**Preface**

We thank the Federal Aviation Administration Office of the Chief
Scientist for Human Factors (AAR-100) for supporting this and other air
traffic control (ATC) communications work. Bryan Brett of Science
Applications International Corporation (SAIC) coordinated the voice-tape
and raw-data analysis, and, together with Dave Malek, promptly fielded
many follow-up questions. Ben Cameron, John Chevalier, Joe Jarboe, and
Joe Moyer, also from SAIC, analyzed the voice tapes. We appreciate their
effort in this difficult and sometimes tedious task. Dr. Kim Cardosi
provided insightful comments on an earlier draft of this report.
Finally, we thank the quality assurance specialists at the participating
ATC facilities who supplied the tapes for this analysis.

**METRIC/ENGLISH CONVERSION FACTORS**

  ------------------ ------- --------------- ----------------- ------- -----------------
  **ENGLISH TO                               **METRIC TO               
  METRIC**                                   ENGLISH**                 

  **LENGTH                                   **LENGTH                  
  (APPROXIMATE)**                            (APPROXIMATE)**           

  **1 inch (in)**    **=**   **2.5           **1 millimeter    **=**   **0.04 inch
                             centimeters     (mm)**                    (in)**
                             (cm)**                                    

  **1 foot (ft)**    **=**   **30            **1 centimeter    **=**   **0.4 inch (in)**
                             centimeters     (cm)**                    
                             (cm)**                                    

  **1 yard (yd)**    **=**   **0.9 meter     **1 meter (m)**   **=**   **3.3 feet (ft)**
                             (m)**                                     

  **1 mile (mi)**    **=**   **1.6           **1 meter (m)**   **=**   **1.1 yards
                             kilometers                                (yd)**
                             (km)**                                    

                                             **1 kilometer     **=**   **0.6 mile (mi)**
                                             (km)**                    
  ------------------ ------- --------------- ----------------- ------- -----------------

  ------------------ ------- --------------- ----------------- ------- -----------------
  **AREA                                     **AREA                    
  (APPROXIMATE)**                            (APPROXIMATE)**           

  **1 square inch    **=**   **6.5 square    **1 square        **=**   **0.16 square
  (sq in, in^2^)**           centimeters     centimeter                inch (sq in,
                             (cm^2^)**       (cm^2^)**                 in^2^)**

  **1 square foot    **=**   **0.09 square   **1 square meter  **=**   **1.2 square
  (sq ft, ft^2^)**           meter (m^2^)**  (m^2^)**                  yards (sq yd,
                                                                       yd^2^)**

  **1 square yard    **=**   **0.8 square    **1 square        **=**   **0.4 square mile
  (sq yd, yd^2^)**           meter (m^2^)**  kilometer                 (sq mi, mi^2^)**
                                             (km^2^)**                 

  **1 square mile    **=**   **2.6 square    **10,000 square   **=**   **1 hectare (ha)
  (sq mi, mi^2^)**           kilometers      meters (m^2^)**           = 2.5 acres**
                             (km^2^)**                                 

  **1 acre = 0.4     **=**   **4,000 square                            
  hectare (he)**             meters (m^2^)**                           
  ------------------ ------- --------------- ----------------- ------- -----------------

+------------------+---+--------------+---------------+---+----------------+
| **MASS - WEIGHT  |   |              | **MASS -      |   |                |
| (APPROXIMATE)**  |   |              | WEIGHT        |   |                |
|                  |   |              | (A            |   |                |
|                  |   |              | PPROXIMATE)** |   |                |
+------------------+---+--------------+---------------+---+----------------+
| **1 ounce (oz)** | * | **28 grams   | **1 gram      | * | **0.036 ounce  |
|                  | * | (gm)**       | (gm)**        | * | (oz)**         |
|                  | = |              |               | = |                |
|                  | * |              |               | * |                |
|                  | * |              |               | * |                |
+------------------+---+--------------+---------------+---+----------------+
| **1 pound (lb)** | * | **0.45       | **1 kilogram  | * | **2.2 pounds   |
|                  | * | kilogram     | (kg)**        | * | (lb)**         |
|                  | = | (kg)**       |               | = |                |
|                  | * |              |               | * |                |
|                  | * |              |               | * |                |
+------------------+---+--------------+---------------+---+----------------+
| **1 short ton =  | * | **0.9 tonne  | **1 tonne (t) | * | **1.1 short    |
| 2,000 pounds     | * | (t)**        | =**           | * | tons**         |
| (lb)**           | = |              |               | = |                |
|                  | * |              | **1,000       | * |                |
|                  | * |              | kilograms     | * |                |
|                  |   |              | (kg)**        |   |                |
+------------------+---+--------------+---------------+---+----------------+

