The Outcome of ATC Message Length 

and Complexity on En Route Pilot 

Readback Performance

O. Veronika Prinzo 

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

A.M. Hendrix

R. Hendrix 

Hendrix & Hendrix

Roswell, NM 88201

January 2009

Final Report

DOT/FAA/AM-09/2

Office of Aerospace Medicine

Washington, DC 20591

OK-09-0434
NOTICE

This document is disseminated under the sponsorship 

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

of information exchange. The United States Government 

assumes no liability for the contents thereof.

___________

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


Technical Report Documentation Page 

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

DOT/FAA/AM-09/2 

4. Title and Subtitle 5. Report Date 

The outcome of ATC message length and complexity on en route pilot January 2009 

readback performance 6. Performing Organization Code 

 

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

Prinzo OV,1 Hendrix AM,2 Hendrix R2 

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

 

11. Contract or Grant No. 

1

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 

This work was performed under Task AM-B-06-HRR-516. 

16. Abstract 

Field data and laboratory studies conducted in the 1990s reported 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 yrs have passed since a comprehensive analysis quantified the types and 

frequency of readback errors and communication problems that occurred in the en route operational 

environment. Hence, a content analysis was performed on 51 hrs of pilot and controller messages that were 

transmitted from five en route facilities in the contiguous United States between March and August 2006. This 

report contains detailed and comprehensive descriptions of routine air traffic control (ATC) transmissions and 

how ATC message complexity and message length affected pilot readback performance. The results show that 

message complexity had a statistically significant effect on the production of errors of omission only, while 

message length affected both the production of errors of omission and readback errors (substitution and 

transposition errors). When pilots requested that controllers repeat their messages, often these messages included 

the names of fixes, waypoints, and intersections, as well as the name of the next controlling sector or facility. Five 

recommendations are made: (1) No more than three aviation topics should be included in an ATC transmission. 

(2) A route clearance should be given as a stand-alone transmission. (3) The names of all fix, waypoint, location, 

and other identifiers should be repeated, and if necessary, spelled out following their first recitation. For example, 

“CLEARED DIRECT COBAD THAT’S CHARLIE OSCAR BRAVO ALFA DELTA” or “CLEARED DIRECT COBAD C-O-BA-D.” (4) Slang should not be used or accepted as part of aviation phraseology. (5) Effort should be undertaken 

to reduce excessive words/phrases — on, your, to, is, etc. 

 

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 38 

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

iii

Contents

The Outcome of ATC Message Length and Complexity on En Route Pilot Readback Performance. . . . 1

METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Subject Matter Experts (SMEs). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Sector Descriptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Routine ATC Communication. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Analysis of Pilot Faulty Readback Performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Message Length and Pilot Readback Performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Appendix A

Guide to the Computation of Complexity: ATC Instruction/Clearance Aviation Topics. . . . . . . . A1

Appendix B

Guide to the Computation of Complexity: ATC Advisory Aviation Topics. . . . . . . . . . . . . . . . . . B1

Appendix C

Guide to the Classification of Pilot Readback Errors: Instruction/Clearance Aviation Topics. . . . C1

Appendix D

Readback Error Classification Guide: Advisory Aviation Topics. . . . . . . . . . . . . . . . . . . . . . . . . . D1

EXECUTIVE SUMMARY

Thisreportdescribes andsummarizes51hrs of airtraffic control (ATC) communication that occurred during

normal, day-to-day operations at five different en route

centers between March and August 2006. On average,

an aircraftrequested and received airtraffic servicesevery

2 min 4 s, with approximately 10 messages exchanged

from initial contact to the transfer of communications. 

The most frequently transmitted instructions involved

changes in altitudes and switching pilotsto anotherradio

frequency. 

Unlikethe findingsreported byCardosi (1993)where

78% of the ATC messages involved one or two pieces of

information (e.g., “contact center on one one two point

threefour”=1pieceof information,while “contact[name]

center on one one two point three four” = 2 pieces of

information), wefound that less than 1% of the controllers’ messages had only one or two pieces of information

(of the two examples, the first had a complexity value of

6, and the second had a complexity value of 7). In this

report, 84% of the controllers’ transmissions contained

only one aviation topic (AT) (e.g., altitude, heading,

speed), while messages with two ATs occurred in 14%

of the other transmissions.

We also report a 10.2% increase in fullreadbacks and a

3.4% decrease in partialreadbacks. Moreover, pilotswho

provided a full readback also included the complete call

sign in approximately 64% of their responses. The fact

that call signs were excluded in 10% of their readbacks

and .2% ofthespoken callsignswere unintelligibleleaves

roomfor improvement. Unfortunately,bothreportsshow

the number of unacknowledged ATC transmissions at

slightly more than 3%.

Approximately 71% of the pilot responses contained

faulty readbacks — 67.4% contained one or more errors of omission that could be attributed to increases in

the complexity of ATC messages. Changes in message

complexity alone had no statistically significanteffect on

the production of readback errors (e.g., the substitution

or transposition of numbers in the controller’s original

transmissions). 

Readback errors comprised the remaining 4.0% of the

pilots’ faulty readbacks — 78% resulted from a combination of an error of omission plus one or more substitutions

(89%) ortranspositions(11%). Unlikemessage complexity,

message length affected both the production of errors of

omission and readback errors. There were more errors of

omissionasATCmessagelength increasedfromshort(1AT),

to moderate (2ATs), and long (3 or more ATs). Redundant

information might be eliminated to increase the economy

of capacity limitations of verbal working memory. 

Readbackerrors increasedonceATCmessages included

two or more ATs. The most common readback errors

involved altitude and altitude restrictions, followed by

changes inradiofrequency,route/positionclearances, and

altimeter settings. Some of the readback errors involved

slang. That is, an ATC instruction to “contact center on

one one niner point zero,” was occasionally read back as

“nineteen zip” or “nineteen nothing.”What isworrisome

for aviation safety isthat other pilotsmay not understand

these types of readbacks (especially pilots who are less

proficient in aviation English or making their first flights

into the U.S.).

A common practice among pilots is to acknowledge

controller instructions/advisories with either their call

sign only or their call sign plus roger/wilco. Also common is a response such as “Down to two five zero”

(when acknowledging an instruction to reduce speed

or assignment of a new altitude) and partial readback

(responding to only one of two or more instructions). 

Theseexamples illustrate pilotreadbacksthatmightleave

the controller in doubt of pilot understanding and may

thus adversely affect safety.

Possibly to avoid the risk of either reading back the

contents of an ATC transmission meant for another aircraft (stolen transmission) or reading back its contents

incorrectly, pilots asked the controller for either a full

or partial repeat of the original message. Of these 112

pilot requests, five involved a call sign misspoken by the

controller to which the pilot wanted confirmation of

the message’s recipient. A closer look at requests showed

that 67.5% had either a fix/intersection or facility name

included as part of their content — and many of these

messages involved route clearances and the transfer of

communications. The names of location identifiers,

fixes, intersections, navigational aides, etc. are critical

to navigation; thus, pilots were prudent in requesting

either the repetition or confirmation of previously heard

identifiers.

Based on our findings, we recommend that:

1) No morethan three ATs should be included in an

ATCtransmission. 2)Route clearancesshould be given

separately as stand-alone transmissions. 3) The names

of all fixes, waypoints, locations, and other identifiers

should be repeated, and if necessary, spelled out following their first recitation. For example, “Cleared

direct cobad – charlie-oscar-bravo-alfa-delta,”

or “Cleared direct cobad – c-o-b-a-d cobad.” 4)

Slang should not be accepted by ATC as part of a

pilot readback.

The Outcome of ATC Message Length and Complexity

on En Route Pilot Readback Performance

Many attempts to communicate are nullified by saying too much.

— Robert Greenleaf

Founder, Modern Servant Leadership Movement

Morethan 10 years havelapsed since a comprehensive

analysis was undertaken to quantify and report thetypes

and frequencies of pilot readback errors and air traffic

control(ATC) communicationproblemsthatoccur inthe

operationalenvironment. In particular, it is important to

determinewhetherthe findingsfromthe previous decade

(Cardosi, 1993; Cardosi, Brett, & Han 1996; Prinzo,

1996; Prinzo & Britton 1993) remain representative of

routine communications in 2006. 

This is thesecond of two reports that investigated the

role of ATC message complexity and message length on

operational communications between controllers and

pilots. In the first report, Prinzo, Hendrix, and Hendrix

(2006) describe and summarize the controller-pilot

communication process that takes place during normal,

day-to-day operations in the terminal radar approach

control(TRACON)environment. Inthisreport,we apply

the same theoretical approach and methodology to the

analysis of operational communications in the en route

environment using digital audio tapes (DATs) provided

by five Air Route Traffic Control Centers (ARTCCs). 

In their 2006 report, Prinzo and colleagues took

the concept of message complexity that was originally

proposed by Cardosi (1993) for the ARTCC domain,

modified by Cardosi et al. (1996) for the TRACON

domain, further refined it and provided a theory-driven

rationalefor computingmessage complexity. Inparticular,

Wasow (1997) suggested that an utterance’s complexity

can be derived fromthe amount of information expressed

in its constituents as measured by the number of words,

syntactic nodes, or phrasal nodes in the constituent. 

They further cite the seminal work of Miller (1956) and

Baddeley (1987, 2000) and Baddeley and colleagues

(1974, 1975), who provide a strong theoretical basis for

capacity and processing components of verbal working

memory. Miller proposed that verbal working memory

is limited in the amount of information that can be actively processed (7±2), while Baddeley added a temporal

constraint — external information encoded as mental

representations must be actively rehearsed; otherwise,

they begin to decay in about 2 s or are overwritten by

incoming information. 

Prinzo et al. (2006) attempted to remove as much

of the subjective component as possible when counting

the complexity present in communication elements. As

noted in Prinzo (1996), communication elements are

the fundamental unit of meaningful verbal language. In

aviation, communication elements are identified accord-

ing to their functionality (Address/Addressee, Courtesy,

Instruction/Clearance, Advisory/Remark, Request, and

Non-Codable) and are restricted with regard to their

aviationtopic (AT: altitude,heading,speed,traffic,route,

etc.: Prinzo, Britton, & Hendrix 1995).

For Prinzo et al. (2006), the complexity value of a

communication element is defined by eachword orset of

wordstransmittedbyATCto theflightdeck that contains

anewpieceof informationcriticaltotheunderstandingof

that communication element. Identifying the amount of

information in ATC utterances begins by parsing a message into its communicationelements,thendefining each

communication element according to its speech act and

AT, assigning a complexity valueto each communication

element, and finally, summing the complexity values to

arrive at the level of complexity for each message. They

developedseveraluser guidesfordetermining themessage

complexity for the TRACON (Prinzo et al., 2006) and

En Route (see Appendices) environments.

Their scoring scheme assumes that communication

elements that contain more information should be assigned larger values. This assumption holds, particularly

for altitude instructions. Forexample,thenumericalvalues

present in altitude instructions such as “three thousand

fivehundred,” “one-zero thousand,” and“fourthousand”

arelikely to impose quantitativelydifferentloads onwork-

ing memory. In particular, it takes longer to pronounce

‘three thousand five hundred’ than ‘four thousand’ (e.g.,

articulatory loopproposedbyBaddeley,2000) andutilizes

more capacity because it contains more words (Miller,

1956). In fact, Morrow and Prinzo (1999) found that

when serial reproduction is required, numerical content

thatutilizesmoreresourcesmaybepartiallyor completely

omitted or lead the pilot to request a repetition.

In this report, we describe and summarize the

controller-pilot communication process that takes place
during normal, day-to-day operations in the en route

environment. We alsoapply thesametheoretical approach

and methodology that was developed for the TRACON

environment to the analysis of operational communications intheenrouteenvironment(seePrinzoet al.,2006). 

Like the TRACON report, the purpose of this report is

to (1) provide current information regarding routine en

route communication practices, (2) document the types

of transmissions that are exchanged between pilots and

the certified professional controllers who provide them

with en route ATC services, and (3) record communication problems by type and frequency of occurrence. Like

the TRACON report, we restricted the analysis of ATC

messagesto instructions/clearances, altimetersettings,(or

both), and the pilots’ responses to these messages. 

This report is similar to Cardosi’s (1993), in that (1)

each report examined voice tape samples from Chicago,

Los Angeles, New York, and Oakland ARTCCs; (2) the

sampled ARTCCs had a relatively high proportion of

foreign carriers; and (3) the communication samples

included both high- and low-altitude sectors. 

