**PREFACE**

This research was sponsored by the Federal Aviation Administration\'s
Office of the Chief Scientific and Technical Advisor for Human Factors
(AAR-100). We thank Larry Cole of that office for his support and
helpful suggestions. We are also grateful to the knowledgeable and
dedicated analysts at the Battelle-ASRS office who performed the
searches for the relevant ASRS reports. Their contribution toward the
advancement of aviation safety is invaluable and this type of report
would not be possible without their talented assistance.

**TABLE OF CONTENTS**

[Section]{.underline} [Page]{.underline}

1\. INTRODUCTION 1

1.1 Previous ASRS Research on Pilot-Controller Communications 2

1.2 Method 3

2\. RESULTS 4

2.1 Types of Communication Errors 4

2.2 Factors Contributing to Communication Problems 5

2.3 Results of Communication Errors 6

3\. READBACK/HEARBACK ERRORS 7

3.1 Factors Contributing to Readback/Hearback Errors 7

3.2 Results of Readback/Hearback Errors 8

4\. THE IMPORTANCE OF READBACKS 11

4.1 Factors Associated with No Readbacks 11

4.2 Results of No Readbacks 12

5\. HEARBACK TYPE II ERRORS 13

6\. THE CALL SIGN PROBLEM 15

7\. ANALYSIS OF RESULTS OF COMMUNICATION ERRORS BY CONTRIBUTING FACTORS
17

8\. COMPARISON OF THE RESULTS OF THE ANALYSES OF ASRS REPORTS WITH THE
RESULTS OF TAPE ANALYSES AND OTHER STUDIES 20

8.1 Effect of Message Complexity 20

8.2 Types of Errors 21

9\. IMPLICATIONS OF FUTURE TECHNOLOGY 22

SUMMARY OF RECOMMENDATIONS 23

REFERENCES 25

GLOSSARY 26

**LIST OF FIGURES**

[Figure]{.underline} [Page]{.underline}

1\. Types of Communication Errors 4

2\. Factors Perceived to Contribute to Communication Errors 5

3\. Results of Communication Errors 6

4\. Factors Perceived to Contribute to Readback/Hearback Errors. 8

5\. Results Associated with Readback/Hearback Errors 9

6\. Results Associated with Hearback Type II Errors 13

7\. Altitude Deviations. 17

8\. Loss of Standard Separation 18

9\. Aircraft Accepted a Clearance Intended for Another Aircraft 18

10\. Runway Transgressions 19

11\. Operational Errors 19

**EXECUTIVE SUMMARY**

Safe and expeditious air traffic depends upon accurate and efficient
communications between pilots and controllers. This requirement for
effective communication becomes even more critical as the amount and
complexity of air traffic increases with attempts to increase capacity.

Studies of actual pilot and controller communications (e.g., Cardosi,
1994; Morrow, Lee, and Rodvold, 1993) reveal an astonishingly low error
rate. These studies analyzed voice tapes from ATC facilities and found
that less than one percent of these voice communications resulted in a
communication error that affected the accuracy of the communication.
This low error rate does not detract from the potential seriousness of
even a single error. Also, in terms of communication errors per hour,
the numbers are quite startling; across ATC environments (en route,
TRACON, local control and ground control) the number of readback errors
on a single frequency was close to one per hour. The first step in
reducing the incidence of communication errors is to understand why they
happen.

The purpose of this study was to identify the factors that contribute to
communication errors. Reports submitted to the Aviation Safety Reporting
System (ASRS) offer detailed accounts of specific types of errors and a
great deal of insight as to why the pilots and controllers involved
believe these errors occurred. Because of this, ASRS reports provide an
efficient means of studying communication errors at a level of detail
that is not feasible from more objective materials (i.e., voice tapes).

A total of 386 reports were included in this analysis. The communication
errors found in this study could be classified into three types:

1\) Readback/Hearback errors - the pilot reads back the clearance
incorrectly and the controller fails to correct the error - accounted
for 47% of the errors found in this analysis.

2\) No pilot readback. A lack of a pilot readback contributed to 25% of
the errors found in this analysis.

3\) Hearback Errors Type II - the controller fails to notice his or her
own error in the pilot\'s correct readback or fails to correct critical
erroneous information in a pilot's statement of intent - accounted for
18% of the errors found in this analysis.

Many factors contributed to these errors. The most common factors were:
similar call signs on the same frequency, pilot expectations (e.g.,
leading them to accept a clearance that they expected rather than what
the controller actually said), and high controller workload. The
identified results of these communication errors were (in order of
prominence): altitude deviations, loss of standard separation, ATC
operational errors[^1], pilots landing on the wrong runway, and runway
transgressions.

It is important to realize that the proportions presented in these data
**in no way** represent the proportions of results of communications in
the operational environment. Most communication errors are corrected on
the spot and are not reported to ASRS. Usually, only the errors with
potential consequences (such as a pilot deviation) would merit a report.
However, the data contained in ASRS reports are useful in understanding
the nature of communication errors and the factors that contribute to
them, and in identifying corrective measures to reduce their incidence.

The following recommendations are offered for reducing the number of
communication errors between pilots and controllers:

-   **Controllers should be encouraged to speak slowly and distinctly.**
    In a laboratory study, the rate of pilot readback errors and
    requests for repeats more than doubled when the same controller gave
    the same complex clearances in a faster speaking voice. With a
    normal rate of speech (156 words per minute), 5% of the
    controller\'s instructions resulted in a readback error or a request
    for repeat. This rate rose to 12% when the controller spoke somewhat
    faster (210 words per minute) (Burki-Cohen, personal communication).
    (As a reference, the average newscaster speaks at about 180 words
    per minute.)

-   **Controllers should be encouraged to keep their instructions short
    with no more than four instructions per transmission.** The
    complexity of the controller\'s transmission has a direct effect on
    the pilot\'s ability to remember it - there are fewer pilot errors
    with the less complex transmissions.

-   **Pilots should respond to controller instructions with a full
    readback of critical components.** An altitude, heading, or airspeed
    should be read back *as an altitude, heading, or airspeed*. For
    example, \"Climbing to 230, Aircraft XYZ\" contains critical
    information that \"Roger, 230 for Aircraft XYZ\" does not. When more
    than one runway is in use, clearances to takeoff and land should be
    acknowledged with a readback of the clearance that includes the
    runway number.