  ------------------ ------- --------------- ----------------- ------- -----------------
  **VOLUME                                   **VOLUME                  
  (APPROXIMATE)**                            (APPROXIMATE)**           

  **1 teaspoon       **=**   **5 milliliters **1 milliliter    **=**   **0.03 fluid
  (tsp)**                    (ml)**          (ml)**                    ounce (fl oz)**

  **1 tablespoon     **=**   **15            **1 liter (l)**   **=**   **2.1 pints
  (tbsp)**                   milliliters                               (pt)**
                             (ml)**                                    

  **1 fluid ounce    **=**   **30            **1 liter (l)**   **=**   **1.06 quarts
  (fl oz)**                  milliliters                               (qt)**
                             (ml)**                                    

  **1 cup (c)**      **=**   **0.24 liter    **1 liter (l)**   **=**   **0.26 gallon
                             (l)**                                     (gal)**

  **1 pint (pt)**    **=**   **0.47 liter                              
                             (l)**                                     

  **1 quart (qt)**   **=**   **0.96 liter                              
                             (l)**                                     

  **1 gallon (gal)** **=**   **3.8 liters                              
                             (l)**                                     

  **1 cubic foot (cu **=**   **0.03 cubic    **1 cubic meter   **=**   **36 cubic feet
  ft, ft^3^)**               meter (m^3^)**  (m^3^)**                  (cu ft, ft^3^)**

  **1 cubic yard (cu **=**   **0.76 cubic    **1 cubic meter   **=**   **1.3 cubic yards
  yd, yd^3^)**               meter (m^3^)**  (m^3^)**                  (cu yd, yd^3^)**
  ------------------ ------- --------------- ----------------- ------- -----------------

  ------------------- ------- --------------- ---------------- ------- -----------------
  **TEMPERATURE                               **TEMPERATURE            
  (EXACT)**                                   (EXACT)**                

  **\[(x-32)(5/9)\]   **=**   **y °C**        **\[(9/5) y +    **=**   **x °F**
  °F**                                        32\] °C**                
  ------------------- ------- --------------- ---------------- ------- -----------------

![](media/image1.emf)**1**

![](media/image2.emf)**2**

**For more exact and or other conversion factors, see NIST Miscellaneous
Publication 286, Units of Weights and Measures. Price \$2.50 SD Catalog
No. C13 10286 Updated 1/23/95**

**Table of Contents**

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

1\. INTRODUCTION 1

2\. MATERIALS 3

3\. CONTROLLER INSTRUCTIONS 5

4\. PILOT RESPONSES 7

5\. COMMUNICATION PROBLEMS 11

> 5.1 Overview 11
>
> 5.2 Requests for Repeats 11
>
> 5.3 Readback/Hearback Errors 12
>
> 5.4 Callsign Confusion 15
>
> 5.4.1 Overview 15
>
> 5.4.2 Callsign Confusion by Controllers 16
>
> 5.4.3 Callsign Confusion by Pilots 17
>
> 5.5 Conceptual Errors 17
>
> 5.5.1 Conceptual Errors by Controllers 18
>
> 5.5.2 Conceptual Errors by Pilots 19

