sampled TRACON facilities, one 

aircraft requested and received air traffic services every 

1 min 26 s in the approach sectors and 1 min 6 s in the 

departure sectors. The number of ground-to-air transmissions averaged 7.25 messages per aircraft for approach 

control and 4.7 ground-to-air messages for departure 

control. Approximately 13 messages were exchanged 

(from initial contact until the aircraft was switched to the 

next controller in sequence) that involved an allocation 

of about 1 min 16 s of airtime per aircraft.

For approach control, typically transmitted messages 

involved heading, speed, and altitude instructions, 

while for departure control, heading, altitude, and radio frequency instructions were commonplace. Rarely 

did messages from approach control contain aviation 

topics related to the altimeter, heading modification, 

or transponder aviation topics. Likewise, departure 

controllers seldom transmitted messages containing 

an altimeter setting, altitude restriction, or a heading 

modification.

Unlike the findings reported by Cardosi et al. (1996) 

where 59% of the ATC messages involved one or two 

pieces of information, we found that when controllers transmitted only instructions, almost half of their 

messages had a fairly low level of complexity (ranging 

between 4-7 pieces of to-be-remembered information). 

There did not seem to be a pattern in the frequency of 

occurrence for advisories or messages that combined 

instructions with advisories as a function of level of 

complexity.

Since the publication of Cardosi et al.’s report 10 

years ago, there has been an increase in the percentage 

of full readbacks made by pilots — up by 22.7%. Also 

encouraging is the trend among pilots to provide either 

the full or a partial call sign in the majority of their replies. 

In fact, pilots who provided a full readback also included 

the complete call sign in 61% of their responses (Cardosi 

et al. reported 37%). Where Cardosi et al. reported that 

24% of the full readbacks included a partial call sign, 

we found only 18.8% (suggesting that most pilots included all of the call sign’s numbers/letters). Again, pilots 

seemed to be doing a better job at providing complete 

and accurate information in response to controller messages. The fact that call signs were excluded in 7.6% of 

the readbacks and 0.1% of the spoken call signs were 

unintelligible leaves room for improvement.
25

Although unacknowledged ATC messages increased by 

1.8%, this finding may be partially due to random variation, sampling error, or factors independent of message 

length or complexity. It is unlikely that message length 

was a factor since more than 75% of the unacknowledged 

ATC transmissions had either one instruction or advisory. 

Since one-third of the unacknowledged single-topic instructions involved a change in radio frequency, it may be 

that some pilots preset the next radio frequency assignment 

on their radio transceivers. Radio frequency assignments 

are provided on standard approach and departure charts. 

When the controller provides the numbers, pilots verify 

them against their settings and may simply switch to the 

next frequency.

Miscommunications

As is often the case, ATC messages contain multiple 

communication elements. The information content 

present in a communication element contributes to the 

level of complexity of that message. The development of 

the concept of message complexity is a work-in-progress. 

For Cardosi et al. (1996), the aircraft’s call sign was not 

included as an element since it served only to draw the 

pilot’s attention to the incoming transmission. Their rationale was that the aircraft’s call sign was like one’s name 

— it should not increase the pilot’s memory load. Unlike 

one’s own name that doesn’t change, it is not uncommon 

for commercial and cargo airline pilots to receive three or 

more different call signs in a regularly scheduled workday — depending upon the flight number assigned to a 

particular flight. It is unlikely that these pilots have time 

to learn, let alone memorize them. In fact, many pilots 

have developed the habit of writing their flight numbers 

on a post-it, tape it to the inside of their hats and then, 

upon entering the flight deck, sticking it onto the front 

panel in line with their forward field of view. Hence, we 

argue that a message’s complexity is partially determined 

by the sum of the values assigned to the information 

content of individual communication elements. Other 

factors that could affect a message’s complexity include 

message structure, information value, importance, as 

well as the number of communication elements requiring pilot action. 