It differs from the Cardosi report in the following

ways: (1) Cardosi measured message complexity “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.” [p3]. For

example, “... contact Minneapolis Center 118.8” was

considered one piece of information if the pilot was

already on a Minneapolis Center frequency and as two

pieces of information if the pilot received it while communicating with a different ARTCC. We, on the other

hand, treated “... Contact Minneapolis Center 118.8” as

one communication element with a complexity value =

6. Our rationale was thateach digit comprising theradio

frequency imposes amemory load that could result in the

piloterroneouslyreadingbackthewrongfacility/function;

omitting, transposing, or substituting any (or all) of the

digits in the radio frequency; or both. Figure 1 shows

howweevaluated communication elements and assigned

complexity values. Each anchor word is also assigned a

value = 1; in this case, they are the words “contact” and

“point.” (2) Cardosi evaluated voice tapes from eight

different ARTCCs during high and moderate traffic

situations, and we examined voice tapes representative

of high traffic situations from five different ARTCCs. (3)

Cardosi examined 48 hrs of voicetapes, andweexamined

51 hrs of voice tapes. 

This report is also similar to the Cardosi (1993)

and Prinzo et al. (2006) reports in that they provided

descriptions and descriptive statistics related to three

general types of information: Routine ATC Communications, Pilot Faulty Readback Performance, and Pilot

Requests for Repeat of an ATC Transmission. Like Cardosi (1993) and Prinzo et al. (2006), complexity values

were determined for the transmissions that contained

a) instruction/clearance speech acts that involved head-

ing,headingmodifier, altitude, altituderestriction,speed,

approach/departure, frequency, holding, route/position,

and transponder ATs, and b) advisory speech acts that

involved the altimeter AT. 

In this report, separate analyses were performed for

routine ATC communications and pilot requests for

repeat. Routine ATC Communications arerelated to (1)

types of information included inATCmessages,(2)ATC

messagelengthandcomplexity,(3)pilotresponsestoATC

messages, and (4) pilot use of call signs in readbacks. The

analysis of Pilot Faulty Readback Performance involves

(1) piloterrors of omission, (2) pilot readback errors, (3)

message complexity andpilotreadbackperformance, and

(4) messagelength and pilot readback performance. The

analysis of PilotRequestsforRepeat of an ATCtransmission were further subdivided into four types of requests:

(1) requests for the repeat of a specific aviation topic,

(2) requests for the repeat of an entire transmission, (3)

requests for confirmation/verification of a specific aviation topic, and (4) requests for confirmation that the

transmission was for them. 

Figure 1. Assigning a Complexity Value to an Aviation Topic. 
METHOD

Materials

Audiotapes. Eachenroutefacilitywas askedtoprovide

10 hrs of voice communications representing heavy concentrations of international arrivals, departures, or both. 

The tapes were of communications recorded between

March and August 2006. The facility representative

made copies of their Digital Audio Tape (DAT) record-

ings using the NiceLoggerTM Digital Voice Recorder

System. DATs contained separate voice records of all

communication transmitted on the radio frequency assigned to a particular sector position on the left channel. 

The NiceLoggerTM Digital Voice Reproducer System

decoded and displayed time and correlated it with the

voice stream in real time.

Guides to the Computation of En Route Message 

Complexity. To compute ATC message complexity, the

Guide to the Computation of Complexity: ATC Instruction/

Clearance Aviation Topics and the Guide to the Computation of Complexity: ATC Advisory Aviation Topics were

developed and applied to each aviation topic in controller

transmissions. These guides appear in Appendix A and

Appendix B, respectively. Table 1 shows an example of

the complexity value assigned to the phraseology used to

deliver an altitude instruction. The first column presents

theAT,thesecond column presentsthe complexity value,

and thethird column presents the phraseology extracted

from FAA Order 7110.65P (2004)to supportthe delivery

of that service. In several cases, the phraseology used by

the speaker did not appear in FAA Order 7110.65 (e.g.,

good rate) but was used so frequently that they were assigned values. Capitalized words designate anchors and

italicized words identify qualifiers that vary according to

the geographical location and aircraft position. 

Todetermine complexity value, anchors,qualifiers, and

excessive verbiage are assigned a value indicative of new

information or importancetowardsthe understanding of

an instruction or 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 in the case “turn left/right

heading (degrees).”The anchor “turn left/right”provides

the direction of the turn while “heading” indicates the

aircraft’sbearing. Also,numericalqualifiersmustbeevaluated according to the phraseology used by the speaker. 

That is, the number ‘three thousand five hundred’ was

Table 1. Excerpt from the Guide to the Computation of Complexity.

AVIATION 

TOPIC COMPLEXITY PHRASEOLOGY

Altitude 

6

6

6

5

4

*4-8 

*4-8 

*3-7 

*3-6 

*2-5 

*1-3 

4=FLIGHT LEVEL + (altitude) three digits 

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 (altitude) THOUSAND

one two twelve 

DESCEND/CLIMB & MAINTAIN FLIGHT LEVEL (altitude) 

 two three zero 

DESCEND/CLIMB & MAINTAIN (altitude) THOUSAND 

one zero 

DESCEND/CLIMB & MAINTAIN (altitude) THOUSAND 

four

CONTINUE CLIMB/DESCENT TO (altitude)

AMEND YOUR ALTITUDE DESCEND/CLIMB MAINTAIN (altitude) 

AMEND YOUR ALTITUDE MAINTAIN (altitude) 

DESCEND/CLIMB TO (altitude)

MAINTAIN (altitude)

(altitude, omitted “THOUSAND” “HUNDRED” “FLIGHT LEVEL”) 

* The complexity value varies as a function of the altitude 
assigned a value of 4 (a value of oneforeach number and

a value of one for the anchor ‘hundred’), since it would

bemoredemanding thaneither one-zero thousand(value

= 3) or four thousand (value = 2). Finally, oneelement of

complexity should be added forexcessive verbiage. Excessive verbiage is determined by comparing the speaker’s

utterance againstthephraseologydesignated inFAA Order 

7110.65. The coding procedures used to evaluate the

controllers’transmissionswere appliedtopilotreadbacks.

Thelevel of complexity of an ATC message is thesum of

the values across each aviation topic in the transmission. 

Thus, complexity value (CV) is to aviation topic as level

of complexity (CL) is to an ATC message.

Guides to the Classification of Pilot Readback 

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

unsuccessful attempt by a pilot to correctly read back the

information contained in the communication elements

that comprisetheoriginalmessagetransmittedbyATC. To

aid in the classification ofreadback errors,theGuide to the 

Computation of Pilot Readback Errors:Instruction/Clearance 

Aviation Topics and the Guide to the Computation of Pilot 

Readback Errors: Advisory Aviation Topics were developed

and applied to each pilot readback. They are presented

in Appendix C and Appendix D, respectively. 

Many of the readback error types are common to all

ATs. The more typical ones include substitution, transposition, and omission errors. Presented in Table 2 is an

example of nine possible readback errors for an instruction to change altitude. The column to theright displays

the various types of altitude readback errors. Preceding

each example of a particular type of readback error is the

originalATCmessage. Forexample,ATCmighttransmit

thefollowingmessagetoAAL10, “AmericanTen turn left

heading two one zero.” Ifthe pilotread back either “three

one zero” or “six zero,” it is coded as a substitution error

since the numbers in the original heading instruction

include neither a three nor a six.

Some types of readback errors may pose a greater risk

to safety than others. Forexample, transposing a number

in an aviation topic may be more of a threat in somesituations than the omission of a number or thesubstitution

of an anchor word with its synonym. To ensure safety,

controllers listen to pilot readbacks while monitoring

aircraft along theirroute of flight andwill intervenewhen

necessary. Infact,Prinzo et al. (2006)suggestthat correct-

ing a faulty readback might be a conservative component

of the hearback process. As such, it may be reserved for

readbackswithpotentially adverse consequencestosafety,

aircraft performance, or traffic flow.

Subject Matter Experts (SMEs)

The first author has 14 yrs of experience analyzing

pilot controller communications. The second author

was an instrument-rated pilot and former controller

who had worked as an FAA Academy instructor for 8

yrs and had worked for 12 yrs in FAA supervision and

management. The third author has assisted the second

author in encoding pilot-controller communications for

more than 11 yrs.

Procedure

Data Transcription.Onesetof audiocassettetapeswas

dubbed from each DAT and provided to thetranscribers

who used themto generatethe verbatimtranscripts. Each

message was preceded by its onset and offset time as it

Table 2. Excerpt from the Guide to the Computation of Pilot Readback Errors.

Classification of Readback Errors Examples 

Readback Errors Type (ALT) 

ATC: “American Ten climb and maintain one two thousand”

1 = Substitution of message numbers/flight level vs. thousand

1-“maintain one three thousand” 

 “maintain flight level one two”

2 = Substitution of climb with descend or descend with climb 2-“descend maintain one two thousand”

3 = Substitution of message numbers with incorrect climb/descend 3-“descend maintain one three thousand”

4 = Transposition of message numbers with incorrect climb/descend 4-“descend maintain two one thousand”

5 = Transposition of message numbers 5-“climb maintain two one thousand”

6 = One type of information read back as another type of 

information 6-“AAL Ten one two zero knots”

7 = Omission of anchor word(s) 7-“one two”

8 = Omission of number elements 8-“climb maintain”

9 = Omission of anchor word(s) and some number elements 9-“climb two thousand”
was typed onto an electronic copy of the Aviation Top-

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

Prinzo,Britton,&Hendrix,1995). Oncethetranscribers

finished a set of tapes for an ARTCC, the second and

third authors received copies of the transcripts, video

maps, air carrier identifiers, and routes for use during

the encoding process.

Message Encoding Process. Messageencoding was a

four-stage process. Some of thestages had multiple parts. 

Each stage is described.

Step 1: Parsing and Labeling Communication Elements.In Step 1,each controllermessagewas parsed into

communication elements and categorized by speech act

andaviationtopicusingtheprotocolestablishedbyPrinzo,

et al. (2006). In Table 3 under the column labeled “T1”

is the receiver identification, under “T2” is the speaker

identification, “T3” contains a speed instruction(IS), and

“T4” shows that the last communication element is an

instruction to change altitude (IA). There are four communication elements: two instructions and the speaker

(SID) and receiver (RID) of the transmission.

Step 2: Computing Complexity. In Step 2, the Guide 

to the Computation of Complexity was used to look up the

appropriate complexity value according to the phraseology used by the controller for Instruction/Clearance ATs

(see Appendix A) and for Advisory ATs (see Appendix

B). Complexity values were assigned to all instruction

and advisory ATs. As seen in Table 4, neither SID nor

RID wereevaluated for complexity, so a value of “0” was

entered in each of those columns. For the first example,

the complexity value of IS = 2 and IA = 6. The message’s

complexity level(CL) isthesumofthe complexity values. 

For that example, CL = 8. In the second example, there

is a speed (IS), altitude (IA), and altimeter (AW) issued

by ATC. The complexity values are IS = 5, IA = 5, and

AW = 4. The transmission has a CL = 14.

Step 3: Identifying Message Couplets.In Step 3,each

ATCtransmissionwas pairedwith the pilot’sreply to that

message. The aircraft identifier andmessage contentswere

usedtomatchthe controller’stransmissionwiththepilot’s

response. As shown inTable 5, the controllertransmitted

a message to the flight deck (FD) of Ownship 6410, to

which the pilot replied with a general acknowledgement,

the readback of the speed and altitude instructions, followed by the aircraft’s call sign.

Step 4: Identifying Readback Performance. In Step

4, each readback was evaluated for accuracy. This is a

multistage encoding process. As shown in Table 6, if no

problem was present then a ‘0’ was entered under the

column labeled “Com Prob”; otherwise, the number of

communication problems was recorded for the entire

Table 3. ATC Messages Parsed and Categorized by Speech Acts and Aviation Topics.

SPKR Message T1 T2 T3 T4 

ATC

OWNSHIP SIXTY FOUR TEN / [FID] / RESUME NORMAL SPEED / 

CLIMB MAINTAIN FLIGHT LEVEL TWO THREE ZERO RID SID IS IA 

Table 4. Complexity Values Assigned to Instruction and Advisory Aviation Topics. 