-   Controllers should try to treat the readbacks as they would any
    other piece of incoming information - use it. **Actively listen to
    the readback and check it against the flight strip notations** to
    ensure that the message that the pilot got was the one you wanted
    him or her to get.

-   **When there are similar call signs on the frequency, controllers
    should continue to announce this fact**; this will alert pilots and
    may help to reduce the incidence of pilots accepting a clearance
    intended for another aircraft.

-   **When there are similar call signs on the frequency, pilots should
    be encouraged to say their call sign before and after each
    readback**. This gives the controller added information as to which
    aircraft accepted the clearance.

-   **Controllers should refrain from issuing \"strings\" of
    instructions to different aircraft.** Issuing strings of
    instructions to different aircraft without allowing the opportunity
    for each aircraft to respond directly after the controller\'s
    transmission has two undesirable effects. First, it increases the
    likelihood of a miscommunication. A pilot\'s memory for an
    instruction can be hindered by extraneous information presented
    before or after it. Second, it decreases the likelihood of a pilot
    readback, as it sends the message, \"I\'m too busy for your readback
    to be important right now\". A pilot\'s readback is the quickest
    mechanism for ensuring that the message was received by the intended
    aircraft and understood. There is another important aspect to the
    readback/hearback process that is often overlooked. While a
    controller\'s hearback of the pilot\'s readback is intended to
    ensure that the pilot \"*got* it right\", it also offers an
    opportunity to help ensure that the controller *gave* it right.

**1. INTRODUCTION**

Safe and expeditious air traffic depends upon accurate and efficient
communications between pilots and controllers. This requirement for
effective communication becomes even more critical as the amount and
complexity of air traffic increases. In an ideal aviation world,
unambiguous controller transmissions would be followed by a correct
pilot readback and a flawless execution of the controller\'s
instruction. In the real world, however, congested frequencies, faulty
communications equipment, pilot and controller expectations, workload,
and other human factors conspire to challenge effective communications.

Studies of pilot and controller communications (e.g., Cardosi, 1994;
Morrow, Lee, and Rodvold, 1993) reveal an astonishingly low error rate.
These studies analyzed voice tapes from ATC facilities and found that
less than one percent of these voice communications resulted in a
communication error that affected the accuracy of the communication.
Such a low error rate is a tribute to the pilots and controllers in the
National Airspace System. This does not detract, however, from the
potential seriousness of even a single error. Also, in terms of
communication errors per hour, the numbers are quite startling; across
ATC environments (en route, TRACON, local control, and ground control)
the number of readback errors on a single frequency was close to one per
hour. The first step in reducing the incidence of communication errors
is to understand why they happen.

The purpose of this study was to identify the factors that contribute to
communication errors. Reports submitted to the Aviation Safety Reporting
System (ASRS) offer detailed accounts of specific types of errors and a
great deal of insight as to why the pilots and controllers involved
believe these errors occurred. Because of this, ASRS reports provide an
efficient means of studying communication errors at a level of detail
that is not feasible from more objective materials (i.e., voice tapes).

ASRS reports are submitted voluntarily by pilots, controllers, and
others on events they encountered that may affect air safety. ASRS
reports on communication difficulties often contain very detailed
information on the factors that, in the reporter\'s opinion, contributed
to the error. These reports, filed by pilots and controllers, usually
describe an error that they themselves made. While there is a provision
for limited immunity for pilots who file ASRS reports, these reports are
usually submitted in the hopes that the information will be used to
enhance aviation safety by contributing to the study of human error and
to help prevent others from making the same mistakes.

ASRS reports provide an invaluable database for studying the causes and
consequences of communication errors. These reports have inherent
limitations since the events described are not routinely independently
verified and usually present only one side of the story. However, they
do offer detailed accounts of specific types of errors and a great deal
of insight as to why they believe these errors occurred. Because of
this, ASRS reports provide a means of studying communication errors at a
level of detail that is not feasible from more objective materials
(i.e., voice tapes). However, the analysis of these reports must be
interpreted in the context of the constraints of the database. While
these constraints do not detract from the value of the data, they do
require that care be taken in how they are interpreted. Since the
reports rely on self-report, the accuracy of the data is subject to the
accuracy of the reporters\' perceptions and memory for the events. If
human perceptions were 100% accurate, there would be no communications
errors. Human memory is as fallible as human perception. We cannot
expect ASRS reports to be as precise as analyses of ATC voice tapes.
Fortunately, some of the reports do include the results of the
controllers listening to the tapes; this is the only opportunity to
objectively determine what occurred.

While the proportions of categories of incidents may suggest trends,
they cannot be interpreted as indicative of the proportions that occur
in actual operations. For example, 49% of the readback/hearback errors
found in this analysis resulted in altitude deviations. This cannot be
interpreted to mean that 49% of all readback errors result in altitude
deviations; it only indicates that communication factors are an
important underlying factor in understanding altitude deviations.

1.1 PREVIOUS ASRS RESEARCH ON PILOT-CONTROLLER COMMUNICATIONS

Communication failures between pilots and controllers is a common theme
in reports submitted to ASRS. ASRS reports have been used to take a
broad look at the types of communications problems that arise (Grayson
and Billings, 1981) and to investigate the nature of specific problems
such as readback/hearback errors (Monan, 1986), and the problem of
similar call signs (Monan, 1983).

The first use of ASRS reports to examine communication errors was
published in 1981. Grayson and Billings were the first to report on
communications problems between the cockpit and ATC, as identified in
ASRS reports. They offer a broad description of the problems of pilot
expectations; similar sounding names and numbers; transposing numbers in
a clearance; garbled phraseology; unsent, inaccurate, or incomplete
messages; and equipment failures.

In 1983, Monan reported specifically on the \"Call Sign Problem\". This
report identified the following factors as contributing to communication
errors: poor microphone technique, dangling phraseology, use of
incomplete call signs, controllers issuing strings of instructions to
different aircraft, and similar sounding call signs. He cautioned pilots
not to read back a \"doubtful\" clearance and expect the controller to
correct the error.

In 1986, Monan reiterated this caution to pilots when he identified and
examined the controller \"hearback error\", where a pilot reads back a
clearance intended for another aircraft, or makes another error in the
readback, and the erroneous readback goes unchallenged by the
controller. Monan examined 417 instances of communication errors and
described instances of inadequate acknowledgments, readback errors, and
hearback errors. The analysis is descriptive and informative, but
contains little data on the factors that contributed to the
readback/hearback errors.