5.6 Communication Problems and Transmission Density 20

6\. SUMMARY AND COMPARISON WITH PREVIOUS STUDIES 21

7\. RECOMMENDATIONS 23

8\. REFERENCES 25

**List of Figures**

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

1\. Percent Controller Instructions at Each Complexity Level 5

2\. Percent Pilot Responses for Each Type of Response 7

3\. Percent of the Three Most Frequent Pilot Responses to Instructions
at Each

> Complexity Level 8

4\. Percent Requests for Repeat of the Instructions at Each Complexity
Level 11

5\. Percent Errors in the Readback and Hearback of the Instructions at
Each Complexity

> Level 12

6\. Number of Readback and Hearback Errors for Each Type of Information
13

7\. Number of Callsign Confusions for Each Type of Confusion 15

8\. Number of Conceptual Errors by Controllers for Each Type of
Information 18

9\. Number of Conceptual Errors by Pilots for Each Type of Information
19

**List of Tables**

[Table]{.underline} [Page]{.underline}

1\. Transmissions and Transmission Densities at Each Facility 3

2\. Transmission Distribution by Senders and Receivers 4

**Executive Summary**

This analysis of ground-control communications is part three of a
four-part effort to capture not only the qualitative, but also the
quantitative aspects, or incidence, of air traffic control
miscommunications. The approach taken is to examine representative
samples of pilot-controller communications recorded in four air traffic
control (ATC) environments; tower-ground control (this report), air
route traffic control centers (Cardosi, 1993), tower-local control
(Cardosi, 1994), and terminal radar control (TRACON) (in progress).

Despite initial applications of datalink,[^1] voice communications
between air traffic controllers and pilots remain at the center of ATC
operations. Given this prominent role, it is not surprising that
communication problems have been identified as a factor in over 70
percent of operational errors and pilot deviations in the FAA\'s
Operational Error and Deviation System (OEDS) (Danaher, 1993). Accidents
such as the 1977 collision between two Boeing 747s at Tenerife,
demonstrate the potentially fatal consequences of inadequate
communication. (One aircraft taxied down the runway while the other used
the same runway for an unauthorized takeoff.) Additional evidence for
communication problems stems from incidents reported to the Air Safety
Reporting System (ASRS; see, e.g., Monan, 1988). Although evidence from
such (potential) accidents and incidents is very valuable for
pinpointing and/or investigating specific areas of concern, it
represents a biased sample, counting only the cases in which something
went awry.

At the Department of Transportation's Volpe Center for Transportation
Human Factors Research, we are taking a three-pronged approach to the
investigation of air-traffic control communications. In addition to
communication problems reported to the ASRS, we are examining voice
tapes recorded at ATC facilities during actual operations. This endeavor
is the subject of the present report. Furthermore, we are verifying some
of the conclusions drawn from the two previous approaches and
investigating other issues relevant to effective controller-pilot
communication in carefully controlled laboratory studies (see, e.g.,
Bürki-Cohen, 1995 a, b).

This report is based on an analysis of over 48 hours of pilot-controller
communications recorded from the ground-control frequency at twelve air
traffic control towers. We examined the complexity of controller
instructions, that is, how many pieces of information a single
controller transmission contains. We also looked at how pilots respond
to these instructions, and whether the type of response is affected by
the complexity of the instructions. Particularly, we studied the effect
of complexity of the instructions on communication problems, such as
when pilots ask controllers to repeat their instructions or when they
make an error in the readback. Other communication problems examined
include aircraft callsign discrepancies and conceptual errors (e.g.,
when a controller sends a pilot to the wrong runway or a pilot dials in
the wrong frequency.)

The following recommendations to further improve ATC communications and
thus the margin of safety in the ground-control environment arise from
this investigation:

1.  Controllers should keep instructions short. The shorter an
    instruction, the more likely will it be correctly read back by the
    pilot.

2.  Controllers should listen to what a pilot reads back, especially
    regarding hold-short and taxi instructions and frequencies. More
    emphasis should be given to hearback during controller training
    (ASRS Callback, 1992).

3.  Controllers should try to speak slowly *especially* when they are
    under pressure and don't have time to repeat information.