SPKR Message RID SID IS IA AW CL 

ATC

OWNSHIP SIXTY FOUR TEN / [FID] / RESUME NORMAL 

SPEED / CLIMB MAINTAIN FLIGHT LEVEL TWO THREE 

ZERO 0 0 2 6 0 8 

ATC

OWNSHIP FOUR THREE SIX / [FID] / MAINTAIN TWO 

FIVE ZERO KNOTS / DESCEND AND MAINTAIN ONE 

FOUR THOUSAND / [LOCATION] ALTIMETER IS TWO 

NINER THREE EIGHT 0 0 5 5 4 14 

Table 5. ATC Message Couplets. 

SPKR Message T1 T2 T3 T4 

ATC

OWNSHIP SIXTY FOUR TEN / [FID] / RESUME NORMAL SPEED / 

CLIMB MAINTAIN FLIGHT LEVEL TWO THREE ZERO RID SID IS IA 

FD6410 

OKAY / NORMAL SPEED /AND UP TO FLIGHT LEVEL TWO FOUR 

ZERO /OWNSHIP SIXTY FOUR TEN IGA IS IA SID 
message. There was one identified communication

problem in the couplet, so the value of “1” appears in

that column. 

Then thetype of communication problem was coded

underthe columnlabeled“TypeProb.”Thetypes of communicationproblemswere codedasreadbackerror(RBE)

= 1, request for repeat (RfR) = 2, or their combination

= 3. If a communication problem had been identified

that did not match the pre-defined classifications, then

it would have been assigned a new value and included

in the classification scheme.

If faulty readback performance involved instructions/

clearances then the Guide to the Classification of Pilot 

Readback Errors: Instruction/Clearance Aviation Topics was

used (see Appendix C); if it involved an advisory then

we referred to Pilot Readback Errors: Advisory Aviation 

Topics (see Appendix D). As shown inTable 6, there is an

error involving the readback of the altitude instruction. 

Using the Guide to the Classification of Pilot Readback 

Errors: Instruction/Clearance Aviation Topics (Appendix

C), the readback error was assigned a particular value

that depended upon its specific type oferror. There were

three general classes of readback errors — substitution,

transposition, and omission — and each one had a

particular value that depended upon the nature of the

readback error. 

In the example, the controller instructed the pilot

to “climb maintain flight level two three zero” and

the pilot erroneously read back “flight level two four

zero.”Thereadback error was classified as a substitution

error since none of the numbers in the original altitude

instruction contained the number four, and it was assigned the value = 1.

The last part of the identification of readback errors

definedwhichoftheATswerereadback incorrectly. Since

thefaulty readback involvedthe altitude instruction, “IA”

was coded under the column labeled “Type AT.”

Encoding Reliability.Inter-raterreliabilitywasevaluated by having the first and second author randomly

encode the same randomly selected set of 125 messages

(25 for each facility). Since the first and second author

both used the guides presented in Appendices A and

B to compute complexity, it was expected that there

would be a high percentage of agreement between them. 

Krippendorff’s alpha (α),1 a reliability coefficient, was

performed on their ratings as each set of data was completed and after all the data were encoded. Treating the

ratings as ordinal data producedα= .945, indicating high

inter-coder agreement.

Sector Descriptions

Chicago ARTCC.Thetranscriptions are ofpilot/controller communication from two en route sectors. They

are low-altitude arrival and departure sectors, surface to

flight level two three zero joining a terminal facility. The

arrival sector received traffic from other en route sectors

and descended the aircraft to 8,000 feet via vectors and

arrival routes and transferred control to the approach

control. The departure sector received aircraft from the

approach control and climbed them to higher altitudes

and transferred control to other en route sectors. There

were a few arrival aircraft to outlying airports in the departure sector requiring descent and transfer of control

to an approach facility or other en route sectors.

Los Angeles ARTCC. The transcripts are of 10 en

route sectors. The sectors are north, east, and south of

the greater Los Angeles area. In the low-altitude sectors

(below flight level two four zero) the traffic is climbing/

descending to/from approach areas to en route altitudes

or high altitude. In the high-altitude sectors (flight level

two four zero and above) the traffic is also climbing and

descending, but to and from low-altitude sectors plus

en route traffic.

Miami ARTCC.Thetranscripts are of pilot controller

communications of arrival and departure sectors. The

airspace is the outer peripheral of the Florida peninsula. 

The airports are Fort Myers, Key West, Marathon, Miami, and Palm Beach. One sector transcription includes

coordination and communication change with Havana

1 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 (Krippendorff, 1980). Hayes

(2005) developed an SPSS procedure to compute Krippendorff’s

alpha that is available on the Internet.

Table 6. Identification of Communication Problems.

SPKR Message 

Com 

Prob

Type

Prob

Type

RBE

Type

AT

ATC

OWNSHIP SIXTY FOUR TEN/ [FID] / RESUME NORMAL SPEED 

/ CLIMB MAINTAIN FLIGHT LEVEL TWO THREE ZERO 1 1 1 IA 

FD6410 

OKAY / NORMAL SPEED /AND UP TO FLIGHT LEVEL TWO

FOUR ZERO /OWNSHIP SIXTY FOUR TEN 1 1 1 IA 
Center. The traffic is climbing and descending; there is

some VFR traffic and a few military aircraft flying en

route to warning areas. 

New York ARTCC.Thetranscripts areofpilot/controller communicationsfromoffshoresectors. The area isjust

east ofthe northeast coast ofthe U.S. The flowoftraffic is

inbound to Boston/New York terminal areas, with a few

overflights to Canada. The outbound traffic is to Europe

and the Caribbean Islands, plus a refueling track.

Oakland ARTCC.Thetranscripts areofpilot/controller communications in sectors to the west of the Continental United States. The traffic is mostly high altitude

and climbing and descending. The sectors are setting up

the outbound traffic for their oceanic routes/altitudes

and receiving traffic to the U.S. from the Orient and

Northwest U.S. The outbound traffic is handed-off to an

Ocean sector with a communication transfer to ARINC,

en routefrequencies, orCPDLC, whilethe inbounds are

handed off and communication transferred to another

Oakland sector. The traffic is mostly domestic air carriers and several foreign air carriers. There also are several

military aircraft working a refueling area.

RESULTS

Routine ATC Communication

In this report, 51 hr 3 min 20 s of communications

were analyzed. The amount of voice communications

varied from 58 min 55 s on one communication sample

to 5 hr 13 min 49 s on another. Approximately 89% of

the communications occurred between 6:00 a.m. and

6:00 p.m. The data are representative of 77 controllers

who worked arrival, departure, and mixed traffic flows

at five different ARTCCs within the contiguous U.S. 

between March and August 2006.

Presented in Table 7 is the number of transmissions,

numberofU.S. andForeignRegistryaircraft, andduration

of the communication samples foreach ARTCC facility. 

The grand total presented under the heading “Number

of Aircraft” and “Duration ofCommunication Samples”

revealed that, on average, one aircraft requested and

received air traffic services every 2 min 4 s. The number

of ground-to-air transmissions averaged 4.62 messages

per aircraft (Number of ATC Transmissions/Number

of Aircraft). From initial contact to the hand-off to the

next controller in sequence,theentiretransactional communication set involved the exchange of 10 messages

between a controller and pilot (this includes all of the

pilot transmissions to the controller) and an allocation

of 28.24 s of airtime (per aircraft).

Types of Information Included in ATC Messages.

Like the Cardosi 1993 report, the computation of message complexitywasrestricted to include only controllers’

messagesthat transmitted a combination of instructions,

clearances, andadvisories(i.e.,radar contactwithaltimetry

orradarservicesterminatedwith a frequency change). Of

the6,875controller-to-pilottransmissions,4,261metthe

selection criteria and underwent further analysis.

Figure 2 shows that of the 5,158 ATs transmitted to

pilots, the ones most frequently transmitted included

altitudes, radio frequency assignments, and route/positionclearances. Therewere11 instanceswherepilotswere

instructed to hold at a particular location (e.g., “Ownship

one eighty three roger is cleared to pivot hold northeast

as published except one zero mile legs approved ten mile

expect further clearance two two zero five.”).

ATC Message Length and Complexity.Table8shows

that about92%oftheATCtransmissionshadonly instructions/clearances,2%containedonly analtimetry advisory,

andnearly6%combined instructions/clearanceswiththe

Table 7. Number of Transmissions, Number of Aircraft, and Communication Durations Presented by 

ARTCC Facility.

Number of Transmissions Number of Aircraft 

Source ATC Flight 

Deck

Landline

Unsure Total US Foreign 

Duration of 

Communication 

Samples

Chicago 1233 1326 251 2 2812 214 42 9 h 29 min 35 s 

Los Angeles 1837 1998 282 5 4122 442 57 11 h 08 min 59 s 

Miami 1999 2150 222 1 4372 370 32 9 h 39 min 01s 

New York 1028 1035 195 2 2260 125 41 10 h 37 min 39 s 

Oakland 778 865 177 1 1821 91 65 10 h 08 min 06 s 

Grand Total 6875 7374 1127 11 15387 1252 235 51 h 03 min 20 s 
altimetryadvisory. Furthermore,84%ofthetransmissions

had one speech act; rarely did controllers include three

or more speech acts in their transmissions. 

An Analysis of Variance (ANOVA) was conducted

on the mean number of speech acts in ATC transmissions according to facility and traffic flow. The results

indicated that messages were longer when traffic was in

an arrival as compared with either a departure or mixed

flow [F(2,64)=4.98, p=.01]. However, the statistically

significant ARTCC Facility by Traffic Flow interaction

[F(6,64)=2.39, p=.038], displayed in Figure 3, shows the

main effect of traffic flow isrestricted to the controllers at

the Chicago and Los Angeles ARTCCs who sent longer

transmissions only when traffic was on an arrival flow;2

otherwise,messagelengthwas comparable acrossARTCC

facilities and traffic flows. 

Table 9 shows the distribution of ATC messages according to their complexity level(CL) andmessagelength

(ML). An examination of therowtotals(see column “CL

Percentages”) reveals that, unlike the findings reported

by Cardosi (1993), where 78% of the ATC messages had

one (e.g., frequency change) or two pieces of informa2 Post hoc comparisons were performed using the Tukey statistic.

tion (e.g., turn left heading 090), less than 1% of the

transmissions analyzed in this report did (.40% had a

CL = 1 [e.g., normal speed, ident], and another 9% had

a CL = 2 [e.g., start your decent now]). 

Table9revealsthat51%oftheATCtransmissionswith

a ML = 1 had a CL = 6-7. For example a route clearance

such as “Cleared direct camrn” (CL = 2) is less complex

than“Cross hector at and manintain flight level two four

zero twenty four,” (CL = 8), which is less complex than

“Cleared to houston after dovey direct nantucket direct

boston direct charlie mike kilo jet seven five to golf victor

echo ject three seven to sierra juliett india direct hotel

romeo victor direct lima lima alfa direct victor uniform

hotel direct papa sierra xray and the gland three arrival

to houston”(CL = 24).

The most complex ATC message had a CL = 31 and

it contained three speech acts (route clearance, altitude,

and speed instructions): “Cleared to delta golf alfa alfa

airport after champ direct creeq direct kwlty three five

three zero north six zero west three three five zero north

five five west three one five zero north five zero west two

niner five zero north four five west two seven two zero

north four zero west two four three zero north three

31%

9%

4%

0%

26%

14%

6%

3%

7%

Altitude

Altitude Restrictions

Heading

Hold Instructions

Radio Frequency

Route/Position Clearance

Speed

Transponder

Altimeter

Figure 2. Percentages of ATC Instruction/Clearance Aviation Topics. 

Table 8. Frequency Distribution of the Types of Speech Acts Presented by Message Length.

Types of Speech Acts 

Message Length 

Clearances and 

Instructions 

Altimeter 

Advisory Combination Total Percentage 

1 82.2% 2.1% 84.3% 

2 9.0% 5.1% 14.1% 

3 .8% .6% 1.4% 

4 .1% .1% 

Total Percentages 92.1% 2.1% 5.8% 100.0% 
Table 9. ATC Messages Presented by Level of Complexity and Message Length.

Message Length 

Complexity Level 1 2 3 4 CL Percentages 

1 .40% 0.40% 

2 9.00% 9.00% 

3 5.72% 5.72% 

4 7.80% .31% 8.11% 

5 9.13% .19% 9.32% 

6 28.67% .74% 29.41% 

7 20.54% 1.04% 21.58% 

8 2.94% 3.75% .02% 6.71% 

9 .02% 4.00% .26% 4.28% 

10 .02% 1.45% .17% 1.64% 

11 .05% 2.04% .24% 2.33% 

12 .33% .12% .02% .47% 

13 .21% .10% .31% 

14 or more1 .10% .52% .10% .72% 

ML Percentages 84.32% 14.15% 1.42% .12% 100.00% 

1

 CL ranged from 1 – 31. There were only 3 transmissions with 4 communication elements and a CL  12. 