It is interesting to note that while little has changed since the \"Call
Sign Problem\" was first reported by Monan almost 15 years ago, in terms
of the nature of the problem, there has been a tremendous change in our
attitude toward it. An increased awareness of the ease with which these
communication errors can occur, because of similar call signs on the
same frequency, and an appreciation for the potential seriousness of the
consequences of these errors, has brought us a long way from the
lackadaisical attitude that Monan originally reported:

"The typical reaction of any airman or controller to the subject of ATC
radio communications probably parallels the typical reaction of a
housewife to the subject of the kitchen sink. Both items represent
important job functions - must do activities - but the tasks involved
are too commonplace, mundane, and repetitive to be intellectually
challenging or capable of arousing emotional enthusiasm. Yet, the
routine of controller-cockpit - controller message exchange is of the
most - if not the most - important factors in navigating an aircraft
safely through the airspace and the ATC structure.\" (Monan, 1983, p.1)

Fortunately, this complacent attitude toward communications is as
outdated as the \"housewife\" reference. Both pilots and controllers
take communications extremely seriously. The purpose of this report is
to examine communication errors in detail to be able to identify
procedures that will assist these dedicated professionals in further
reducing the incidence of communication errors.

1.2 METHOD

A search of the ASRS database was performed for reports involving
problems related to voice communications between pilots and controllers
in which equipment malfunctions were not a factor. A total of 504
reports resulted from this search and were examined. All of these ASRS
reports were submitted between July 1991 and May 1996, and specifically
identified communication problems attributable to human factors (i.e.,
not relating to equipment). Many reports were excluded from analysis
either because they did not specifically identify a miscommunication
between pilots and controllers that was not related to equipment
problems or because they lacked sufficient detail to determine the
course of events. This left a total of 386 unique reports that were
included in this analysis. Three hundred twenty three (76%) of these
reports were submitted by pilots and 97 (23%) were submitted by
controllers. (Thirty-two reports on the same incidents were submitted by
both pilots and controllers, but the report was only counted once in the
analysis.)

**2. RESULTS**

2.1 TYPES OF COMMUNICATION ERRORS

The communication errors found in this analysis can be divided into
three distinct types. The first is an absence of a pilot readback. In
most of these instances, the pilot merely acknowledged the clearance
that he/she misunderstood. The second is the well-known
readback/hearback error, where a pilot reads back a clearance
incorrectly and the controller fails to catch the readback error. The
third is a new classification of communication errors called *Hearback
Type II* Errors. In this type of error the pilot correctly repeats the
clearance that was issued, but the controller fails to notice that the
clearance issued was not the intended one. This type of error also
included the few instances where the pilot made a statement of action or
intent that the controller should have noticed was problematic. These
types of errors are mutually exclusive in that a single error can only
be put into one of these categories. However, a single ASRS report can
reference more than one error. For example, if a controller issued an
altitude and heading, and the pilot failed to read back the heading
*and* made an error in reading back the altitude clearance (and both of
these turned out to be problematic in that the pilot flew the wrong
heading and the wrong altitude), then this single report would contain
two communication errors. There were 407 communication errors identified
in the 386 reports.

As can be seen in Figure 1, readback/hearback errors were the most
common type of communication errors, accounting for 47% of the errors
found in this sample of reports. Errors precipitated by the absence of a
pilot readback accounted for 25% of the errors and hearback type II
errors accounted for 18% of the errors. All other errors, such as a
pilot misunderstanding a clearance, accounted for the remaining 10% of
the errors.

**Figure 1. Types of Communication Errors**

2.2 FACTORS CONTRIBUTING TO COMMUNICATION PROBLEMS.

A frequency distribution of the factors contributing to communication
errors is seen in Figure 2. There was often more than one factor cited
as contributing to the error described in the report. The most commonly
cited factors that contributed to communication errors were: similar
call signs, pilot expectations and controller workload. The two most
commonly cited contributing factors were similar call signs (on the same
frequency) and pilot expectations. Similar call signs were cited in 15%
of the reports and most commonly resulted in a pilot accepting a
clearance intended for another aircraft. Instances of pilots hearing
what they wanted or expected to hear was cited in 13% of the reports and
most commonly resulted in runway transgressions; 36% of the reports
citing pilot expectation as a contributing factor resulted in runway
transgressions. Twenty-eight percent of the reports citing pilot
expectation as a contributing factor resulted in altitude deviations.
Controller workload, cited in 11% of the reports was the third most
frequently cited contributing factor. Forty-five percent of the reports
that cited controller workload as a contributing factor resulted in
altitude deviations, 36% resulted in less than standard separation, and
29% stated that they resulted in operational errors[^2]. (One report can
have more than one result associated with it. For example, a report that
identifies loss in standard separation, as a result, is also likely to
identify an operational error as a result.)

**Figure 2. Factors Perceived to Contribute to Communication Errors**

2.3 RESULTS OF COMMUNICATION ERRORS

The results of the reports described here are the ones specifically
listed in the reports. One report can have more than one result
associated with it, but only the ones specifically listed in the reports
are cited. For example, a report may identify less than standard
separation as a result. It cannot be assumed that this also resulted in
an operational error, unless it is also specifically identified as a
result. A frequency distribution of the results of the communication
errors found in this analysis is seen in Figure 3. The most commonly
cited results of the communication errors in this analysis were:
altitude deviations, less than standard separations, operational errors,
pilots accepting a clearance intended for another aircraft, and runway
transgressions.

**Figure 3. Results of Communication Errors**

The results of this analysis can be summarized in a discussion of four
types of problems in controller-pilot communications: readback/hearback
errors, absence of effective readbacks, hearback type II errors, and
similar call signs.

**3. READBACK/HEARBACK ERRORS**

A readback error is defined as an important discrepancy between the
clearance that the controller issued and what the pilot read back. When
the controller fails to correct this discrepancy, this is a \"hearback\"
error. Readback errors that are corrected by controllers are not likely
to result in ASRS reports since the correction of the readback usually
precludes further problems. Readback/hearback errors, however, can have
serious consequences; they can start a chain of events that can
jeopardize safety and they contribute to operational errors. Even when
the error does not result in an unsafe situation, the consequences for
the controller can be serious. The following excerpt speaks to this and
the perceived injustice in the consequences of these errors: \"the pilot
makes an error \[readback error of altitude\] and nothing happens. I
miss one readback out of probably 100 during that rush and I get
decertified and placed into a performance improvement program.\"
(Accession number 209860).