4.  When talking to foreign pilots, controllers should take into account
    the potential for phraseology differences and reduced English
    language proficiency. The FAA should compile a list phraseology
    differences to be distributed to controllers *and* pilots,
    especially those flying internationally. Controllers also should
    speak *"staccato,"* that is, to break the instruction up into its
    component words by inserting short pauses. Recognizing where one
    word ends and the next begins is notoriously difficult for any
    inexperienced listener of a foreign language. Repeating numbers in
    grouped format, i.e., "seven-teen," instead of sequential format,
    "one seven," as recently authorized for emphasis of altitudes (FAA,
    1992), may backfire with foreign pilots who group numbers
    differently in their native language.

5.  Pilots should ask when they are not sure about a piece of
    information. But even if pilots are sure that they have heard and
    remembered correctly, they should at least read back hold-short
    instructions and frequency changes.

6.  Whenever possible, controllers should point out similar callsigns on
    the same communication frequency. All instructions and readbacks
    should include the full callsign.

7.  Both controllers, when listening to readbacks, and pilots, when
    taking instructions, should be aware of how their expectations may
    affect what they hear. Pilots expecting certain instructions must
    wait for complete aircraft identification before taking action on
    the instructions.

**1. INTRODUCTION**

Despite initial applications of datalink,[^2] voice communications
between air traffic controllers and pilots remain at the center of air
traffic control (ATC) operations. Given this prominent role, it is not
surprising that communication problems have been identified as a factor
in over 70 percent of operational errors and pilot deviations in the
FAA\'s Operational Error and Deviation System (OEDS) (Danaher, 1993).
Accidents such as the 1977 collision between two Boeing 747s at
Tenerife, where one aircraft taxied down the runway while the other used
the same runway for an unauthorized takeoff, demonstrate the potentially
fatal consequences of inadequate communication. Additional evidence for
communication problems stems from incidents reported to the Air Safety
Reporting System (ASRS; see, e.g., Monan, 1988). Although evidence from
such (potential) accidents and incidents is very valuable for
pinpointing and/or investigating specific areas of concern, it
represents a biased sample, counting only the cases in which something
went awry. At the Department of Transportation's Volpe Center for
Transportation Human Factors Research, we are taking a three-pronged
approach to the investigation of air-traffic control communications. In
addition to communication problems reported to the ASRS, we are
examining voice tapes recorded at ATC facilities during actual
operations. This endeavor is the subject of the present report.
Furthermore, we are verifying some of the conclusions drawn from the two
previous approaches and investigating other issues relevant to effective
controller-pilot communication in carefully controlled laboratory
studies (see, e.g., Bürki-Cohen, 1995 a, b).

This analysis of ground-control communications is part three of a
four-part effort to capture not only the qualitative, but also the
quantitative aspects, or incidence, of air traffic control
miscommunications. The approach taken is to examine representative
samples of pilot-controller communications from four ATC environments:
tower-ground control (this report), air route traffic control centers
(Cardosi, 1993), tower-local control (Cardosi, 1994), and terminal radar
control (TRACON) (in progress).

The Federal Aviation Administration (FAA) lists communications as a
primary factor causing runway incursions (Runway Incursion Plan, 1991).
Although runway incursions are rare events, the same document reports a
39 percent increase in runway incursions at towered airports for the
first eleven months of 1990 (249 incursions) compared to 1988 (179
incursions). The present report is based on a study of over 48 hours of
pilot-controller communications recorded from the ground frequency at
twelve air traffic control towers. It examines the complexity of
controller instructions, that is, how many pieces of information a
single controller transmission contains. It looks at how pilots respond
to these instructions, and whether the type of response is affected by
the complexity of the instructions. Particularly, it studies the effect
of complexity of the instructions on communication problems, such as
when pilots ask controllers to repeat their instructions or when they
make an error in the readback. It also examines the incidence and
possible causes of callsign confusions as well as of conceptual errors
(such as when a pilot dials in the wrong frequency or a controller sends
a pilot to the wrong runway) in pilot-controller communications. Lastly,
it compares the incidence of communication problems with the
transmission density (transmissions per minute) at a facility. It
relates these findings to what has been found in the en-route (Cardosi,
1993), terminal-radar (TRACON; Morrow, Lee, and Rodvold, 1993), and in
the tower-local control (Cardosi, 1994) environment. In conclusion, a
series of recommendations is presented.