1

1.4

1.8

ZAU

Chicago

ZLA Los

Angeles

ZMA

Miami

ZNY 

New

York

ZOA

Oakland

ARTCC

Mean Number of Speech Acts 

per Message

Arrival

Departure

Mixed

Figure 3. ATC Message Length Presented by ARTCC Facility and Traffic Flow. 
10

five west eltin gamba maintain flight level three two zero

for now maintain mach point eight five.”

Pilot Responses to ATC Messages. In response to

the 4,261 ATC messages, there were 3,437 full readbacks, 363 partial readbacks, 126 acknowledgment only

(e.g., “Roger,” “Wilco”), 26 courtesies such as “Thank

you,” 160 messages with no acknowledgment, and 17

transmissions were retransmitted by ATC because they

were previously blocked. In addition to these messages,

there were 152 other replies, of which 76.3% involved

uncertainty regarding previous ATC instructions. That

is, they included transmissionssuch as “Confirm that was

twenty eight point one five,” “Verify that’s up to three

three zero,” and “Say again secondaries.”

Table 10 shows that 81.2% of the pilots’ responses

contained full readbacks of the controller instructions,

advisories, or both. Whereas in Cardosi’s 1993 report,

full readbacks occurred for 71% of previously issued

ATC messages, the data presented here indicate a 10.2%

increase in full readbacks with a corresponding decrease

in partial readbacks — down from 12% in the Cardosi 

report to 7.9%. We took the category ‘Other Replies’

which constituted another 8% of pilot responses in

the Cardosi report and split it into ‘Other Replies’ and

‘Courtesy.’Together,they accountedfor3.3%ofthepilot

responses. Only 2.8% of the pilots responded with an

acknowledgment only.

Of the 3.8% ATC messages that received no pilot

acknowledgement, 91.9% of them had one (86.3%) or

two (5.6%) instructions while another 5.0% involved

only altimeter settings. Of the unacknowledged singletopic instructions, 49.0% involved changes to radio

frequency. Altitude(16.0%),transponder(11.0%),head-

ing (5.0%), route/position (8.0%), speed (6.0%), and

altitude restriction (5.0%) comprised the remainder of

unacknowledgedsingle-topic instructions. Theremaining

3.1% unacknowledged messages were a combination of

instructions and advisories that contained two (2.5%)

or more than two topics (0.6%).

Pilot Use of Call Signs in Readbacks. The types of

callsigns used by pilots and theirrepresentativeexamples

are shown in Figure 4. In Table 11, 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 to 4,261 ATC messages. 

Table 11 shows that pilots provided either a complete (74.5%) or partial (14.5%) call sign in 89% of

their responses to ATC. Production of incorrect call

signs constituted 1.3% of their responses. There were

59 transmissions where pilots provided incorrect call

signs (replacement of the assigned call sign with that of

another). In 27 of these transmissions, the incorrect call

signsresultedfrom importingnumbersorlettersnotfound

in the actual call sign. Forexample, the pilot of Ownship

2577 responded with, “Ownship twenty twenty-seven.”

In 32 other transmissions, pilots omitted some numbers

either at the beginning (Ownship 049 was called ‘Ownship four nine’), middle(Ownship 901 became‘Ownship

ninety one’), or at theend of the call sign (Ownship 1512

became ‘Ownship fifteen’) failed to include the prefix

(‘Ownship’), or gave only a partial prefix.

Analysis of Pilot Faulty Readback Performance

As noted in Table 11, 89% of the selected controller transmissions were responded to with either a full

or partial readback. Of these 3,799 readbacks, 28.7%

(n=1,089) were read back correctly, complete with all

anchor words. The remaining 71.3% faulty readbacks

Table 10. Pilot Responses to ATC Messages.

Types of ATC Messages 

Types of Pilot Response 

Instructions 

Only

Altimeter

 Only 

Instructions and 

Altimeter

Percent of all 

Messages 

Full Readback 76.6% 1.4% 3.1% 81.2%*

Partial Readback 6.4% .1% 2.2% 8.6% 

Acknowledgment Only 2.5% .2% .0% 2.8% 

Other Replies 2.5% .0% .3% 2.8% 

Courtesy .4% .1% .0% .5% 

No Acknowledgment 3.3% .2% .1% 3.8% 

Blocked .3% .0% .1% .4% 

Table Total 92.1% 2.1% 5.8% 100.0% 

*

Subject to rounding error 
11

Call Sign Usage ACID Example 

 Complete UAL56H UNITED FIFTY SIX HEAVY 

 Partial 

Prefix w/ some numbers/letters 

Inc. prefix w/ all numbers/letters 

No prefix w/ all numbers/letters 

No prefix w/ some numbers/letters 

DAL884 

ACA1017 

TRS467 

GWY256 

DELTA EIGHTY FOUR 

CANADA TEN UH SEVENTEEN 

FOUR SIXTY SEVEN 

FIFTY SIX 

 Incorrect call sign N21828CG TWO CHARLIE GOLF 

 Unintelligible AAL538 AMER (UNINTELLIGIBLE) 

 No call sign 

Figure 4. Examples of Various Types of Pilot Call Sign Usage. 

Table 11. 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 Blocked Percent

 Complete 63.6% 7.3% 1.6% 2.0% 74.5% 

 Partial 12.8% .8% .5% .4% 14.5% 

 Incorrect call sign .9% .3% .1% 1.3% 

 Unintelligible .1% .1% .2%

 No call sign 6.3% .4% .9% 1.4% .6% .4% 10.0% 

Table Total 83.7% 8.9% 3.1% 3.8% .6% .4% 100.5%*

*Rounding errors
12

were categorized according to three types of errors: Errors of omission only (67.4%), Readback errors only

(0.9%), and Readback errors combined with Errors of

omission(3.1%). Pearsoncorrelationsrevealedthaterrors

of omissions increased significantly as the complexity, r

(3799) = .46 and message length (i.e., number of ATs),

r(3799) = .29 in a controller’s message increased, p<.05. 

Similarly,thenumber ofreadback errors ina transmission

increased with its complexity, r (3799) = .11 and length,

r (3799) = .14, p<.05.

Pilot Errors of Omission. Readbacks are predicated

on the Aeronautical Information Manual § 4-4-6b,which

suggests that the pilot read back the numbers in altitude

assignments or vectors. FAA Order 7110.65 § 2-4-3

instructs the controller to verify the accuracy and completeness of any readback. § 2-4-3, while instructing the

controller to ensure the completeness, has by common

usage and knowledge omitted frequency readbacks from

this requirement. This is a logical move in communication, as the frequency band used throughout these

transcriptions is in the 100 range, i.e., a readback of a

frequency instruction “one one nine point two five” read

back as “one nine two five” would be considered an error

of omission (i.e., failure to include the anchor ‘point’ or

‘decimal’ but NOT a readback error. 

Errors of omission were classified into threetypes: (1)

exclusion of anchor words (absence of ‘point’ in a radio

frequency, ‘flight level’ or ‘thousand’ in altitude, etc.),

(2) exclusion of some digits (in response to “Contact

center one one nine point two five,” thepilotreplies “point

twenty five”), and (3) the exclusion of anchor word(s)

and digit(s) (in responseto “Contact center one one nine

point two five,” the pilot replies “nine twenty five”).

Therewere2,559pilotfaulty readbacksthat contained

errors of omission.The data presented inTable 12 under

the columnlabeled“Percentagesof allPilotTransmissions”

were computed using the 3,799 full and partialreadbacks

in the denominator(e.g., 2,506/3,799),while data in the

columnlabeled “Percentage ofTransmissionswithOmissions” used the 2559 faulty readbacks in the denominator

(e.g., 2,506/2,599). When pilots attempted either a full

or partial readback, 67.4% of their readbacks contained

oneormoreerrors ofomission. Ofthese omissions,nearly

98% involved the absence of an anchor word. 

Aviation Topics with Errors of Omission. Table 13

presents the distribution of errors of omission according

to the types of ATs read back incorrectly. Column (a)

Table 13. Distribution of Pilot Errors of Omission by Aviation Topic.

Type of Aviation Topic 

Number of AT in 

all Pilot 

Readbacks 

(a) 

Number of 

Errors of 

Omission

(b)

Proportion of 

Errors of 

Omission

(c)

Percentage of 

Errors of 

Omission

(d)

 Altimeter 244 185 6.83 75.82 

 Altitude 1394 933 34.44 66.93 

 Altitude restriction 396 255 9.41 64.39 

 Heading 205 109 4.02 53.17 

 Holding 9 1 0.04 11.11 

 Radio frequency 1158 876 32.34 75.65 

 Route/Position 650 134 4.95 20.62 

 Speed 276 154 5.68 55.80 

 Transponder 102 62 2.29 60.78 

 Total 4434 2709 100.00 

Table 12. Prevalence of Pilot Transmissions with Errors of Omission. 

Types of Omission Errors 

Number of 

Readbacks 

Percentage of all 

Pilot Transmissions 

Percentage of Transmissions 

with Omissions 

Omit anchor word (s) 2506 66.0% 97.9% 

Omit digit (s) 9 .2% .4% 

Combination of both 44 1.2% 1.7% 

Total 2559 67.4% 100.0% 
13

presents the number of ATs read back in all of the 3,799

pilot readbacks. Column (b) presents the total number

of omission errors contained in the 2,559 pilot transmissions with one or more omissions. The proportion

of omission errors that are presented in Column (c) was

computed using the number of instances a particular

aviation topic was read back with an omission divided

by thetotal number oferrors of omission (e.g., Altimeter

readback errors = 185/2,709*100). 

Column (c) of Table 13 shows that 43.85% of the

2,709 identified omission errors involved an altitude

(34.44%) or altitude restriction (9.41%). Changing to

another radio frequency captured an additional 32.34%

of the errors of omission. The remaining 24% errors of

omission were fairly well distributed among altimeter,

speed, route/position, and heading instructions, with

transponder and holding instructions accounting forless

than 3% of these errors of omission.

The percentages presented in Column (d) were computedbydividing thenumberof instancesthat aparticular

aviation topicwasread back incorrectly by thetotal number of readbacks of that aviation topic found in Column

(a) (e.g., Altimeter readback errors = 185/244*100). It

showsthatslightly morethan 75% of altimeter and radio

frequency ATs contained errors of omissions, followed

by more than 60% of the altitude, altitude restriction,

and transponder readbacks, and greater than 50% of the

heading and speed assignments. Pilots were less likely to

omit some of the contents of their readback of holding

and route/position instructions.

Pilot Readback Errors. During periods of heavy traffic,radio frequency congestion is often cited as a problem

affecting communication efficiency (Data Link Benefits

Study Team, 1995). Following the delivery of an ATC

transmission, the controller listens for the pilot to read

back accurately the original message. The presence of

a mistake is called a readback error. Failure to detect a

readback error is a hearback error. Improperly phrased

readbacks that contain the correct information are not

readback errors. 

Each pilot readback was evaluated for accuracy. Also,

the type of readback error and the number of readback

errors were recorded (e.g., a zero indicated no error,

while a value of 3 indicated three errors). Also examined

was the prevalence of pilot readback errors as a function

of ATC message complexity and message length. Each

ATCtransmission that met theselection criterion (i.e., it

contained an instruction, an advisory, or a combination

of instruction and advisory) was paired with the pilot’s

responseto thattransmission. When a pilotread back the

contents of a controller’s transmission, the same criteria

found in FAA Order 7110.65 was applied to the evaluation of that readback. 

Theresultspresented inTable14werederivedfrom151

pilottransmissionswith one ormoresubstitutions,transpositions, or their combination with errors of omission. 

The data presented under the column labeled “Percentages of All PilotTransmissions”were computed using the

3,799full andpartialreadbacks inthedenominator,while

data in the column labeled “Percentage ofTransmissions

with Readback Errors” used the 151 pilot transmissions

with readback errors in the denominator. Four percent

of the pilots’ readbacks contained one or more readback

errors, of which 78% resulted from a combination of an

error of omission with one or more substitution (89%)

or transposition (11%) errors.