Studies of voice tapes from actual operations reveal that readback
errors occur in less than one percent of all controller transmissions
(Cardosi, Brett, and Han, 1996). Because the number of readback errors
is so few in these observational studies, the factors associated with
them could not be easily determined. The studies do show that, on
average, 66% of these readback errors are corrected by the controller
(Cardosi, Brett, and Han, 1996). The proportion of readback errors
corrected by the controller varies widely with the ATC environment.
While controllers corrected 89% of the readback errors en route, only
50% of the readback errors on the ground frequency were corrected. On
the TRACON and local control frequencies, controllers corrected 60% and
63%, respectively. It may be that the probability of controllers
catching readback errors is directly related to their momentary
workload, however, a tape analysis (such as the ones from which these
data were derived) is not a good vehicle to study the causes of hearback
errors. First, the database is limited due to the small number of
readback errors. Second, the tapes provide insufficient insight into
what the controller was doing at the time of the error.

3.1 FACTORS CONTRIBUTING TO READBACK/HEARBACK ERRORS

Readback/hearback errors were the most common type of communication
error found in this analysis of ASRS reports, accounting for 47% of the
communication errors in the sample. Many of the reports on
readback/hearback errors did not specify any contributing factors, nor
could any be inferred. As Figure 4 shows, in the reports of
readback/hearback errors in which a causal factor was cited, the most
common contributing factor was similar call signs, followed by
controller workload. Similar call signs contributed mostly to the
instances in which a pilot accepted a clearance that was intended for
another aircraft and the controller failed to notice that the readback
had the wrong call sign associated with it.

**Figure 4. Factors Perceived to Contribute to Readback/Hearback
Errors**

The most fundamental reason behind hearback errors is perhaps too
obvious to be listed as a factor in the ASRS report. We are all set up
to hear what we expect to hear. Pilots notice this when they realize
that the clearance they \"heard\" was not the clearance that was issued.
Instead, they heard what they expected to hear. Controllers suffer the
same fate. Expecting to hear a correct readback (after all, this is what
occurs over 99% of the time), they miss the rare discrepancy between
what the pilot said and what he/she should have said. Another
contributing factor to hearback errors involves the nature of the job.
While the controller is listening to the readback, he/she may also be
thinking about anything from their next transmission to solutions to a
potential conflict. As one controller noted, \"I was giving a position
relief briefing to a new controller and was receiving information on a
configuration change from a supervisor at the same time. This is why I
missed the readback.\" (Accession Number 330258).

Even when a controller catches a readback error, he/she may not be
completely out of the woods, as the following example illustrates:

\"Training was in progress. Traffic was heavy. Aircraft X was told,
\"Climb and maintain 10,000, leaving 3,000 turn right Direct MXE,
Maintain speed 250\". He read back, \'OK, its 13, Direct MXE out of 2
and 250.\" We caught the incorrect readback on the turning altitude, but
not the 13.\" (Accession Number 201259)

3.2 RESULTS OF READBACK/HEARBACK ERRORS

As can be seen in Figure 5, the most common result associated with
readback/hearback errors was altitude deviations, followed by less than
standard separation. Forty-nine percent of the reports involving
readback/hearback errors listed an altitude deviation as a result. Less
than standard separation was the second most commonly reported result,
and was specified in 35% of the reports. These were followed by
instances of an aircraft accepting a clearance that was intended for
another aircraft (30%), ATC operational errors (23%), and heading and
track deviations (10%). (\*Note: These percentages do not add to 100
since more than one result can be specified in a report.)

**Figure 5. Results Associated with Readback/Hearback Errors**

[What can be done?]{.underline}

Hearing what we expect to hear is a natural human tendency. Experience
and skill cannot compensate for this perceptual limitation. As one pilot
said, \"After 27 years of flying I now find it becoming easier to
\"hear\" things in a clearance that are not really there. More diligence
is required.\" (Accession Number 199764).

When a clearance is different from what the pilot was told to expect, it
is imperative that the controller emphasize the difference. For example,
if the pilot was told to expect runway 23 *left* and is now cleared on
23 *right,* the controller should emphasize this in some way, e.g.
\"clear to land runway 23 right, that\'s 23 *right*\". If the difference
is not emphasized, the pilot will be set up to hear what he/she expects
rather than what is actually said.

An effective readback can mitigate the effects of expectation, as it
gives the controller an opportunity to correct the error. In the
following example, a readback (of what was expected, rather than what
was said) saved the pilot from an unauthorized landing.

\"The initial approach controller told us to expect an approach to
runway 18R. This \"expect\" call, plus recent flights into \[airport X\]
with construction on Runway 18L had us all thinking runway 18R. The
final controller apparently cleared us for 18L. We had the ILS set up
for Runway 18R, were expecting 18R, and the captain read back, \"Cleared
for 18R\". I headed for Runway 18R. The tower \[then\] cleared us to
land on 18R. On landing roll, the tower advised us to contact the
approach controller about a little problem with our approach. We were
all wearing headsets, but heard what we expected instead of what was
really coming over the headset.\" (Accession Number 162629).

Pilots should NEVER read back what they *thought* they *might* have
heard expecting that the controller will inform the pilot of any
discrepancies. **When in doubt, ASK.** As one reporter lamented, \"I
broke my own \"cardinal rule\" by not taking questions over clearances
out of the cockpit to the source.\" (Accessison Number 300858).

It would be futile to suggest that controllers should guard against
being distracted during readbacks and hearing what they expect to hear;
it is the nature of the controller\'s job to plan one action while they
are executing another. It is more practical to suggest that controllers
regard the readbacks as they would any other piece of incoming
information - by using it. Actively listen to the readback and check it
against the flight strips notations to ensure that the message the pilot
received was the correct one.

**4. THE IMPORTANCE OF READBACKS**

A pilot readback presents the first and most efficient opportunity to
catch miscommunications. It provides a \"reality check\" in two ways. It
tells the controller \"this is what the pilot heard\" to help ensure
that 1) that\'s what the controller said, and 2) that\'s what the
controller MEANT to say. When a clearance is acknowledged without a
readback, this safety net of communications is taken away. While there
are some frequencies that are occasionally too congested to accommodate
full readbacks, readbacks should never be sacrificed to pilot
complacency. Acknowledging a controller transmission with only a flight
number gives the controller no useful information and removes any
possibility that an error will be caught at the readback stage. This is
illustrated in the following example:

\"Over the St. Thomas VOR the first officer requested \"direct EMI\".
The controller responded with the altimeter and what I thought was,
\"when able, direct EMI\". In reality, he said \"unable, direct EMI\".
The first officer responded with (only) the flight number. I should have
been more aware that he (first officer) did not read back \"Direct
EMI\". The solution is obvious: read back all clearances.\" (Accession
Number 163826).