**2. MATERIALS**

Over 48 hours of ground air-traffic communications were examined. Twelve
different ground‑control facilities provided the tapes. The facilities
were selected to cover a wide range of geographical locations, possible
regional accents, workload levels, and traffic mixes (general aviation,
airlines, and international traffic). Table 1 lists the facilities with
the number of hours and transmissions examined, and also the average
transmission density (transmissions per minute). The facilities are
sorted in ascending order of the average transmission density. The
average transmission density ranges from four transmissions per minute
at Baltimore-Washington ground control to 12 transmissions per minute at
Boston Logan ground control. All tapes were recorded during periods of
busy traffic, as defined by the facility.[^3]

**TABLE 1. TRANSMISSIONS AND TRANSMISSION DENSITIES AT EACH FACILITY.**

  ---------------------- ------- ---------------- ---------------------------
  TOWER                  HOURS   TRANSMISSIONS    TRANSMISSIONS PER MINUTE

  Baltimore-Washington   3.63    858              3.94

  Albuquerque            3.11    1,074            5.75

  Atlanta                6.90    2,636            6.36

  Dallas-Ft. Worth       4.13    1,590            6.42

  Dulles                 2.06    795              6.45

  Seattle                4.13    1,758            7.09

  Chicago                4.11    1,845            7.48

  Philadelphia           2.05    988              8.03

  Pittsburgh             4.07    2,226            9.12

  Los Angeles            4.15    2,447            9.84

  Miami                  4.03    2,575            10.64

  Boston                 6.15    4,432            12.01

  All                    48.51   23,224           7.98
  ---------------------- ------- ---------------- ---------------------------

As shown in Table 2, the total of 23,224 transmissions examined included
10,208 controller-to-pilot transmissions and 12,221 pilot-to-controller
transmissions. Transmissions between controllers and ground vehicles
were not examined in this study. A transmission is a single continuous
communication by a speaker, whatever its length.

**TABLE 2. TRANSMISSION DISTRIBUTION BY SENDERS AND RECEIVERS.**

+--------------+--------------+---------------+---------------+-------+
| Cont         | Pilo         | Controller-   | Ground        | All   |
| roller-Pilot | t-Controller |               | Vehicle-      |       |
|              |              | Ground        |               |       |
|              |              | Vehicle       | Controller    |       |
+--------------+--------------+---------------+---------------+-------+
| 10,208       | 12,221       | 403           | 392           | 2     |
|              |              |               |               | 3,224 |
+--------------+--------------+---------------+---------------+-------+

Of the 10,208 controller-pilot transmissions, two thirds (6,841) were
taxi instructions. The remaining transmissions were initial contacts,
questions or requests, answers to questions and requests, and weather or
traffic advisories.

Of the 12,221 pilot-controller transmissions, more than half (6,432)
were responses to instructions. The remaining transmissions were initial
contacts, position reports, questions or requests, and answers to
questions and requests.

The tape analysis was conducted by a former controller and three pilots.
They identified and transcribed all communication errors, which were
then analyzed separately by the author.

**3. CONTROLLER INSTRUCTIONS**

Figure 1 shows the relative frequencies of controller instructions at
each complexity level. The complexity level was computed by counting all
elements containing information a pilot has to remember, such as
taxiways, runways, who to follow, but not items such as aircraft and
facility identification, "Roger," or salutations. For example, the
instruction "(Aircarrier) 3890, (Facility) Ground, give way to the
second Dornier inbound, then taxi runway 32 left, intersection departure
at Gulf, via outer, Charlie, Gulf" was coded as containing the following
eight elements: Give way, Traffic, Runway, Other, Location, Taxiway 1,
Taxiway 2, Taxiway 3. Although most of the instructions contained three
or fewer pieces of information, over 35 percent contained four or more
elements.