Some communication elements are ancillary – they 

do not affect the pilot’s ability to aviate or navigate 

(e.g., general acknowledgments, greetings). The more 

important ones provide pilots with new information, 

confirm pilot expectations, verify existing information, 

or negate that information (e.g., heading, altitude, speed 

instructions; approach/departure clearances; traffic advisories). For example, Rantanen and Kokayeff (2002) 

reported no apparent correlation between the number 

of elements and the complexity ranking among a set of 

28 ATC clearances on the ability of a sample of airline 

pilots to accurately copy down previously recorded clearances. For example, in one clearance neither of the two 

elements (complexity rank of 12 out of 28) was copied 

correctly while in another with eight elements (complexity rank of 26) 91.67% were correctly reproduced. They 

suggest that factors such as familiarity with the operating 

procedures within a domain (air carrier, general aviation) 

and geographical location (knowing the names of the 

navaids, fixes, etc.) affected what pilots in their study 

copied accurately and what was discarded.

Several studies documented the vulnerability of pilot 

memory and readback performance. For example, Cardosi 

(1999) reported that message complexity directly affects 

pilot memory. Several field studies have shown fewer pilot 

readback errors and requests for repeats when controllers’ 

messages were short and simple (e.g., Cardosi, 1993; 

Morrow & Rodvold, 1993). Likewise, laboratory studies 

(Morrow & Prinzo, 1999; Morrow, Rodvold, McGann, & 

Mackintosh, 1994) found that readback errors and pilot 

requests were more likely to occur in response to longer 

ATC messages. Finally, the operational data analyzed 

here provide additional evidence that readback errors 

and pilot requests increased with increases in complexity 

and message length (when measured by the number of 

aviation topics in a controller’s message). Of particular 

interest, but not surprising, was the finding that pilots 

experienced the most difficulty reading back ATC messages when flying the approach segment of their flight. 

Adding to their workload the reading back of a message 

with more than one aviation topic or a complexity value 

of 10 or greater rapidly increased readback errors. 

Readback errors generally fell within three major groupings — omission, substitution, and transposition errors. 

The type of readback error produced seemed to be related 

to the type of information read back. For example, pilots 

were more likely to omit an anchor word or phrase when 

reading back a heading and either exclude a number or 

leave out the point/fix in a speed instruction. They were 

more likely to substitute an anchor word(s) when reading 

back either an altitude restriction or speed assignment 

than a heading or approach clearance. When instructed 

to either switch frequencies, change to a new heading, or 

alter the aircraft’s speed, pilots were likely to substitute 

numbers. Finally, a majority of the transposition errors 

involved reversing the order of one point/fix with that 

of another within the same message.

It was surprising that controllers only corrected 8% of 

the readback errors. Why were so few corrected? It would 

seem that during the hearback process, controllers might 

evaluate the intrinsic safety component of each readback 

and then decide whether or not to correct a detected error. 

It would follow that some communication elements may 
26

have little or no impact on safety, and if corrected, add 

to radio frequency congestion and task load. In such a 

situation, the controllers might elect not to alert the pilot 

to the presence of a readback error since aircraft track and 

position information are available on their situation displays. In fact, when given the opportunity for researchers 

to listen in on a frequency while observing controllers, it 

is common to hear a controller whisper “close enough” 

when some readback errors occur. Apparently, such 

readback errors were not sufficient to warrant another 

transmission. Since controllers monitor the progress of 

aircraft along their route of flight, they will intervene 

when it is necessary to maintain safety. Consequently, 

actively correcting a faulty readback might be a conservative process with corrections reserved for transmissions 

that have a direct or immediate affect on safety, aircraft 

performance, traffic flow, or similar factors.

It may be that some types of readback errors are more 

safety-critical than others — especially when situational 

factors are taken into account (e.g., reading back “runway 

four-left approach” when “four-right” was given following the instruction “turn left”). Controllers were more 

likely to correct transposition errors more often than 

either substitutions or errors of omission. By correcting 

the pilot as soon as possible, the controller can prevent 

down-stream consequences — such as potential increases 

in workload and frequency congestion. For example, if 

radio frequency number substitution errors went uncorrected, the pilot might switch to the wrong frequency. 

Typically, the pilot will come back on frequency and 

request the radio frequency again; the controller gives it, 

and the pilot reads it back. This adds to the controller’s 

workload and frequency congestion.