In another instance, a copilot who \"does not believe in complete
readbacks of clearances, as that tends to clutter the frequency\"
(Accession Number 217581) responds \"Roger\" to a clearance to
\"position and hold\" that was intended for another aircraft. This
runway transgression necessitated a go-around for an incoming aircraft.
Clearly, pilots should use their complete call signs to acknowledge such
clearances.

4.1 FACTORS ASSOCIATED WITH NO READBACKS

The most common factor associated with a lack of a readback that
resulted in a communication error was pilot expectation. In 29% of these
errors, the pilot heard what they expected to hear (rather than what was
actually said), gave no readback, and then did something other than what
they were instructed to do. Many of these pilots were certain that they
heard what they expected to hear and did not bother to confirm it with
an acknowledgement. Sadly, there were also ASRS reports of one of the
pilots doubting what the other pilot thought he/she heard and, yet, even
in the face of this uncertainty, the transmission is STILL acknowledged
without a readback. Any readback is better than a vote in the cockpit
(as to what the controller said).

No other factors were strongly associated with the absence of a pilot
readback. However, it is important to note that in 11% of the cases, the
lack of a readback was not attributable to complacency or pilot error.
In 8% of the cases, a lack of a readback was associated with frequency
congestion that made it impossible to give a readback in a timely
manner. In 3% of the cases, the lack of a readback was associated with
the controller issuing a \"string\" of instructions to different
aircraft with no pauses between the transmissions to allow for
readbacks.

Controllers should refrain from issuing \"strings\" of instructions to
different aircraft whenever possible. Issuing strings of instructions to
different aircraft without allowing the opportunity for each aircraft to
respond directly after the controller\'s transmission has two
undesirable effects. First, it decreases the likelihood of a pilot
readback, as it sends the message, \"I\'m too busy for your readback to
be important right now\". Perhaps more importantly, it increases the
likelihood of a miscommunication. A pilot\'s memory for an instruction
can be hindered by extraneous information presented before or after it.
Increasing the likelihood of the pilot not getting the message
correctly, combined with a limited or nonexistent opportunity for a
readback, removes the safety net that the readback provides and sets the
stage for a potentially dangerous situation.

4.2 RESULTS OF NO READBACKS

The most common results of errors involving no readbacks were altitude
deviations and runway transgressions. Each of these results accounted
for 32% of the errors involving no pilot readback. No other results were
strongly associated with this type of error.

[What Can be Done?]{.underline}

A readback that omits critical information such as the runway number,
when more than one runway is in use, is little better than no readback
at all - all it tells the controller is that Aircraft X thinks they are
cleared for takeoff. While this gives the controller more information
than a \"roger\" does, it doesn\'t afford the protection of a complete
readback. Similarly, an altitude, heading, or airspeed should be read
back *as an altitude, heading, or airspeed*. For example, \"Climbing to
250, Aircraft XYZ\" tells the controller that the pilot heard 250 *as an
altitude* whereas \"Roger 250 for Aircraft XYZ\" could be a readback of
a heading, altitude, or speed. Similarly, when more than one runway is
in use, clearances to takeoff and land should be acknowledged with a
readback of the clearance that includes the runway number.

The following report illustrates the importance of including more than
numbers in a readback; clearly, a little noun can go a long way toward
preventing a communication error.

\"The sector was extremely busy with over 17 aircraft. I told aircraft
X, \"Turn left, heading 270\". He said, \"Roger 270\". Next, I said,
\"Aircraft Y Turn left fly heading 270 vector for traffic.\" He said,
\"OK, leaving 310 for 270\". I thought he meant that he was leaving a
310 heading for a 270 heading.\"

(Accession Number 169149).

This resulted in an operational error as the aircraft descended and
standard separation was lost.

Pilots should respond to controller instructions with a full readback of
critical components. An altitude, heading, or airspeed should be read
back *as an altitude, heading, or airspeed*. When more than one runway
is in use, clearances to takeoff and land should be acknowledged with a
readback of the clearance that includes the runway number. Finally,
readbacks should *always* contain the aircraft\'s call sign.

**5. HEARBACK TYPE II ERRORS**

Hearback Type II Errors are controller errors in which the pilot
correctly repeats the clearance that was issued, but the controller
fails to notice that the clearance issued was not the clearance that
he/she *intended* to issue. This type of error also included the few
instances in which the pilot made a statement of action or intent that
the controller should have noticed was problematic.

There were 73 reports of Hearback Type II errors in this data set. Most
reports involving this type of error did not specify a contributing
factor. Of the reports that did offer a reason as to why these errors
occurred, the most common contributing factor was controller workload.
Controller workload was identified as a contributing factor in 11% of
the incidents of Hearback Type II errors. As can be seen in Figure 6,
the most common result of these errors was less than standard
separation; 32% of the Hearback Type II errors were described as
resulting in less than standard separation. Thirty percent were listed
as resulting in altitude deviations and 22% specifically cited
operational errors as a result. (Again, a report can specify more than
one result and often the results are related such as altitude deviations
and loss of standard separation.)

**Figure 6. Results Associated with Hearback Type II Errors**

The following excerpts from reports filed by controllers illustrate the
nature of these errors:

\"In issuing the clearance, I gave him a wrong initial heading of 200
degrees instead of 050 degrees. The pilot read it back and I missed it
on the readback.\" (Accession Number 199586)

\"A right turn was issued when a left turn was intended. I did not
notice the readback.\" (Accession Number 214761)

\"I reissued the clearance to descend now to FL310, then at pilot\'s
discretion maintain FL240, thinking all the time I had said FL280.\"

\"Aircraft X was issued a clearance to 14000 that was meant for company
Y. I even put the \[wrong\] altitude in the data block.\"

\"I descended aircraft X to 4,000 feet, but meant to descend aircraft Y
to 5,000. I said 4,000. \[The pilot\] readback 4,000. I didn\'t catch
it\....I mispoke.\" (Accession Number 227572)

[What Can be Done?]{.underline}

Saying something different from what we intended to say is all too easy
to do. That is why it is useful for controllers to treat the readbacks
as they would any other piece of incoming information - use it. Actively
listen to the readback and check it against the flight strip notations
to ensure the message the pilot received was the correct one.