**Figure 1. Percent Controller Instructions at Each Complexity Level.**

**4. PILOT RESPONSES**

4.1 OVERVIEW

Figure 2 shows pilots\' responses to the 6,841 taxi instructions issued
by ATC. One third (32 percent) of the instructions were acknowledged
with a full readback, allowing the ground controller to verify pilots'
correct perception and recall of the information through "hearback."
Another third (33 percent) of the instructions were acknowledged with an
equivalent of \"Roger,\" \"O.K., thanks,\" or callsign only. The
remaining instructions were followed by a partial readback (16 percent),
requests for repeat (fewer than two percent), other replies (10
percent), or no response at all (8 percent). The fact that only one
third of the instructions were acknowledged with a full readback needs
to be kept in mind when looking at the low incidence of readback errors
(Section 5).

**Figure 2. Percent Pilot Responses for Each Type of Response.**

4.2 EFFECT OF COMPLEXITY ON TYPE OF RESPONSE

Figure 3 shows the distribution of full and partial readbacks and
one-word acknowledgments over complexity levels. The effect of
instruction complexity on requests for repeats will be discussed in the
section on communication problems.

**Figure 3. Percent of the Three Most Frequent Pilot Responses to
Instructions at Each Complexity Level.**

As can be clearly seen, when instructions became more complex, the
percentage of partial readbacks sharply increased, whereas the
percentage of full readbacks decreased (Pearson r~xy~=.96 and -.90,
respectively; i.e., 92 and 81 percent of the change was accounted for by
the increase in complexity).[^4]

The percentage of one-word acknowledgments also decreased with
increasing complexity, but only slightly more than half of this decrease
was accounted for by an increase in complexity of the ATC instructions
(Pearson r~xy~=.72).

Similarly, fewer of the more complex instructions were not acknowledged,
but now less than half of this decrease was accounted for by the
complexity increase (Pearson r~xy~=.70). Other types of responses (such
as comments, questions, or requests other than for repeating the
instructions) very slightly increased with instruction complexity
(Pearson r~xy~=.47).

**5. COMMUNICATION PROBLEMS**

5.1 OVERVIEW

In this section, any interruption of the ideal controller-pilot
communication sequence, that is, accurate instructions from the
controller that are understood and correctly read back by the pilot to
whom they were addressed, will be discussed. This includes requests from
the pilot that the controller repeat the instructions, misunderstandings
by the pilot that result in erroneous readbacks, failure of the
controller to recognize erroneous readbacks (hearback errors), confusion
of the callsign either by the controller or the pilot, and conceptual
errors such as a controller sending a pilot to the wrong runway or a
pilot dialing in the wrong frequency. Some errors were corrected within
a transmission, e.g., "Ground ASE 395, 5 north, correction, 4 north with
tango". While this type of error may represent a cognitive load on the
receiver of the message and contribute to congestion of the
communication frequencies, it was not classified as a communication
problem.

5.2 REQUESTS FOR REPEATS

For 76 of the 6,841 taxi instructions (one percent), pilots requested
full or partial repeats. Figure 4 shows the percent requests for repeats
at each complexity level. The need for full or partial repetition of
instructions increased with the complexity of the instructions from one
to eight pieces of information in a single instruction (Pearson
r~xy~=.73, i.e., over 50 percent of the increase was accounted for by
the increase in complexity).

**FIGURE 4. PERCENT REQUESTS FOR REPEAT OF THE INSTRUCTIONS AT EACH
COMPLEXITY LEVEL.**

5.3 READBACK/HEARBACK ERRORS

Twenty-six of the 3,313 full and partial readbacks contained errors,
that is, fewer than one percent. Two of these communications contained
two readback errors. The total error count was thus 28.

The distribution of readback and hearback errors as a function of the
complexity of the instructions that were read back is shown in Figure 5.
The errors are expressed in percent of the combined full and partial
readbacks (i.e., opportunities for errors), at a given complexity level.
The Pearson r~xy~ of .87 (calculated over complexity levels 1 through 7)
shows that three quarters of the increase in readback errors are
accounted for by the complexity increase.