Finally, controllers may be less likely to correct pilots’ 

errors of omission than substitution errors since immediacy of reply and context mitigate the potential for 

misunderstanding created by missing digits (“one seven 

zero knots” read back as “seventy knots”), anchor words 

(“one seventy” in response to a speed instruction), or 

other omissions. Also, controllers’ prior knowledge (i.e., 

knowing that aircraft slow down on approach and speed 

up on departure; and at certain speeds aircraft fall out 

of the sky), coupled with redundant visual information 

(observing aircraft trajectories on their situation displays), 

assist them as they monitor and verify pilot compliance 

with their instructions.

Another recurring problem involved aircraft call signs. 

Aircraft identification can be presented visually or aurally 

using alphanumeric characters and can be received either 

as text, using line printers or visual displays (e.g., radar 

displays and avionics such a cockpit display of traffic information) or aurally over the voice radio communications 

system. The FAA authorization, assignment, and use of 

aircraft identifiers can be found in FAA Order 7110.65, 

The Handbook of Air Traffic Control. Approximately half 

of the misspoken call signs came from controllers, half 

from pilots, and the majority were corrected. 

When controllers produced an incorrect call sign, it 

often came about from the replacement of some numbers, 

letters, or prefixes with others not found in the call sign. 

It may be that similarity in the structure of numbers/

letters, data block overlap, or both contributed to call 

sign problems for controllers (e.g., COM355 replaced 

COM855; AAL for UAL, DAL for AAL, EJA for BTA). 

When pilots detected a disparity, they either asked the 

controller if the message was for them using their aircraft 

call sign as part of the query (e.g., “THAT FOR OWNSHIP ONE TWENTY THREE MAYBE?”) or they 

explicitly corrected the controller’s error as illustrated 

by the following dialogue. In response to the altitude instruction, “ALASKA SEVEN NINETY EIGHT CLIMB 

AND MAINTAIN ONE FIVE THOUSAND” the pilot 

said, “SIR THAT’S AIR CANADA SEVEN NINETY 

EIGHT FOR ONE FIVE THOUSAND” in which case 

the controller followed up with, “AIR CANADA SEVEN 

NINETY EIGHT THANK YOU SIR CLIMB AND 

MAINTAIN ONE FIVE THOUSAND.” 

As noted previously, pilots are at their busiest during 

the approach phase of their flights. They must simultaneously aviate, navigate, and actively monitor the radio 

frequency —listening for their aircraft’s call sign, anticipating an approach clearance as they near the airport. 

Generally, problems arising from call sign discrepancies 

such as similar sounding call signs are well documented 

(Monan, 1983; Wright & Patten, 1996; Civil Aviation 

Authority, 2000) and although rare, have been cited 

by the National Transportation Safety Board (NTSB, 

2003) in aircraft mishaps. After carefully reviewing the 

transcripts, there are several factors that may shed light on 

why stolen transmissions occur. For the few instances that 

we have identified, the factors that seemed to go together 

in each case shared similar characteristics between call 

signs (either in the name of the airline or flight numbers), 

prior knowledge of the frequency of misspoken call signs 

that might lead some pilots to think that the controller 

misspoke the call sign, pilot expectations during a flight 

segment, and pilot confidence. To illustrate, two aircraft 

are flying for the same company and both are on final 

approach. The pilots expect to be switched to the tower 

shortly after receiving their approach clearance. Upon 

hearing the company name and the tower frequency, each 

pilot might assume that they are the intended recipient 

of the transmission — especially if not listening for the 

flight number. One would be right and the other wrong. 

Had one pilot called to verify/confirm the identity of the 

receiver, there would not have been a problem.
27

Chapter 4 of the Aeronautical Information Manual

(FAA, 2006) provides pilots with good information about 

basic communication techniques, communication procedures, and phraseology. The key concept is that good 

communication skills promote safety through a mutual 

understanding between the pilot and air traffic service 

personnel. When pilots make their first radio call to a given 

air traffic control facility or controller within a facility, that 

message is to be spoken in a defined format. The message 

begins with the name of the facility being called, followed 

by the full aircraft identification. As stated in the AIM, 

“If radio reception is reasonably assured, inclusion of your 

request, your position or altitude, and the phrase ‘(ATIS) 

Information Charlie received’ in the initial contact helps 

decrease radio frequency congestion. Use discretion; do 

not overload the controller with information unneeded 

or superfluous5

”. Regardless of whether making initial 

contact or receiving a new altitude assignment, most 

domestic and foreign air carrier and cargo pilots and 

the majority of general aviation pilots included altitude 

information as part of their reports and readbacks. Approximately 63% of the pilots who received the current 

altimeter included it in their readback.