**6. THE CALL SIGN PROBLEM**

Most pilots and controllers are aware of the confusion that similar
sounding call signs can cause. Call signs can sound similar because they
have: the same flight number (such as AAL 123 and UAL 123), numbers that
sound similar (such as \"two\" and \"ten\"), or the same numbers in
different positions (such as \"four thirty-two\" and \"three
forty-two\"). The problem is compounded when the flights with similar
sounding numbers also have the same airline name. Similar call signs are
inevitable, particularly at airline hubs, where many aircraft will have
the same company name.

Similar call signs was the single most contributing factor to
miscommunications between pilots and controllers. There were 56 reports
involving similar call signs found in this study. Fourteen of these
involved the same company name. In only one of these cases were the
pilots informed by the controller that there was a similar call sign on
the frequency. Again, this does not mean that controllers only inform
pilots that similar call signs are on the frequency in 2% of the cases
in actual operations. Rather, it points to the benefit of controllers
specifically advising pilots that there is a similar call sign on the
frequency.

Similar call signs on the same frequency was the major causal factor in
pilots accepting a clearance intended for another aircraft. Of the 68
reports of the wrong aircraft accepting a clearance, similar call signs
was cited as a contributing factor in 54% of them. In the 106 reports of
a loss of standard separation, 21% stated that similar call signs were
involved. More seriously, there were seven instances of reported near
mid-air collisions (NMAC) in this sample; similar call signs was listed
as a contributing factor in 3 (43%) of them.

[What can be done?]{.underline}

Controllers should continue to inform pilots when there are similar call
signs on the frequency. Pilots should always use their full call signs
in their acknowledgements. When there are similar call signs on the
frequency, stating the call sign at the beginning, and again at the end
of the readback, helps to ensure that the controller knows who has
accepted the clearance. Finally, controllers should NEVER assume that
the pilots are aware of the similar call signs, just because there
hasn\'t been a problem.

As one controller who filed a report of an incident that resulted in a
loss of standard separation states:

\"I did not issue \"similar call signs\" because both pilots seemed to
be aware of the other. I issued several clearances to each with no
confusion." He ends his report with the following recommendation,
\"Solution: Don\'t let your guard down, even for one second.\"
(Accession Number 169395).

Similarly, when it comes to similar call signs, we must remember that to
err is human and, unfortunately, to err repeatedly is also human. The
following excerpt from a report filed by a controller describes an
incident that resulted in an operational error:

\"I never specifically told the aircraft to be careful of similar call
signs and I assumed that after catching the first mistake, they would
not make the same mistake again.\" (Accession Number 190010)

**7. ANALYSIS OF RESULTS OF COMMUNICATION ERRORS BY CONTRIBUTING
FACTORS**

> The five most common results of the communication errors found in this
> study (listed sequentially from "most" to "least" common) were:
> altitude deviation, loss of standard separation, pilot accepting a
> clearance intended for another aircraft, operational error, and runway
> transgression. Remember that one report can identify no outcome or
> more than one outcome (such as altitude deviation and loss of standard
> separation). Similarly, a report may not identify any contributing
> factors or may identify more than one.
>
> Figures 7 - 11 chart the outcomes of the communication errors by the
> factors that were identified as contributing to them. We can see from
> these figures that similar call signs on the same frequency,
> controller workload, and pilot expectations were the three most common
> factors cited in altitude deviations. Reports citing loss of standard
> separation identified similar call signs and controller workload as
> contributing factors. The factor most commonly cited as contributing
> to communication errors resulting in pilots accepting a clearance
> intended for another aircraft was similar call signs on the same
> frequency. Similar call signs was identified in 54% of these
> instances. For runway transgressions, the only clearly associated
> factor identified was pilot expectation (identified in 32% of the
> reports). The reports of operator errors most often identified similar
> call signs and controller workload as contributing factors.
>
> **Figure 7. Altitude Deviations**
>
> **Figure 8. Loss of Standard Separation**
>
> **Figure 9. Aircraft Accepted a Clearance Intended for Another
> Aircraft**
>
> **Figure 10. Runway Transgressions**
>
> **Figure 11. Operational Errors**

**8. COMPARISON OF THE RESULTS OF THE ANALYSES OF ASRS REPORTS WITH THE
RESULTS OF TAPE ANALYSES AND OTHER STUDIES**

8.1 EFFECT OF MESSAGE COMPLEXITY

In the studies of actual ATC voice tapes, the only factor that clearly
affected the accuracy of the controller-pilot communication (as measured
by the probability of a correct readback) was the complexity of the
transmission; the less information that was contained in a single
transmission, the greater the probability of a full and correct readback
(Burki-Cohen, 1995; Cardosi, Brett, and Han, 1996; Cardosi, 1994;
Cardosi, 1993; and Morrow and Rodvold, 1993).

The study of en route communications showed a 1-3% miscommunication rate
(i.e., of readback errors and requests for repeats) for clearances
containing one to four pieces of information and a 8% rate for
transmissions containing five or more elements (Cardosi, 1993). Although
clearances containing five or more pieces of information constituted
only 4% of the messages examined, it accounted for 26% of the readback
errors found in the sample. In the TRACON environment, Morrow, Lee, and
Rodvold (1993) found that incorrect readbacks were more frequent for
communications containing two or more pieces of information than those
containing only one. In a part-task simulation study, Morrow and Rodvold
(1993) found that incorrect readbacks and requests for clarification
were more frequent after long messages (i.e., those containing four
pieces of information) than for shorter messages. While this relation
between message complexity and miscommunications was striking in the en
route environment, it was not as strong in the ground control study
(Burki-Cohen, 1995) and weaker, still, in the study of local tower
communications (Cardosi, Brett, and Han, 1996). This is probably due to
the fact that pilot familiarity with the airport is such a powerful
determining factor in the pilot\'s ability to comprehend and recall
clearances issued by tower controllers.

The effect of message complexity does not stop with communication
errors. In a study of altitude deviations, pilots said that almost half
(49%) of their altitude deviations involved multiple instructions being
given in the same controller transmission (Carlow and MiTech, 1992).