Aviation Topics With Readback Errors. Table 15

presents the distribution of readback errors according

to the types of ATs read back incorrectly. Column (a)

presents the number of ATs read back in all of the 3,799

pilotreadbacks. Column (b) presentsthetotal number of

readback errors contained in the 151 pilot transmissions

containing one ormorereadback error. Theproportionof

readback errors presented in Column (c) was computed

using the number of instances a particular aviation topic

was read back incorrectly divided by the total number

of readback errors (e.g., Altimeter readback errors =

15/162*100). 

Table 14. Prevalence of Pilot Transmissions With Readback Errors.

Type of Readback Error 

Number of 

Transmissions with 

Readback Error (s) 

Percentage of All 

Pilot Transmissions 

Percentage of 

Transmissions with 

Readback Errors 

Transposition 3 .1% 2.0% 

Substitution 30 .8% 19.9% 

Combination of readback error types 118 3.1% 78.1% 

Total 151 4.0% 100.0% 
14

Column (c) of Table 15 shows that 30.25% of the

162 identified readback errors involved an instruction

to change to another radio frequency. Altitude and

altitude restrictions captured an additional 36.42% of

thereadback errors. Route/position clearances wereread

back incorrectly 10.49%, altimeter contributed another

9.26%,while heading and speed instructionseach added

another 5.56%to thetotal proportion ofreadback errors. 

Finally, holding and transponder instructions captured

the remaining readback errors. These findings replicate

those of Cardosi (1993) who reported radio frequency,

followed by crossing restrictions, altitude, and altimeter

ATs were the most common readback errors.

The results presented in column (c) of Table 15 fail

to take into account the frequency of delivery of those

ATs by controllers. There may be more opportunities to

incorrectly readbacka radiofrequencyor altitude instruction simply because controllers issued them more often. 

Therefore, another analysiswasperformedthat compared

the number of readback errors of a particular aviation

topic (e.g., altimeter) to the total number of readbacks

of that AT. The percentages presented in Column (d)

were computed by dividing the number of instances

that a particular aviation topic was read back incorrectly

by the total number of readbacks of that aviation topic

found in Column (a) (e.g., Altimeter readback errors =

15/244*100). The data show that 22% of the holding

instructions were read back incorrectly, although they

were few in number. 

Message Complexity and Pilot Readback Performance. As seen in Figure 5, pilots read back correctly

72% of the ATC messages that had a CL that was no

greater than 3, and their performance dropped to less

than 15% accuracy once the CL of an ATC message

­increased to 7 or greater. To determine whether the

increase in faulty readbacks was statistically reliable,

ANOVAs were conducted separately on errors of omission and readback errors.

The number of pilot errors of omission and number

of readback errors were aggregated by facility, sector,

and complexity value. The One-Way ANOVA revealed

that Message Complexity had a statistically significant

effect on the production oferrors of omission [F(4,89) =

52.41] but not readback errors [F(4,89) = 1.97]. As seen

in Figure 6, Fisher LSD post hoc comparisons revealed a

steady increase in the mean number oferrors of omission

as the complexity value of ATC messages increased. The

mean number of omission errors were comparable at CL

6 and CL 7 only [(CL 8 > CL 7, CL 8 > CL 6, CL 8 >

CL 4-5, and CL 8 > CL ≤ 3); (CL 7 = CL 6, CL 7 > CL

4-5, CL 7 > CL ≤ 3); (CL 6 > CL 4-5, CL 6 > CL ≤ 3);

(CL 4-5 > CL ≤ 3)].

Message Length and Pilot Readback Performance

The number of pilot errors of omission and number of

readback errors were aggregated according to thefacility, sector, andmessagelength. TheOne-WayANOVArevealed that

Message Length affected the production oferrors of omission

[F(2,45) = 5.97] and readback errors [F(2,45) = 3.61]. As

seen in Figure 7, Fisher LSD post hoc comparisons revealed

more errors of omission as ATC message length increased

from one to two or more aviation topics, [(NAT 3-4 > NAT

2, NAT 3-4 > NAT 1); (NAT 2 = NAT 3-4)]. Likewise, an

examination of the mean number of pilot readback errors us-

ing Fisher LSD post hoc comparisons indicates a statistically

significant increase in the production of pilot readback errors

onceATCmessagelength exceedstwo aviation topics,[(NAT

3-4 > NAT 2, NAT 3-4 > NAT 1; NAT 3-4 = NAT 2)].

Table 15. Distribution of Pilot Readback Errors by Aviation Topic. 

Type of Aviation Topic 

Number of AT in 

all Pilot 

Readbacks 

(a) 

Number of 

Readback 

Errors

(b)

Proportion of 

Readback 

Errors

(c)

Percentage of 

Readbacks in Error

(d)

 Altimeter 244 15 9.26 6.15 

 Altitude 1394 36 22.22 2.58 

 Altitude restriction 396 23 14.20 5.81 

 Heading 205 9 5.56 4.39 

 Holding 9 2 1.23 22.22 

 Radio frequency 1158 49 30.25 4.23 

 Route/Position 650 17 10.49 2.62 

 Speed 276 9 5.56 3.26 

 Transponder 102 2 1.23 1.96 

 Total 4434 162 100.00 
15

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

CL 3

N=546

CL 4-5

N=652

CL 6

N=1143

 CL 7 

N =826

CL  8

N=632

ATC Message Complexity Level

Pilot Readback Performance

Readback Errors

Errors of Omission

Correct Readback

Figure 5. Pilot Readback Accuracy Presented by ATC Message Complexity. 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

CL 3 CL 4-5 CL 6 CL 7 CL  8

ATC Message Complexity Level

Mean Number of Errors

Errors of Omission

Readback Errors

Figure 6. Mean Number of Pilot Omission and Readback Errors Presented by 

Message Complexity. 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

SHORT (NAT 1) MODERATE (NAT 2) LONG (NAT 3-4)

ATC Message Length (Number of Aviation Topics)

Mean Number of Errors

Errors of Omission

Readback Errors

Figure 7. Mean Number of Pilot Omission and Readback Errors Presented by 

Message Length. 
16

Analysis of Pilot Requests for Repeat of an ATC 

Transmission

There aretimeswhilepilots arebusy completing checklists or performing otherstation-keeping taskswhen they

hear, or think they hear, their aircraft’s call sign and will

ask ATC if they are calling them. At other times, 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 still other instances,

they may request confirmation of the aviation topics that

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

ten thousand). 

There were 112 requests for ATC to repeat an earlier

transmission. We identifiedfourdifferenttypesofrequests:

(1) 25% requests for the repetition of a specific AT, (2)

23.2% requests for the repetition of an entire transmission, (3) 39.3% confirmation/verification of a specific

AT, and (4) 12.5% confirmation that the transmission

was for them.

Requests for Repeat of a Specific AT. A detailed

examination of 28 pilot transmissions requesting that

ATCrepeat a specific aviation topic is presented in Figure

8. It shows that route clearances (43%) and instructions

to switch to another radio frequency (29%) were more

frequently requested “say agains” than altimeter (11%),

speed (7%), altitude(4%), altituderestriction (4%), and

transponder assignments.

Types of Aviation Topics Requested: Message Length 

and Complexity. Amore comprehensiveexamination of

the pilot and controller pairs was performed that took

into account both ATC message length and complexity

value. It revealed that when messages were short (NAT

= 1) and less complex (CL > 3) only radio frequencies (n

= 1) and route (n = 7) were requested. When messages

werestill short but somewhat complex (CL = 6-7), pilots

requested radio frequencies (n = 6) and a route clearance

berepeated (n = 1). Therewas only onerepeatrequest for

a short message that was considered high in complexity

(CL ≥ 8), and it was a route clearance.

AsATCmessages increasedtoamoderatelength(NAT

= 2) but were less complex (CL ≤ 3), there were no say

agains for a specific aviation topic. There was one ‘say

again altitude’ for an ATC message that was somewhat

complex (CL = 6-7). Messages that were high in complexity (CL ≥ 8) had one request each for the repetition

of an altitude restriction, speed, radio frequency, and

transponder; two requests for a route; and three requests

for the altimeter.

ForlongATCmessages(NAT= 3),requestsforrepeats

only occurred for complex messages from ATC(CL ≥ 8). 

There was one each for speed and route.

Requests for the Repetition of an Entire Transmission. There were 26 pilot requests to have ATC repeat

an entire message. When the ATC transmission was

short (NAT = 1) and increased in complexity, there was

a general increase in the number of ‘say agains’ (CL ≤ 3

n= 3; CL = 4-5, n = 2; CL = 6-7, n = 12; CL ≥ 8, n =

3). This same pattern was present for moderate length

ATC Messages (CL = 6-7, n = 1; CL ≥ 8, n = 3) and for

long messages (CL ≥ 8, n = 2).

Confirmation/Verification of a Specific AT. There

were 44 pilot requests for confirmation for a specific

aviation topic. Figure 9 shows that the more frequently

requestedconfirmations involvedradiofrequency(27.3%)

and altitude (22.7%) assignments, route clearances

(25.0%), and altitude restrictions (11.4%). Infrequent

were confirmations that involved altimeter and transponder settings (each 2.23%) or heading and holding

instructions (each 2.23%).

Types of Aviation Topics Requested: Message Length 

and Complexity. A closer examination of pilot requests

for confirmation also took into account ATC message

length and complexity value. It revealed that when messages were short and less complex (NAT = 1, CL < 3),

Figure 8. Requests for Repeat. 
17

pilotswanted confirmation oftheirroute(n = 6), altitude

restriction (n = 2), altitude(n = 1), and transponder (n =

1). Whenmessageswerestillshortbut increasedslightly in

complexity (CL = 4-5), pilots againwanted confirmation

of their speed (n = 2), route (n = 1), and altitude (n = 1)

assignments. Shortbutsomewhat complexATCmessages

(CL = 6-7) entailed confirmation for radio frequency (n

= 12), altitude(n = 5), route(n = 2), and heading (n = 1)

aviation topics. Therewere no requestsforshortmessages

that were considered high in complexity.

As ATC messages became moderate in length (NAT

= 2) and weresomewhat complex (CL = 6-7), there were

requests for confirmation of an altitude (n = 2) and a

route (n = 1). Once ATC messages were high in complexity, pilots made three requests for the repetition of

an altitude restriction and one request each for altitude,

altimeter, holding, and route. There were no requests for

the confirmation of any ATs for long messages.

Pilots Request Confirmation That the Transmission 

was for Them.Therewere 14 pilotrequeststhat the controller confirm that the preceding message was for them. 

Themessage pair generally took theformofATCsending

a message to Ownship1234 and the pilot replying with

“... that for Ownship 1234?” Of these queries, five were

in response to the controller calling Ownship1234, but

with an incorrect call sign. Each of these ATC messages

were short (NAT = 1) but varied in complexity (CL ≤ 3,

n = 1; CL = 4-5, n = 1; and CL = 6-7; n = 3). 

There were seven other ATC short (NAT = 1) transmissions that varied in complexity (CL ≤ 3, n = 3; CL

= 4-5, n = 1; CL = 6-7; n = 2; CL ≥ 8, n = 1). Finally,

there were two ATC transmissions of moderate length

that were high in complexity (CL ≥ 8). For all of these

14 requests, rather than executing an instruction that

might have been intended for another aircraft, the pilot

appropriately questioned the controller.

DISCUSSION

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 en route environment. Five different

geographical areas in the contiguous United States were

contacted for 10 hrs each of pilot-controller voice communications that resulted in the analysis of 51 hrs of

communications. Both U.S. and foreign registry aircraft

were represented, as well as general aviation aircraft. On

average, across the five sampled ARTCC facilities, one

aircraft requested and received air traffic services every

2 min 4 s. The number of ground-to-air transmissions

averaged 4.62messages per aircraft and approximately 10

messages were exchanged (from initial contact until the

aircraft was switched to the next controller in sequence)

that involved an allocation of about 28 s of airtime per

aircraft. The most frequently transmitted instructions

involved changes in altitudes and switching pilots to

another radio frequency. 

UnlikethefindingsreportedbyCardosi (1993),where

78% of the ATC messages involved one or two pieces of

information, we found that less than 1% of the controllers’ messages did. In fact, our data revealed that 51% of

the controllers’ transmissions had a Complexity Level =

6-7 and generally ranged from very low (CL = 2) to very

high (CL = 31) in complexity. The messages very high

in complexity involved route clearances. The number of

aviationtopics(messagelength)wasmore inkeepingwith

Cardosi’s approach to determining message complexity. 