Whether unintentional or purposeful, many pilots 

also made number/letter substitutions. A new trend 

that is occurring in pilot and controller communications is the tendency to round the numbers in the call 

sign and aviation topics. For example, Ownship67H 

became Ownship60H and Ownship528 became Ownship520. Some pilots truncated or otherwise abbreviated the numerical values in speed (“TWENTY FIVE 

KNOTS”), heading (“one four” for a heading of 

one four zero), or altitude assignments (“DOWN TO 

FIVE HUNDRED”). It is possible that some of the abbreviations were due to delivery technique or equipment 

use, while others may reflect a heightened workload. As 

reported by Prinzo and McClellan (2005), disruptions 

to efficient information transfer from blocked, steppedon, and clipped transmissions occurred in 1.16% of the 

8,000 sampled transmissions. The premature release of 

the mic key clipped the end of the call sign in a few of 

the transmissions. 

Other forms of nonstandard phraseology were also 

associated with readback errors. It may be that some of 

the phraseology used (or heard) by pilots during international flights is making its way into the NAS. Some 

pilots used the “point” designation associated with radio 

frequencies when reading back altitudes (e.g., “THREE 

POINT FIVE” instead of “THREE THOUSAND FIVE 

HUNDRED”) and speeds (e.g., “TWO POINT SEVEN 

ON THE SPEED” for “two hundred and

5 Aeronautical Information Manual § 4-2-3. Contact Procedures.

seventy knots”) or substituted “decimal” for the 

word “point” when reading back a radio frequency. Also, 

several pilots flying for foreign air carriers displayed some 

problems in English proficiency and language production — for example, reading back a speed instruction as 

“two zero hundred” instead of “two hundred 

knots,” or responding to “maintain visual from traffic” 

as “MAINTAIN VISUAL APPROACH.”

In summary, a comparison between the voice communications analyzed by Cardosi et al. with those presented 

in this report revealed differences in message complexity 

and readback/hearback error rates. As noted in the introduction, we conducted a more detailed, and objectively 

driven, content analysis that reflected greater information 

density than Cardosi et al. It may be that Cardosi et al.’s 

definition of message complexity was more congruent 

with the approach Prinzo, Britton, and Hendrix (1995) 

used to count the number of aviation topics present in 

messages. When the data were compared, the findings 

show more than 50% of controllers’ messages are fairly 

short but information-rich.

Similarly, the differences in the degree of faulty pilot 

readbacks and controller hearback errors may be partially 

due because of the approach used to evaluate the message 

content. We applied the FAA Order 7110.65 whereas 

Cardosi et al. do not describe their evaluation criteria. A 

liberal criterion reveals only a minimal increase in pilot 

readback errors (up 0.3%) between the two reports. Both 

reports show that aircraft headings and radio frequency 

changes still are the most frequently occurring readback 

errors. Likewise, there is no change in how often pilots 

request that controllers repeat all or some portions of their 

transmissions. The most notable disparity between the 

reports is the percentage of hearback errors — Cardosi 

et al. reported 40% and we reported 92%. It may be 

that some readback errors are relatively ‘harmless’ for the 

controller and are viewed as commonplace. To correct all 

readback errors could put a strain on the communications 

system and increase the controllers’ workload. However, 

disregarding them all could result in unsafe acts.