There were too few communication errors found in these tape analyses to
identify contributing factors in each of these studies. Furthermore,
some factors such as pilot expectation and workload are impossible to
infer from tape analysis. The only contributing factor that could be
identified on the tapes as being coincident with the miscommunications
observed was similar call signs on the same frequency. At most, (on the
TRACON frequencies) this was coincident with 6% of the miscommunications
observed (Cardosi, Brett, and Han, 1996).

While the complexity of the message is a powerful determining factor in
the probability of a successful transmission, it is not a factor that
can be examined in the ASRS reports. First, the reports tend to focus on
the part of the communication in error rather than offer a quotation of
the entire transmission. Secondly, even if the report did appear to
describe the entire clearance, the accuracy of the description would be
questionable (since it is dependent on the accuracy of the reporter\'s
memory for the clearance). For these reasons, message complexity cannot
be identified as a contributing factor, unless the reporter specifically
mentions it in the report. Clearly, we must look to the results of the
tape analysis to understand the effects of message complexity and look
to ASRS reports to understand the other factors that contribute to
communication errors.

8.2 TYPES OF ERRORS

The most common types of errors found in the tape analyses were
readback/hearback errors and call sign discrepancies. Readback/hearback
errors were also the most common type of error found in the ASRS reports
on communication errors. The tape analyses found that pilots are least
likely to make a readback error on altitude information, yet the most
commonly reported readback/hearback error in the ASRS reports is for
altitude information. In fact, 47% of the readback/hearback errors found
in this study involved altitudes. This is indicative of the fact that
readback errors of altitude, even though they are far less common than
readbacks of other information such as radio frequencies, have much more
serious (i.e., reportable) consequences than readback errors of other
types of information, such as headings or frequency changes.

In the tape analyses there was a one percent incidence of pilots
responding to a controller transmission with a different call sign than
the controller used. Over half of these call sign discrepancies were
never corrected as the controller continued to contact the aircraft with
one call sign and the pilot responded to the transmission with another
call sign (Cardosi, Brett, and Han, 1996). In most cases, such call sign
discrepancies do not result in any ill effects, or even ambiguity, since
there are other cues that controllers use to identify aircraft. In
addition to the visual information that the controllers have in front of
them on the flight (as to the location of the aircraft), they also have
the pilot\'s voice. Without a call sign, the pilot\'s voice and the
content and the context of the message are the only cues that the
controller is still talking to the same aircraft. While this presents
obvious opportunities for errors, it should be noted that it was a rare
case when one of the instances of call sign discrepancies found in these
studies resulted in a communication error. In only one case was there
another aircraft on the frequency with that (erroneously used) call
sign. In this case, an aircraft accepted a clearance to land that the
controller intended for another aircraft. This is probably why there
were so few instances of call sign errors found in the ASRS reports; it
is a situation that rarely results in a serious error.

**9. IMPLICATIONS OF FUTURE TECHNOLOGY**

> "Regardless of the level of sophistication that the air traffic system
> achieves by the turn of the century, the effectiveness of our system
> will always come down to how successfully we communicate." (Linter and
> Buckles, 1993)

Data link is often touted as a technology that will reduce or eliminate
communication errors. Certainly, data link technology presents
capabilities for reducing some forms of errors and greatly reducing
frequency congestion. However, no technology can resolve all
communication errors. Clearly, the pattern of errors will change.
Sending clearances directly to the aircraft will eliminate the
possibility of a pilot accepting a message intended for another
aircraft, however, it is possible for the controller to send the message
to the wrong aircraft or make another error in the message sent. For the
pilot, seeing clearances in writing has several advantages over hearing
them; it gives the advantage of being able to attend to it when the
pilot is ready and to double check it whenever necessary. However, it is
still possible to misinterpret written clearances (e.g., by transposing
numbers). While no communication technology can totally prevent
communication errors, careful use of data link is likely to reduce
frequency congestion and reduce the likelihood of communication errors.
The relative error rates observed with data link (compared to voice)
will depend on the design of the equipment and the procedures used to
implement it. Careful attention to human factors issues in the design
and implementation of data link systems will help to realize the
benefits of data link without a potentially deleterious increase in
controller workload.

**SUMMARY OF RECOMMENDATIONS**

The following is a compilation of results from the results of this
analysis and of analyses of voice tapes of ATC facilities.

For those pilots and controllers already using good communication
practices, admonishments to \"pay attention\" to controller
transmissions or pilots readbacks will have no effect; these
professionals are already performing at their peak. However, there are
many pilots and controllers whose performance could benefit from
following the communication practices recommended below.

-   **Controllers should be encouraged to keep their instructions short
    with no more than four instructions per transmission.** The
    complexity of the controller\'s transmission has a direct effect on
    the pilot\'s ability to remember it - there are fewer pilot errors
    with the less complex transmissions.

-   **Controllers should be encouraged to speak slowly and distinctly.**
    In a laboratory study, the rate of pilot readback errors and
    requests for repeats more than doubled when the same controller gave
    the same complex clearances in a faster speaking voice. With a
    normal rate of speech (156 words per minute), 5% of the
    controller\'s instructions resulted in a readback error or a request
    for repeat. This rate rose to 12% when the controller spoke somewhat
    faster (210 words per minute). (As a reference, the average
    newscaster speaks at about 180 words per minute.)

-   **Pilots should respond to controller instructions with a full
    readback of critical components.** An altitude, heading or airspeed
    should be readback *as an altitude, heading, or airspeed*. For
    example, \"Climbing to 230, Aircraft XYZ\" contains critical
    information that \"Roger, 230 for Aircraft XYZ\" does not. When more
    than one runway is in use, clearances to takeoff and land should be
    acknowledged with a readback of the clearance that includes the
    runway number and the aircraft call sign.

-   Controllers should treat the readbacks as they would any other piece
    of incoming information - use it. **Actively listen to the readback
    and check it against the flight strip notations** to ensure that the
    message that the pilot got was the one you wanted him to get.

-   **When there are similar call signs on the frequency, controllers
    should announce this fact**; this will alert pilots and may help to
    reduce the incidence of pilots accepting a clearance intended for
    another aircraft.

-   **When there are similar call signs on the frequency, pilots should
    be encouraged to say their call sign before and after each
    readback**; this gives the controller added information as to which
    aircraft accepted the clearance.