In this report, short messages with only one aviation

topic occurred in 84% of the controllers’ transmissions,

and messages with two aviation topics occurred in 14%

of the transmissions.

Figure 9. Requests for Clarification. 
18

The disparity between thesereportsresides partially in

how the concept of message complexity was measured. 

ForCardosi (1993),theATCtransmission “contactMinneapolis Center 118.8” (p3) was considered one piece

of information if the pilot was already on a Minneapolis

Center frequency and as two pieces of information if

the pilot received it while communicating with a different ARTCC. Prinzo et al. (2006) considered the same

transmission to contain one communication element

— radio frequency —and it had a complexity value of

6. Therationale was thateach digit comprising theradio

frequency imposedamemory loadthat couldresult inthe

pilot erroneously reading back the wrong facility/function, omitting, transposing, or substituting any (or all)

of the digits in the radio frequency, or both. 

Sincethe publicationofCardosi’s 1993 report 15 years

ago, we report a 10.2% increase in full readbacks and a

3.4% decrease in partial readbacks. It is of no surprise

then that there is also a decline in replies such as ‘Roger,’

‘See ya,’ and ‘Thanks’ in lieu of full and partial readbacks. 

This is encouraging because ‘Roger’ and ‘Thanks’ fail to

providethe controllerwith confirmationthatthe instruction/clearance, advisory, and related information was

received and understood by the pilots. Unfortunately,

both reports show no change in the number of ATC

transmissions that went unacknowledged (3%).

Also encouraging to improvements in aviation safety

is the trend among pilots to include either their full or

a partial call sign as part of their readbacks. Controllers

should never have to infer the identity of a pilot from

voice qualities alone. Overall, pilots included either a

complete or partial call sign in 89% of their readbacks. 

Moreover, pilots who provided a full readback also included the complete call sign in approximately 64% of

their responses. The fact that call signs were excluded in

10% of their readbacks and 0.2% of thespoken call signs

wereunintelligibleleavesroomfor improvement. Theuse

of an incorrect call sign was infrequent — occurring in

1.3% of their responses to ATC. In approximately 45%

of these instances, pilots either included some numbers

or letters that were not part of their call sign; while in the

remaining 55%, numbers or letters were omitted.

As noted previously, 71.3% of the pilot responses to

ATC contained faulty readbacks, of which 67.4% contained one or more errors of omission. The increase in

faulty readbackperformancewas attributedtoa steady rise

in errors of omission brought on by the added complexity

of ATC messages. This is not altogether surprising, given

the high memory load imposed on the pilot’s working

memory capacity and thefactthat verbatimrecall ofATC

messages is not a requirement. It seems that pilots shed

theredundant words present in aviation topics that were

not altogether necessary for navigating the flight plan. 

This is comparableto retaining the gist of a conversation

rather than its exact wording. Omitting non-essential

words during readback still preserves the identity of the

aviation topic and has no adverse affect on pilot’s actions

(Barshi & Healy, 2002).

The remaining 4% faulty readbacks contained at

least one readback error. An examination of these faulty

readbacks revealed that less than 1% involved readback

errorsonly. Theremaining3.1%faultyreadbacks involved

the combination of an error of omission coupled with

one or more substitution (89%) or transposition (11%)

readback errors. Unlike message complexity, message

length affected both the production oferrors of omission

and readback errors. There were moreerrors of omission

as ATC message length increased from short (one aviation topic), to moderate (two aviation topics), and long

(three or more aviation topics). Redundant information

might beeliminated to increasetheeconomy of capacity

limitations of verbal working memory. 

Readbackerrors increasedonceATCmessages included

two ormore aviation topics. Themost common readback

errors involvedaltitude andaltituderestrictions,followed

byradiofrequency,route/positionclearance, andaltimeter

settings. These findings agree with research investigating

the capacity limitations of verbal working memory that

suggest that the upper limit for propositional representations (Barshi & Healy, 2002) and sentence processing

(Lewis, 1996) is three.

Someofthepilotreadbacksthatwerescoredasreadback

errors involved pilot use ofslang. That is, anATC instruction to ‘Contact center on one one niner point zero,’

was occasionally read back as ‘nineteen zip’ or ‘nineteen

nothing.’ Clearly, ‘nineteen nothing’ and ‘nineteen zip’

are not part of the ICAO alphabet or aviation accepted

pronunciation of numbers. Both readbacks areexamples

of substitution errors (and omission of the anchor word

‘point’). What is worrisome about these readbacks is

that other pilots may hear the slang and not understand

what the pilot intends to do (especially pilots who are

less proficient in aviation English).

There were several pilots who chose to read back

flight levels in thousands of feet — the ATC instruction ‘descend and maintain flight level one nine zero’

read back as, ‘nineteen thousand,’ or ‘to level one nine.’

Instructions to ‘reduce speed to two five zero’ read back

as ‘two five’ or ‘down to two five zero’ are ambiguous

and subject to misinterpretation. Again, these examples

illustrate substitution errors made during read back that

may have adverse affects on safety.

The low number of pilot requests for the repeat of an

ATCtransmissionwas particularly encouraging. Possibly

to avoid the risk of either reading back the contents of

an ATC transmission meant for another aircraft (stolen
19

transmission) or reading back the contents incorrectly

(readback error), pilots asked the controller for either

a full or partial repeat of the original message. Of these

112 pilot requests, five involved a call sign misspoken

by the controller to which the pilot wanted confirmation that the message was for them. Of the remaining

107 requests, 37% involved the repetition of the entire

transmission. Acloserlook atthesetransmissionsshowed

that 67.5% had either a fix/intersection or facility name

included as part of their content3 — and many of these

messages involved route clearances and the transfer of

communications.

The inspectionoftheATCmessagesthatpilotswanted

confirmedorrepeatedrevealedthatmanyofthesemessages

also involved route clearances that varied in complexity

from less complex (CL < 3 clear direct [name]) to high

in complexity [CL > 8 Fly heading three four zero / intercept the [name] one six two radial track inbound join

the [name] five arrival off that]. In both examples, pilots

asked forthe name ofthe fix/intersection,the name ofthe

radial, name of the arrival route, etc. Clearly, the names

of location identifiers, fixes, intersections, navigational

aides, etc. are critical to navigation; and the pilots were

prudent in requesting either the repetition or confirmation of previously heard identifiers.

Likewise, the same pattern of results was found when

pilots requested that a radio frequency be repeated or

confirmed. In such instances, radio frequencies were

somewhat complex (CL = 6-7) and varied according to

whetherthe name of thefacilitywas included, alongwith

the numbers in the radio frequency and the number of

digitsfollowing the anchor‘point’(Contact [name] center

on one three four point five five) or not (Contact center

on one three four point five five orContact [name] center

on one two four point seven).

Based onthefindings inthisreport, it isrecommended

that:

1) No more than three aviation topics are present in

any ATCtransmission. Thereview of theliterature

and the findings of this report indicate an increase

in readback errors is more likely with increases in

message length;

2) If a route clearance is given, that it should be given

separately as a stand-alone transmission. This is

especially important when complex route clearances are transmitted by ATC. The findings in

this report show that as the complexity of an ATC

message increases, so does the frequency of errors

of omission during readback;

3 SeeFAAJO7400.2G. Procedures for Handling Airspace Matters(April

10, 2008) for details involved in naming conventions and references

to other FAA Orders.

3) The names of all fix, waypoint, location, etc.,

identifiers be repeated, and if necessary, spelled

out following their first recitation; for example,

‘Cleared direct cobad that’s charlie oscar bravo

alfa delta,’ or ‘Cleared direct cobad c-o-b-a-d.’

This could aid pilots flying for non-U.S. registry

airlines who may not be familiar with how U.S. 

identifiers arepronounced. Likewise,sometimesthe

spelling of a location identifier is not intuitive, or

in agreement with its pronunciation (e.g., CVE =

Cowboy). In such cases, the controller may want

to spell out the 3-letter identifier to aid pilots in

locating it on their maps;

4) Slang should not be accepted as part of a pilot

readback. There is a degree of uncertainty when

slang is introduced into the recitation of an ATC

message. Ambiguity creates doubt and compromises safety;

5) Effort be undertaken to reduce excessive words/

phrases — on, your, to, is, etc. The phraseology

created by the FAA is precise and needs no further

embellishment.

As part of itsmodernization programs,the FAA, along

with avionics developers, may want to consider building

messages sets for complex messages and send them via a

datalink. Likewise,themoreroutineATCmessages could

be up-linked to the flight deck computers according to

application. For example, route clearances that contain

the names of all fix, waypoint, location, and other identifiers could be up-linked to a navigation display (e.g.,

moving map) for pilot consideration and then either

accepted, rejected, or negotiated. Traffic advisories could

be displayed on an ADS-B/CDTI(automatic dependent

surveillance broadcast/cockpit display of traffic information). Also,by changing therepresentationofthemessage

away fromtextto graphic, complexitybecomessimplified

and easier to process.

References

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Baddeley, A.D. (2000). Theepisodic buffer: A new component of working memory? Trends in Cognitive 

Sciences, 4, 417-23.

Baddeley,A.D. and Hitch, G. (1974). Workingmemory. 

In G. H. Bower (Ed.), The Psychology of Learning 

and Motivation (Volume 8, 47-89). New York:

Academic Press.
20

Baddeley, A.D., Thomson, N., and Buchanan, M. 

(1975). Word length and the structure of shorttermmemory. Journal of Verbal Learning and Verbal 

Behavior, 14, 575-89.

Barshi, I. and Healy, A. (2002). The effects of mental

representation on performance of a navigation task. 

Memory & Cognition, 30, 1189-1203.

Cardosi,K. 1993. An analysis ofen route controller-pilot

voice communications. DOT-VNTSC-FAA-93-2. 

Cambridge, MA: Volpe National Transportation

Systems Center.

Cardosi, K., Brett, B., and Han, S. (1996). An analysis

ofTRACON (Terminal Radar Approach Control)

controller-pilot voice communications. DOT/FAA/

AR-96/66. Washington, DC: Federal Aviation

Administration.

Data Link Benefits Study Team (1995). User benefits of

two-way data link communications: Aircraft delay

and flightefficiency in congested en route airspace. 

Final Report, DOT/FAA/CT/95/4. www.tc.faa.

gov/act300/act350/OnLineReports/Dlground/95_

4en_1.pdf Accessed 20 June 2008.

Final Report, DOT/FAA/CT-95-4, Federal Aviation

Administration Technical Center. (pp 126). Federal Aviation Administration. (2004). FAA Order 

7110.65P Air Traffic Control. www.faa.gov/ATPUBS/ATC/. Accessed 4 November 2005.

Federal Aviation Administration. (2008). Procedures for 

handling airspace matters. www.faa.gov/airports_

airtraffic/air_traffic/publications/atpubs/AIR/INDEX.HTM. Accessed May 6 2008.

Hayes, A.F. (2005). An SPSS procedure for computing 

Krippendorff’s alpha [Computersoftware]. Available

from www.comm.ohio-state.edu/ahayes/macros.

htm. Accessed 14 February 2006.

Krippendorff, K. (1980). Content Analysis, an Introduction to Its Methodology. Thousand Oaks, CA: Sage

Publications.

Lewis, R.L. (1996). Interference in short-term memory:

The magical number two (or three) in sentence

processing. Journal of Psycholinguistic Research, 25,

93-115.

Miller, G.A. (1956). The magical number seven plus or

minustwo. Somelimits on our capacity for process-

ing information. Psychological Review, 63, 81-97. 

Morrow, D. and Prinzo, O.V. (1999). Improving pilot/

ATC voice communication in general aviation. 

DOT/FAA/AM-99/21. Washington, DC: Federal

Aviation Administration.

Prinzo,O.V. (1996). Ananalysis of approach control/pilot

voice communications. DOT/FAA/AM-96/26. 

Washington, DC: Federal Aviation Administration.

Prinzo, O.V. and Britton, T.W. (1993). ATC/pilot voice

communications – A survey of the literature. 

DOT/FAA/AM-03/20. Washington, DC: Federal

Aviation Administration.