**FIGURE 5. PERCENT ERRORS IN THE READBACK AND HEARBACK OF THE
INSTRUCTIONS AT EACH COMPLEXITY LEVEL.**

Fourteen or half of the readback errors were both corrected by the
controller and the corrections were acknowledged by the pilot. In 16
cases the controller corrected the readback by repeating the information
in error. Only two of these corrections were not explicitly
acknowledged. Twelve errors, however, were not corrected by the
controller (hearback errors). Some of them contained critical
information. For one of these errors, where the pilot read back an
instruction to hold short of a runway giving the wrong runway, the
controller requested a repeat, but did not notice that the pilot omitted
the runway to hold short of in his repeat.

Figure 6 shows the distribution of errors with regard to the type of
information in error. Eight of the readback errors involved hold-short
instructions (limits), eight the taxi route (including one runway
crossing), five frequencies, three the sequence, three contained an
erroneous runway, and one the wrong ATIS (Automatic Terminal Information
Service).

**FIGURE 6. NUMBER OF READBACK AND HEARBACK ERRORS FOR EACH TYPE OF
INFORMATION.**

Four errors, or 14 percent, may have been caused by interference of
other information contained in the instruction. For example, when the
controller advised \"Expect 27L\" in the same instruction, the
instruction to hold short of runway 27R was read back as hold-short of
27L. Similarly, when the controller advised of traffic on the \"outer\"
and, in the same instructions, asked the pilot to join the inner
taxiway, the pilot read back that he had to join the outer instead of
the inner taxiway. After being told to \"turn left on Quebec and hold
short of Quebec 4,\" a pilot read the limit back as Quebec and omitted
the turn left instructions. When the taxiway was November and the ATIS
was Quebec, the pilot read the ATIS back as November and omitted the
taxiway.

In one case, the controller was apparently very busy, as shown by an
apology for the wait, and might have spoken too quickly. Another error,
concerning the sequence, occurred after multiple changes of the
instruction by the controller, increasing the cognitive workload of the
pilot and, presumably, also an indication of high controller workload.

In another case, a pilot of a Japanese aircraft with a very heavy
foreign accent read back \"pass behind the DC 10 exiting the runway. He
will turn right\" as \"we will turn right.\" His readback of the
controller\'s correction was almost unintelligible.

Other errors involved the confusion of left and right, inner and outer,
V and W, etc., but none occurred more than once.

In summary, two emerging patterns associated with readback errors are
interference from other pieces of information in the instruction and the
complexity of the instructions to be read back. Fluency in English and
controller speech rate were a factor in at least one readback error
each.

5.4 CALLSIGN CONFUSIONS

5.4.1 [Overview]{.underline}

There were 118 cases of callsign (CS) discrepancies in the 23,224
transmissions, that is, exchanges in which pilots and controllers did
not use the same callsign. Figure 7 shows that in the majority of these
transmissions, the controller appears to be using the wrong callsign
(90).[^5]

Fifty-two of the confusions by controllers were resolved. Forty-five of
these confusions were corrected by the pilot of the intended (but
incorrectly named) aircraft in the acknowledgment or with a question,
sometimes only after two or three ATC calls that remained without
response. In two cases, the named (but not intended) aircraft alerted
the controller, one with a question and the other with the statement
\"We already did that.\" In five cases, the controllers corrected the
confusion themselves, three times in a subsequent transmission, once
after the pilot responded, and once after nobody responded to the wrong
callsign. Fourteen of the corrections were not explicitly acknowledged.

**FIGURE 7. NUMBER OF CALLSIGN CONFUSIONS FOR EACH TYPE OF CONFUSION.**

Of the 38 controller-induced callsign discrepancies that remained
unresolved, 13 were answered by the presumably intended, but incorrectly
named aircraft without correcting the callsign. In at least two cases,
the pilot had previously corrected the controller, but the controller
continued to use the discrepant callsign. In one case, a third aircraft
responded, which led to an exchange with multiple callsign confusions
(see below). The remaining 22 confusions appeared to remain unanswered.