When examining pilot transmissions for the presence 

(or absence) of the aircraft call sign, the results show that 

when pilots provided a full readback, the complete call 

sign was included in 61% of their responses (Cardosi et 

al. reported 37%). Where Cardosi et al. reported that 

24% of the full readbacks included a partial call sign, we 

found 18.8%, of which 13.4% excluded the prefix but 

included all the numbers/letters of the call sign. Likewise, 

pilot/controller call sign mismatch has decreased from 

0.8% to 0.3%. Finally, pilots increased their production 

of full readbacks — up from 60% in 1996 to more than 

82% in 2005. Most striking is the finding that 10 years 
28

ago pilots provided a full readback with a complete call 

sign about 37% of the time. In today’s air traffic control 

environment, the full call sign accompanies a full readback 

in 61% of the pilots’ readbacks.

Communicating for safety is the primary objective 

of the phraseology developed for and provided in FAA 

Order 7110.65. With increased international travel and 

the gradual migration of other phraseologies into the 

NAS, pilots and controllers must remain vigilant in the 

accurate production and recitation of ATC clearances, 

instructions, advisories, reports, requests, and other 

communications.

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2005.

A-1

APPENDIX A 

Instruction/Clearance Complexity Guide (*Minimum-Maximum Values) 

Aviation Topic Complexity Phraseology 

Heading

4

4

3

3

3

2

2

1

TURN LEFT/RIGHT HEADING (degrees) 

TURN (degrees) DEGREES LEFT/RIGHT 

TURN LEFT/RIGHT (degrees)

DEPART (fix) HEADING (degrees)

FLY HEADING (degrees) 

FLY PRESENT HEADING 

HEADING (degrees)

(degrees)

Heading

Modification

2

2

1

INCREASE RATE OF TURN 

GOOD LEFT/RIGHT TURN 

TIGHT TURN 

Altitude

6

5

4

*4-8

*4-8

*3-7

*3-8

*2-6

*1-2

3=(altitude) two digits +THOUSAND 

2=(altitude) one digit + THOUSAND 

3=(altitude) two digits + HUNDRED 

2=(altitude) one digit + HUNDRED 

2=(altitude) two digits 

1=(altitude) one digit

DESCEND/CLIMB & MAINTAIN (altitude) THOUSAND (altitude) HUNDRED 

 three five 

DESCEND/CLIMB & MAINTAIN (altitude) THOUSAND 

 one zero 

DESCEND/CLIMB & MAINTAIN (altitude) THOUSAND 

 four

CONTINUE CLIMB/DESCENT TO (altitude)

AMEND YOUR ALTITUDE DESCEND/CLIMB AND MAINTAIN (altitude) 

AMEND YOUR ALTITUDE MAINTAIN (altitude) 

DESCEND/CLIMB TO (altitude)

MAINTAIN (altitude)

(altitude, omitted “THOUSAND” “HUNDRED”)

Altitude Restriction *4-7

*4-7

*4-7

*3-7

*3-6

*3-6

2

2

2

3=(altitude) two digits + THOUSAND 

2=(altitude) one digit + THOUSAND 

3=(altitude) two digits + HUNDRED 

2=(altitude) one digit + HUNDRED 

2=(altitude) two digits 

1=(altitude) one digit

EXPEDITE CLIMB/DESCENT THROUGH/TO (altitude)

CROSS (point) AT/ABOVE/BELOW (altitude)

MAINTAIN (altitude) UNTIL (point) 

(altitude) TIL ESTABLISHED/LOCALIZER/ESTABLISHED ON LOCALIZER

EXPEDITE THROUGH/TO (altitude)

(point) AT (altitude)--(altitude) TIL (point)--HURRY DOWN TO (altitude) 

GOOD RATE DOWN/YOUR BEST RATE 

EXPEDITE CLIMB/DESCENT 

(Speed assignment) “THEN” DESCEND/CLIMB 
A-2

Aviation Topic Complexity Phraseology 

Speed 

6

5

5

5

5

4

4

3

3

3

2

2

1

2=(speed) 

1=(number)

MAINTAIN SPEED (speed) TIL (point) OR MAINTAIN (speed ) KNOTS TIL (point)

MAINTAIN (speed) KNOTS OR GREATER OR MAINTAIN SPEED (speed) OR 

GREATER

AT (point) SPEED (speed) OR AT (point) (speed) KNOTS

REDUCE/INCREASE SPEED TO (speed) OR REDUCE/INCREASE TO (speed) 