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

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AM-95/15. Washington, DC: Federal Aviation

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Language Variation and Change, 9, 81-105. 
A1

APPENDIX A 

Guide to the Computation of Complexity: ATC Instruction/Clearance Aviation Topics 

(*Minimum-Maximum Values) 

AVIATION 

TOPIC COMPLEXITY PHRASEOLOGY

ALTITUDE

6

6

6

5

4

*4-8 

*4-8 

*3-7 

*3-6 

*2-5 

*1-3 

4=FLIGHT LEVEL + (altitude) three digits 

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 (altitude)

THOUSAND

 one two twelve 

DESCEND/CLIMB & MAINTAIN FLIGHT LEVEL (altitude) 

 two three zero 

DESCEND/CLIMB & MAINTAIN (altitude) THOUSAND 

one zero 

DESCEND/CLIMB & MAINTAIN (altitude) THOUSAND 

four

CONTINUE CLIMB/DESCENT TO (altitude)

AMEND YOUR ALTITUDE DESCEND/CLIMB MAINTAIN (altitude) 

AMEND YOUR ALTITUDE MAINTAIN (altitude) 

DESCEND/CLIMB TO (altitude)

MAINTAIN (altitude)

(altitude, omitted “THOUSAND” “HUNDRED” “FLIGHT LEVEL”) 

ALTITUDE

RESTRICTION

*4-7 

*4-7 

*4-7 

*3-7 

*3-7 

*3-6 

*3-6 

2

2

1

1

4=FLIGHT LEVEL + (altitude) three digits

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

INCREASE/DECREASE RATE OF DESCENT THROUGH (altitude) 

EXPEDITE THROUGH/TO (altitude)

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

(Speed assignment) “THEN” DESCEND/CLIMB 

INCREASE/DECREASE RATE OF DESCENT 

GOOD RATE DOWN/YOUR BEST RATE 

EXPEDITE CLIMB/DESCENT -- CLIMB/DESCEND NOW 
A2

AVIATION 

TOPIC COMPLEXITY PHRASEOLOGY

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 

DISREGARD NOT INCLUDED IN COMPLEXITY 

GENERAL

ACKNOW.

 NOT INCLUDED IN COMPLEXITY 

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 

HOLDING

11

8

4

3

1

HOLD (direction) OF (fix/waypoint) ON (specified radial, course, bearing, 

track, airway, azimuth(s), or route) (number of minutes/miles) MINUTE/MILE 

LEG

HOLD (direction) OF (fix/waypoint) STANDARD PATTERN 

HOLD SHORT RUNWAY (number))

EXPECT FURTHER CLEARANCE (time)

HOLD 

RADIO

FREQUENCY

*6-7 

*5-7 

*4-5 

*3-4 

3

3

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)/(facility/function) (frequency w/o point)

(frequency + point)

(frequency) 

FREQUENCY CHANGE APPROVED 

CHANGE TO ADVISORY/MY FREQUENCY APPROVED 

CONTACT (facility/function) 

(facility/function) 

(change point, e.g. now, there, at/over marker/when established)
A3

AVIATION

TOPIC COMPLEXITY PHRASEOLOGY 

ROUTE/

POSITION

*5-7 

5

*3-5 

4

3

3

2

2

2

2

2

2

1

DIRECT (fix) (airway, course, localizer, arrival/departure, etc.) 

INTERCEPT/JOIN RUNWAY (name) LEFT/RIGHT LOCALIZER 

INTERCEPT/JOIN/RESUME (airway, course, localizer, arrival/departure, 

etc.) 

TURN LEFT/RIGHT DIRECT (point/fix) 

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 

Note: Omission of “rest of route unchanged” not encoded.

SPEED

6

*5-7 

5

5

5

5

4

4

3

3

3

2

2

2

1

3=MACH (number--digit point digit), 2=MACH (number--point digit) 

2=(speed) 

1=(number)

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

(point) 

REDUCE/INCREASE SPEED TO MACH (number) 

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 

RESUME NORMAL SPEED 

BEST FORWARD SPEED 

GO FAST -- NORMAL SPEED 

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 

B1

APPENDIX B.

Guide to the Computation of Complexity: ATC Advisory Aviation Topics 

(*Minimum-Maximum Values) 

Note: Plain language – all topics, except where otherwise noted, vary from minimum to 

maximum values. 

AVIATION 

TOPIC COMPLEXITY PHRASEOLOGY 

ALTITUDE

7

6

5

*3-7 

*3-7 

*3-6 

*2-3 

*2-4 

2

4=FLIGHT LEVEL + (altitude) three digits

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

EXPECT DESCEND/CLIMB & MAINTAIN FLIGHT LEVEL (altitude)

two three zero 

EXPECT DESCEND/CLIMB & MAINTAIN (altitude) THOUSAND 

 one zero 

EXPECT DESCEND/CLIMB & MAINTAIN (altitude) THOUSAND 

 four

(altitude) WILL BE YOUR FINAL ALTITUDE 

EXPECT DESCEND/CLIMB TO (altitude)

EXPECT MAINTAIN (altitude)

EXPECT (altitude, omitted “THOUSAND” “HUNDRED”)

EXPECT (altitude) 

HIGHER/LOWER SHORTLY 

ALTITUDE

RESTRICTION

*5-8 

*5-8 

*4-8 

4=FLIGHT LEVEL + (altitude) three digits

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

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

EXPECT MAINTAIN (altitude) UNTIL (point) 

EXPECT (altitude) TIL ESTABLISHED/LOCALIZER/ESTABLISHED ON 

LOCALIZER

APPROACH/

DEPARTURE

6

6

5

5

4

4

3

3

3

2

2

2

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

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

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

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

EXPECT ILS RIGHT/LEFT/CENTER APCH 

EXPECT DEPART RUNWAY (number)

ILS RIGHT/LEFT/CENTER APCH 

EXPECT ILS (name)

EXPECT RWY (name) – PLAN RWY (name) 

EXPECT APCH/LAND 

EXPECT (type) 

EXPECT VISUAL/ILS 

ATIS NOT INCLUDED IN COMPLEXITY 

EQUIPMENT

 AIRCRAFT TYPE WOULD BE IN THE TRAFFIC ADVISORY 

(Complexity of 2 included in TRAFFIC ADVISORY) 
B2

AVIATION 

TOPIC COMPLEXITY PHRASEOLOGY 

GENERAL

SIGHTING

 NOT INCLUDED IN COMPLEXITY 

GENERAL

ACKNOW.

 NOT INCLUDED IN COMPLEXITY 

HEADING

4

4

3

3

3

3

3

2

EXPECT TURN LEFT/RIGHT HEADING (degrees) 

EXPECT TURN (degrees) DEGREES LEFT/RIGHT 

EXPECTTURN LEFT/RIGHT (degrees)

EXPECT DEPART (fix) HEADING (degrees)

EXPECT HEADING (degrees) 

EXPECT TURN BASE/DOWNWIND (point)

EXPECT FLY HEADING (degrees) 

EXPECT VECTORS 

HOLDING

4

8

EXPECT HOLDING (fix) 

EXPECT HOLDING (fix) AS PUBLISHED 

NOTAM NOT INCLUDED IN COMPLEXITY 

RADIO

FREQUENCY

 NOT INCLUDED IN COMPLEXITY 

ROUTE/

POSITION

6

5

*4-6 

4-5 

3

3

3

3

*2-8 

2

2

2=(miles) two digits 

1=(miles) one digit

EXPECT INTERCEPT/JOIN RUNWAY (name) LEFT/RIGHT LOCALIZER 

EXPECT FURTHER CLEARANCE VIA J22 

EXPECT INTERCEPT/JOIN/RESUME (airway, course, localizer, 

arrival/departure, etc.) 

POSITION (miles) MILE(S) IN TRAIL 

EXPECT DIRECT (fix) 

EXPECT FOLLOW THAT TRAFFIC/HIM/THEM/MD80 

EXPECT VICTOR (airway number) J (route number) 

EXPECT TAXIWAY (number/letter)

RADAR CONTACT (position)/RADAR SERVICE TERMINATED (position) 

EXPECT INTERCEPT/JOIN LOCALIZER 

I’LL TURN YOU BACK SHORTLY 

SPEED

*7-8 

6

6

6

*5-6 

4

4

3-5 

3-4 

3

3=MACH (number--digit point digit), 2=MACH (number--point digit) 

2=(speed) 

1=(number)

EXPECT REDUCE/INCREASE SPEED TO MACH (number) 

EXPECT MAINTAIN (speed) KNOTS OR GREATER OR MAINTAIN SPEED 

(speed) OR GREATER 

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

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

TO (speed) KNOTS 

EXPECT (speed) KNOTS PRIOR TO (time/fix/altitude)

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

EXPECT HIGHER/SLOWER (speed) OR EXPECT SLOW TO/GO BACK 

TO/MAINTAIN (speed) 

EXPECT SPEED REDUCTION/INCREASE (time/fix/altitude) 

EXPECT SPEED REDUCTION/INCREASE SHORTLY (fix) 

EXPECT INCREASE/DECREASE (number) KNOTS 
B3

AVIATION

TOPIC COMPLEXITY PHRASEOLOGY 

TRAFFIC

*8-14 

*8-14 

*7-9 

3

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 

4=FLIGHT LEVEL + (altitude) three digits

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) 

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

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

YOU’RE FOLLOWING/GOING TO FOLLOW/YOU’LL BE FOLLOWING 

(type) 

TRAFFIC (NO FACTOR) 

WAKE

TURBULENCE

2 CAUTION WAKE TURBULENCE (Complexity included in ADV TFC except 

when no ADV TFC issued 

WEATHER

(ALTIMETER 

ONLY)

4

3

ROSWELL ALTIMETER (4 digits) 

ALTIMETER (4 digits) 

NOTE: Other weather not included in complexity

C1

APPENDIX C. 

Guide to the Classification of Pilot Readback Errors: Instruction/Clearance Aviation Topics 

Variable Descriptions & Value Labels Explanation/Examples 

ALTITUDE

1 = Substitution of message numbers/flight 

level vs. thousand 

2 = Substitution of climb with descend or 

descend with climb 

3 = Substitution of message numbers with 

incorrect climb/descend 

4 = Transposition of message numbers with 

incorrect climb/descend 

5 = Transposition of message numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of anchor word(s) 

8 = Omission of number elements 

9 = Omission of both anchor word(s) and 

some number elements 

ATC – “AAL Ten climb maintain one two thousand” 

1-“maintain one three thousand” 

 “maintain flight level one two” 

2-“descend maintain one two thousand” 

3-“descend maintain one three thousand” 

4-“descend maintain two one thousand” 

5-“climb maintain two one thousand” 

6-“AAL Ten one two zero knots” 

7-“one two” 

8-“climb maintain” 

9-“climb two thousand” 

ALTITUDE RESTRICTION

1 = Substitution of message numbers/rate of 

descent/climb, substitution of word “expedite” 

2 = Substitution of (point/fix) 

3 = Substitution of message numbers with 

incorrect (point/fix) 

4 = Transposition of message numbers with 

incorrect (point/fix) 

5 = Transposition of message numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of anchor word(s) 

8 = Omission of message numbers and/or 

(point/fix) 

9 = Omission of anchor word(s) and some 

number elements and/or (point/fix) 

ATC – “AAL Ten cross Alpha at or above one two thousand”

1-“cross Alpha at or above one three thousand” 

2-“cross Bravo at or above one two thousand” 

3-“cross Bravo at or above one three thousand” 

4-“cross Bravo at or above two one thousand” 

5-“cross Alpha at or above two one thousand” 

6-“descend to one two thousand” 

Note: “Expedite” clearances – readbacks should include the word 

“expedite.” 