-   **Controllers should refrain from issuing \"strings\" of
    instructions to different aircraft.** Issuing strings of
    instructions to different aircraft without allowing the opportunity
    for each aircraft to respond directly after the controller\'s
    transmission has two undesirable effects. First, it increases the
    likelihood of a miscommunication. A pilot\'s memory for an
    instruction can be hindered by extraneous information presented
    before or after it. Second, it decreases the likelihood of a pilot
    readback, as it sends the message, \"I\'m too busy for your readback
    to be important right now\".

**REFERENCES**

Burki-Cohen, J. 1995. *An analysis of tower (ground) controller-pilot
voice communications*. DOT/FAA/AR-96/19

Cardosi, K., Brett, B., and Han, S. 1996. *An analysis of TRACON
(Terminal Radar Approach Control) controller-pilot voice
communications*. DOT/FAA/AR-96/66.

Cardosi, K. 1994. *An analysis of tower (local) controller-pilot voice
communications*. DOT/FAA/RD-93/11.

Cardosi, K. 1993. *An analysis of en route controller-pilot voice
communications*. DOT/FAA/RD-94/15.

Grayson, R.L. and Billings, C. E., 1981. *Information transfer between
air traffic control and aircraft: Communication problems in flight
operations.* In Billings and Cheaney (Eds.) Information Transfer
Problems in the Aviation System. NASA Technical Paper 1875, National
Aeronautics and Space Administration Scientific and Technical
Information Branch.

Linter, T and Buckles, J. (1993). *Why Can\'t We Talk to Each Other?*
Journal of Air Traffic Control, January-March.

MiTech Inc., Carlow International Inc., and Federal Aviation
Administration\'s Research and Development Service. *Altitude deviation
study: A descriptive analysis of pilot and controller incidents*. Final
Report, October 1992.

Monan, W.P., 1986. *Human factors in aviation operations: The hearback
problem.* NASA Contractor Report 177298.

Monan, W.P., 1983. *Addressee errors in ATC communications: The call
sign problem*. NASA Contractor Report 166462. National Aeronautics and
Space Administration.

Morrow, D., Lee, A., and Rodvold, M. 1993. *Analysis of problems in
routine controller-pilot communication*. International Journal of
Aviation Psychology, 3(4), 285-302.

Morrow, D. and Rodvold, M. 1993. *The influence of ATC message length
and timing on pilot communication*. NASA Contract Report 177621.

**GLOSSARY**

This glossary presents terms and definitions as they are used in this
report. Some of these terms are presented as defined by the Aviation
Safety Reporting System. Such definitions are followed by \"(ASRS)\".

*aircraft call sign* - the complete aircraft ID. For Part 121 carriers
this consists of an airline name and flight number.

*altitude deviation* - a departure from, or failure to attain, an
altitude assigned by ATC.

*ASRS*- Aviation Safety and Reporting System

*ATC* - air traffic control

*complete readback* - a pilot\'s acknowledgement of a controller's
transmission that repeats all of the key information the controller
conveyed. (Note that the information does not have to be repeated
verbatim, or in the same order, in this use of *complete readback*.)
Also, see *partial readback*.

*NMAC* (Near-Midair Collision)- a conflict in which the flight crew
reports (either directly, or as quoted by the controller) that the
reported miss distance is less than 500 feet. (ASRS)

*hearback error* - the failure, on the controller\'s part, to notice or
correct a pilot\'s readback error

*hearback error Type II* - the failure, on the controller\'s part, to
notice his/her own error in the pilot's correct readback. For example,
if the controller instructed an aircraft to descend to 11,000, but
*meant* to descend the aircraft to 10,000 and did not notice his or her
own error when the pilot readback the descent to 11,000, this would be a
hearback error type II.

*less than standard separation* - less than the legal separation between
two airborne aircraft (as defined by the airspace involved) (ASRS)

*operational error -* less than standard separation (between two or more
aircraft, or between aircraft and terrain, obstacles or obstructions,
including vehicles/equipment/personnel on the runway) that occurred as a
result of ATC actions, inactions, or ATC equipment malfunction. (ASRS)

*readback* - a pilot\'s acknowledgement of a controllers transmission
that repeats the information that the controller conveyed.

*readback error* - an incorrect repeat of the controller\'s transmission
by the pilot. For example, if the controller said, \"AirCarrier 123,
Descend and maintain one one thousand\" and the pilot responded with
\"Roger, one zero thousand for AirCarrier 123\", this would be a
readback error since the pilot should have read back the altitude of
11,000.

*reporter* - a pilot or controller who files an ASRS report

*partial readback* - a pilot\'s acknowledgement of a controllers
transmission that repeats some, but not all, of the key information that
the controller conveyed. For example, if the controller issued both an
altitude and heading, but only the altitude was readback, this would
constitute a *partial readback*. Also see *complete readback*.

*runway transgression* - the erroneous or improper occupation of a
runway or its immediate vicinity by an aircraft that poses a potential
collision hazard to other aircraft using the runway, even if no other
aircraft were actually present (ASRS).

Key words: controller-pilot communications, readback/hearback errors,
readbacks, hearback type II errors

Abstract

The purpose of this study was to identify the factors that contribute to
pilot-controller communication errors. Reports submitted to the Aviation
Safety Reporting System (ASRS) offer detailed accounts of specific types
of errors and a great deal of insight as to why they occur. The
communication errors found in this study could be classified into three
types: Readback/Hearback errors (the pilot reads back the clearance
incorrectly and the controller fails to correct the error); a lack of a
pilot readback, and Hearback Errors Type II (the controller fails to
notice his or her own error in the pilot\'s correct readback or
statement of intent).

Many factors contributed to these errors. The most common contributing
factors were: similar call signs on the same frequency, pilot
expectations (e.g., leading them to accept a clearance that they
expected rather than what the controller actually said), and high
controller workload. The identified results of these communication
errors were (in order of prominence): altitude deviations, loss of
standard separation, ATC operational errors, pilots landing on the wrong
runway, and runway transgressions. The report concludes with
recommendations for reducing the number of communication errors between
pilots and controllers.

\"Regardless of the level of sophistication that the air traffic system
achieves by the turn of the century, the effectiveness of our system
will always come down to how successfully we communicate.\" (Linter and
Buckles, 1993)

[^1]:     ^?^ The term \"operational error\" here and throughout this
    report refers to a situation that, in the ASRS analyst\'s judgement,
    would have resulted in an operational error as defined by the FAA.

[^2]:     ^?^ The term \"operational error\" here and throughout this
    report refers to a situation that, in the ASRS analyst\'s judgement,
    would have resulted in an ATC operational error as defined by the
    FAA.