KNOTS

SPEED (speed) TIL (point) OR (speed) KNOTS TIL (point)

MAINTAIN SPEED (speed) OR MAINTAIN (speed) KNOTS

DO NOT EXCEED (speed) KNOTS OR DO NOT EXCEED SPEED (speed) 

MAINTAIN (speed) OR SPEED (speed) OR (speed) KNOTS

DO NOT EXCEED (speed) OR SLOW TO/GO BACK TO/MAINTAIN (speed)

INCREASE/DECREASE (number) KNOTS 

MAINTAIN PRESENT/THAT/NORMAL SPEED 

BEST FORWARD SPEED 

GO FAST 

Approach/

Departure

6

6

5

5

4

3

3

3

2

2

2

2

2

CLEARED ILS RWY (name) R/C/L APCH 

CLEARED VISUAL APCH RWY (name) R/C/L 

CLEARED ILS/VISUAL RWY (name) R/C/L 

CLEARED ILS/VISUAL (name) R/C/L APCH 

CLEARED ILS RIGHT/LEFT/CENTER APCH 

ILS RIGHT/LEFT/CENTER APCH 

CLEARED ILS (name)

CLEARED RWY (name)

CLEARED APCH 

CLEARED (type) 

ILS RIGHT 

RWY (name) 

CLEARED VISUAL/ILS 

Radio Frequency 

*6-7

*5-6

*4-5

*3-4

2

1

1

CONTACT (facility/function) (frequency + point) – could be up to four digits in 

frequency (2 on either side of “point”) 

(facility/function) (frequency + point)

(frequency + point)

(frequency)

CONTACT (facility/function) 

(facility/function) 

(change point, e.g. now, there, at/over marker/when established) 

Position/ Route 

5

*3-5

3

3

2

2

2

2

2

2

2

1

INTERCEPT/JOIN RUNWAY (name) LEFT/RIGHT LOCALIZER 

INTERCEPT/JOIN/RESUME (airway, course, localizer, arrival/departure, etc.) 

MAINTAIN VISUAL FROM THAT TRAFFIC/HIM/THEM/MD80 

KEEP HIM IN SIGHT 

MAINTAIN VISUAL SEPARATION 

DIRECT (fix) 

FOLLOW THAT TRAFFIC/HIM/THEM/MD80 

VICTOR (airway number) 

J (route number) 

INTERCEPT/JOIN LOCALIZER 

RESUME OWN NAVIGATION/PROCEED ON COURSE 

TO JOIN 

Transponder 

4

3

3

2

RESET TRANSPONDER SQUAWK (4 digits) 

SQUAWK (4 digits)/CODE (4 digits)/IDENT

SQUAWK (4 digits)/CODE (4 digits) and IDENT

SQUAWK VFR 
B-1

APPENDIX B 

Advisory Complexity Guide (*Minimum-Maximum Values)

Aviation Topic Complexity Phraseology 

Traffic

*8-14

*7-9

*5-9

2

2

1=TRAFFIC 

1=O’CLOCK

1=one number for O’Clock, e.g. “TWELVE” 

2=two numbers for O’Clock, e.g., “TEN TO TWELVE” 

1=MILES 

1=one digit for Miles 

2=two digits for Miles 

1=ALTITUDE

2=ALTITUDE UNKNOWN 

3=(altitude) two digits +THOUSAND 

2=(altitude) one digit + THOUSAND 

3=(altitude) two digits + HUNDRED 

2=(altitude) one digit + HUNDRED 

2=(altitude) two digits 

1=(altitude) one digit 

1=(direction) 

1=(type)

TRAFFIC (number) O’CLOCK (number) MILES (direction)-BOUND (altitude) (type) 

TRAFFIC (number) MILES (number) O’CLOCK ALTITUDE UNKNOWN 

YOU’RE FOLLOWING (type) (number) O’CLOCK (number) MILES (altitude)

YOU’RE FOLLOWING/GOING TO FOLLOW/YOU’LL BE FOLLOWING (type) 

TRAFFIC (NO FACTOR)

Altimeter 4

3

(source) ALTIMETER (4 digits) 

ALTIMETER (4 digits)