ATC – “AALTen expedite climb to one two thousand” 

1-“hurry up to one two thousand” 

Note: Incorrect rate of climb/descent – “increase rate of climb” for 

“decrease rate of climb” 

7-“cross (point/fix) at one two” 

 “(point/fix) at one two thousand” 

8-“cross at or above one two thousand” 

 “cross (point/fix) at or above” 

9-“cross VOR at one two” 
C2

Variable Descriptions & Value Labels Explanation/Examples 

APPROACH/DEPARTURE

1 = Substitution of message 

numbers/(fix)/(route) 

2 = Substitution – transposition of message 

numbers 

3 = Substitution of approach name 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of anchor word(s) 

8 = Omission of number elements, (fix), 

(route) 

9 = Omission of anchor word(s) and (fix), 

(route), some number elements 

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

1-“cleared ILS runway two two right approach” 

2-“cleared ILS runway one two right approach” 

3-“cleared visual two one right approach” 

6-“maintain two one” 

7-“cleared two one right” 

8-“cleared to the right” 

9-“cleared ILS right” 

FREQUENCY

1 = Substitution of message numbers, 

(facility), (point/fix) 

2 = Substitution – transposition of message 

numbers 

3 = Substitution of message numbers with 

incorrect (facility), (point/fix) 

4 = Transposition of message numbers with 

incorrect (facility), (point/fix) 

5 = Transposition of message numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of anchor word(s) 

8 = Omission of number elements 

9 = Omission of both anchor word(s) and 

some number elements 

ATC – “AAL Ten contact (facility) one one eight point three” 

1-“contact (facility) one eight” 

2-“contact (facility) one eight one point three” 

3- “contact (incorrect facility) one two eight point three” 

4-“contact (incorrect facility) one eight one point three” 

5-“contact (facility) one eight one point three” 

6-“squawk one one eight three” 

Anchor words “contact,” location, facility, “point/decimal” 

ATC – “AAL Ten contact (facility) one one eight point three” 

7-“contact (facility) one one eight three” 

8-“contact (facility) one eight point three” 

9-“(facility) one eight three” 

HEADING

1 = Substitution of message numbers 

2 = Substitution of direction of turn 

3 = Substitution of message numbers with 

incorrect direction of turn 

4 = Transposition of message numbers with 

incorrect direction of turn 

5 = Transposition of message numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of anchor word(s) 

8 = Omission of number elements 

9 = Omission of both anchor word(s) and 

some number elements 

ATC – “AAL Ten turn left heading two one zero” 

1-“zero one zero” 

2-“turn right heading two one zero” 

3-“right turn one three zero” 

4-“turn right one two zero” 

5-“turn left one two zero” 

6-“AAL Ten two one zero knots” 

7-“two one zero” 

8-“turn left heading” 

 “left on the heading” 

9-“two one” 
C3

Variable Descriptions & Value Labels Explanation/Examples 

HEADING MODIFICATION 

1 = Substitution of rate of turn 

ATC – “AAL Ten increase rate of turn descend maintain four 

thousand” 

1-“decrease rate of turn” 

HOLDING

1 = Substitution of message numbers, 

(fix/waypoint), (direction), etc. 

2 = Substitution – transposition of message 

numbers 

3 = Substitution of message numbers with 

incorrect (fix/waypoint), (direction), etc. 

4 = Transposition of message numbers with 

incorrect (fix/waypoint), (direction), etc. 

5 = Transposition of message numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of (fix/waypoint), (direction), 

(course), (minutes/miles), etc. 

8 = Omission of number elements 

9 = Omission of (fix/waypoint), (direction), 

(course), (minutes/miles), etc. and some 

number elements 

ATC – “AAL Ten hold northeast Alpha one two mile legs right 

turns” 

1-“hold southwest Alpha” 

2-“two one” 

3-“southwest Bravo one zero mile legs” 

4-“southwest Bravo two one mile legs” 

5-“two one” 

6-“via Victor twelve” 

7-“hold northeast” 

8-“hold northeast of Alpha” 

9-“hold two mile legs” 

ROUTE

1 = Substitution of message numbers, (fix), 

(route) 

2 = Substitution – transposition of message 

numbers 

3 = Substitution of message numbers with 

incorrect (fix), (route) 

4 = Transposition of message numbers with 

incorrect (fix), (route) 

5 = Transposition of message numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of (fix)/aircraft/ airport 

8 = Omission of part/all of route 

9 = Omission of (fix)/aircraft/ airport and 

part/all of route 

ATC – “AAL Ten via Victor nine J twenty eight Alpha” 

1-“via Victor five J twenty eight Alpha” 

2-“via Victor nine J eighty two Alpha” 

3-“via Victor eight J twenty eight to Bravo” 

4-“via Victor nine J eighty two to Bravo” 

5-“via Victor nine J eighty two Alpha” 

6-“altimeter’s nine twenty eight” 

7-“twelve twenty eight” 

8-“Victor and the J route (fix)” 

9-“Victor and twenty eight” 

ATC – “AAL Ten turn right direct (fix)” 

8-“(fix)” 
C4

Variable Descriptions & Value Labels Explanation/Examples 

SPEED

1 = Substitution of message numbers, 

(point/fix) 

2 = Substitution – transposition of message 

numbers 

3 = Substitution of message numbers with 

incorrect increase/decrease 

4 = Transposition of message numbers with 

incorrect increase/decrease 

5 = Transposition of message numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of anchor word(s) 

8 = Omission of number elements 

9 = Omission of both anchor word(s) and 

some number elements 

ATC – “AAL Ten reduce speed two one zero knots” 

1-“two five zero knots” 

2-“reduce one two zero knots” 

3-“increase speed two five zero knots” 

4-“increase one two zero knots” 

5-“reduce one two zero knots” 

6-“heading two one zero” 

7-“reduce two one zero” 

8-“reduce speed” 

9-“reduce two one” 

TRANSPONDER

1 = Substitution of message numbers 

2 = Substitution – transposition of message 

numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of anchor word(s) 

8 = Omission of number elements 

9 = Omission of both anchor word(s) and 

some number elements 

ATC – “AAL Ten squawk two one two four” 

1-“squawk four two one three” 

2-“squawk one two two four” 

6-“altimeter two one two four” 

Anchor word “squawk,” and 4 digits of transponder code 

ATC – “AAL Ten squawk two one two four” 

7-“two one two four” 

8-“squawk” 

9-“one two four” 
D1

APPENDIX D. 

Readback Error Classification Guide: Advisory Aviation Topics 

Variable Descriptions & Value Labels Explanation/Examples 

ALTITUDE

1 = Substitution of message numbers 

2 = Substitution of climb with descend or 

descend with climb 

3 = Substitution of message numbers with 

incorrect climb/descend 

4 = Transposition of message numbers with 

incorrect climb/descend 

5 = Transposition of message numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of anchor word(s) 

8 = Omission of number elements 

9 = Omission of both anchor word(s) and 

some number elements 

ATC – “AAL Ten expect climb maintain one two thousand”

1-“expect maintain one three thousand” 

2-“expect descend maintain one two thousand” 

3-“expect descend maintain one three thousand” 

4-“expect descend maintain two one thousand” 

5-“expect climb maintain two one thousand” 

6-“AAL Ten expect one two zero knots” 

Anchor words “thousand,” and/or “hundred” or “flight level” 

(dependent upon the altitude) 

ATC – “AAL Ten expect climb maintain one two thousand” 

7-“one two thousand” 

8-“maintain it” 

9-“expecting two” 

ALTITUDE RESTRICTION

1 = Substitution of message numbers 

2 = Substitution of (point/fix)

3 = Substitution of message numbers with 

incorrect (point/fix)

4 = Transposition of message numbers with 

incorrect (point/fix)

5 = Transposition of message numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of anchor word(s) 

8 = Omission of message numbers and/or 

(point/fix) 

9 = Omission of anchor word(s) and some 

message numbers and/or (point/fix)

ATC – “AAL Ten expect cross Alpha at or above one two thousand”

1-“expect cross Alpha at or above one three thousand” 

2-“expect cross Bravo at or above one two thousand” 

3-“expect cross Bravo at or above one three thousand” 

4-“expect cross Bravo at or above two one thousand” 

5-“expect cross Alpha at or above two one thousand” 

6-“expect descend one two thousand” 

Anchor words “cross,” “til,” (point/fix), “at” or “at or above/below,” 

“thousand,” “hundred,” “flight level” 

ATC – “AAL Ten expect cross Alpha at or above one two thousand” 

7-“Alpha at or above one two thousand” 

8-“cross Alpha” 

9-“two thousand” 
D2

Variable Descriptions & Value Labels Explanation/Examples 

APPROACH/DEPARTURE

1 = Substitution of message numbers 

2 = Substitution – transposition of message 

numbers 

3 = Substitution of approach name 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of anchor word(s) 

8 = Omission of number elements 

9 = Omission of both anchor word(s) and 

some number elements 

ATC – “AAL Ten expect ILS runway two one right approach” 

1-“expect ILS runway two two right approach” 

2-“expect ILS runway one two right approach” 

3-“expect visual approach” 

6-“expect maintain two one”

Anchor words approach name/type, the word “ runway” 

ATC – “AAL Ten expect ILS runway two one right approach” 

7-“expect two one right” 

8-“expect ILS” 

9-“expect ILS approach” 

HEADING

1 = Substitution of message numbers 

2 = Substitution of direction of turn 

3 = Substitution of message numbers with 

incorrect direction of turn 

4 = Transposition of message numbers with 

incorrect direction of turn 

5 = Transposition of message numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of anchor word(s) 

8 = Omission of number elements 

9 = Omission of both anchor word(s) and 

some number elements 

ATC – “AAL Ten expect turn left heading two one zero”

1-“zero one zero,” or “six zero” 

2-“turn right two one zero” 

3-“right two two zero” 

4-“expect turn right heading one two zero” 

5-“expect turn left heading one two zero” 

6-“expect two one zero knots” 

Anchor words “heading” or “degrees 

ATC – “AAL Ten expect turn left heading two one zero” 

7-“left two one zero” 

8-“expect left heading two one” 

9-“expect left two one” 

HOLDING 

1 = Substitution of message numbers, 

(facility,) (point/fix) 

2 = Substitution – transposition of message 

numbers 

3 = Substitution of message numbers with 

incorrect (facility), (point/fix) 

4 = Transposition of message numbers with 

incorrect (facility), (point/fix)

5 = Transposition of message numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of any item(s) 

ATC – “AAL Ten expect hold south of (fix) (radial, airway, etc.)

(direction) turns (minutes/hours) delay”

1-Substitution of any of the elements in holding advisory 

2-Transposition of any number elements in holding advisory 

3-Substitution of message numbers with incorrect holding fix 

4-Transposition of message numbers with incorrect holding fix 

5-Transposition of any of the number elements 

6-“cleared for approach” 

7-Omission of any item(s) in holding advisory 
D3

Variable Descriptions & Value Labels Explanation/Examples 

ROUTE

1 = Substitution of message numbers

2 = Substitution – transposition of message 

numbers 

3 = Substitution of message numbers with 

incorrect (point/fix) 

4 = Transposition of message numbers with 

incorrect (point/fix)

5 = Transposition of message numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of anchor word(s), (fix), route 

8 = Omission of number elements 

9 = Omission of anchor word(s), (fix), route, 

and some number elements 

ATC – “AAL Ten expect Victor nine J twenty eight Alpha” 

1-“expect Victor five J twenty eight Alpha” 

2-“expect Victor nine J eighty two Alpha” 

3-“expect Victor five J twenty eight Bravo” 

4-“expect Victor nine J eighty two Bravo” 

5-“expect Victor nine J eighty two Alpha” 

6-“speed two eighty” 

Anchor words/phrases “expect,” “direct,” “intercept/join,” “radial,” 

“maintain visual/visual separation from/with traffic/him/type/ACID,” 

“follow that traffic/him/type/ACID.” 

Use of “direct,” must include a (point/fix)

Use of “intercept/join” must include localizer name. 

Use of “radial” must include identification of the radial. 

SPEED

1 = Substitution of message numbers 

2 = Substitution – transposition of message 

numbers 

3 = Substitution of message numbers with 

incorrect increase/decrease 

4 = Transposition of message numbers with 

incorrect increase/decrease 

5 = Transposition of message numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of anchor word(s) 

8 = Omission of number elements 

9 = Omission of both anchor word(s) and 

some number elements 

ATC – “AAL Ten expect reduce speed two one zero knots”

1-“expect two five zero knots” 

2-“expect reduce zero one two knots” 

3-“expect increase speed two five zero knots” 

4-“expect increase zero one two knots” 

5-“expect reduce zero one two knots” 

6-“expect heading two one zero”

Anchor words “speed” or “knots,” “mach” (including point) 

ATC – “AAL Ten expect reduce one eight zero knots” 

7-“expect one eight zero” 

8-“expect reduce” 

9-“expect eight oh” 

WEATHER (ALTIMETER)

1 = Substitution of message numbers 

2 = Substitution – transposition of message 

numbers 

5 = Transposition of message numbers 

6 = Other-one type of information read back 

as another type of information 

7 = Omission of anchor word 

8 = Omission of number elements 

9 = Omission of both anchor word(s) and 

some number elements 

ATC – “AAL Ten (source) altimeter two nine nine two” 

1-“altimeter nine two nine zero” 

2-“altimeter nine two nine two” 

5-“altimeter nine two two nine” 

6-“squawk two nine nine two” 

Key word “altimeter” and four digits 

7-“(source) two nine nine two” 

8-“(source) altimeter nine two” 

9-“(source) nine nine two”