Of the 28 pilot-initiated callsign confusions, 24 occurred in
transmissions and 4 were responses to wrong callsign. Half of the
callsigns in transmissions were corrected by the controller, sometimes
with a question. Except for two, these corrections were acknowledged.

Only one of the four responses to the wrong callsign was successfully
corrected by the controller. For another erroneously accepted
transmission including a frequency change, the controller tried to
correct the confusion, but the aircraft had already switched to the new
frequency.

5.4.2 [Callsign Confusions By Controllers]{.underline}

Of the 90 callsign confusions by controllers, 23 were part of ten
exchanges involving multiple callsign confusions. Because these
exchanges effectively illustrate the load imposed by callsign confusions
on the ATC system, even when they remain without serious consequences,
some of them will be discussed in detail below.

In one of these exchanges, involving three aircraft from the same
carrier, the controller asks Aircarrier 1407 for a repeat after a
hard-to-understand transmission from 1406 (first confusion). Now 146
mistakenly tries to respond (this was one of the five responses to the
wrong callsign by pilots), but 1406 steps on, giving a correct callsign
and repeating its request for a ramp transition. The controller responds
with a follow-up question (\"taxi or tow\"), using 146 (second
confusion). Aircarrier 1406 responds \"taxi\" without giving its
callsign. The controller gives taxi instructions \"by Lima as
requested,\" and continues to use 146 (third callsign error). Now 146
comes on the frequency with \"(ATC facility), we have Zulu.\" The
controller asks for a repeat, and 146 responds with the full callsign
and location, indicating \"Zulu for (facility).\" The controller
responds with the correct callsign 146, ATC facility identification, and
taxi instructions. They are read back correctly, although without
callsign. No mention of the confusion was made by any of the parties
involved. Contributing factors to the confusions in this exchange were
sound quality of the first transmission, similarity between the callsign
1406 and 146 of the same carrier, response to wrong callsign without
correction (first by 146, then by 1406), and omission of callsign and
readback by 1406.

In another multiple exchange taxing the ATC system, the controller calls
Aircarrier 69 Alpha first by its correct name, but then, when there is
no response, switches to (same) Aircarrier 69 Bravo for the next call.
After two calls without response, (same) Aircarrier 341 asks whether the
call is for 341. The controller answers in the negative and gives up
after two additional attempts to contact 69 Bravo.

The vast majority of callsign confusions involve a difference of only
one number or one letter. Only few differ in the name of the carrier; in
those cases, usually the numbers are identical (Aircarrier A 546 instead
of Aircarrier B 546). Numbers are either dropped (146 for 1406), added
(16-24 for 6-24), substituted (7-98 for 7-28), or transposed (16-17 for
16-71). Sometimes, an aircraft with a similar callsign was indeed on the
same frequency. In two cases, the pilot was hard to understand. In one
case, the pilot called up and said \"Aircarrier 1444 Echo
*[five]{.underline}*,\" which may have induced the controller to call
the aircraft \"Aircarrier 144*[5]{.underline}*.\" In another case, a
controller calls \"Aircarrier 757 on the 18 Bridge\" for an urgent
hold-short instruction, when addressing Aircarrier 1720. Apparently, the
controller used location and aircraft type for rapid identification
instead of the correct callsign. Incidentally, the aircraft read back
\"Go to India\" instead of \"Hold short of India for traffic\" without
correction by the controller. The outcome of this possible
misunderstanding is not known.

5.4.3 [Callsign Confusions By Pilots]{.underline}

The 28 callsign confusions by pilots did not differ very much from the
ones made by controllers. Again, the majority involve changes in the
numbers contained in the callsign. A few are due to the pilot giving up
correcting a controller using a similar, but incorrect callsign and
adopting it themselves.[^6]

In only one case did a pilot respond to a wrong callsign that was
similar. In the remaining three responses to the wrong callsign, the
wrong callsign was quite different from the correct one. In one case,
Aircarrier A 1310 responded to a call for Aircarrier B 707, in another
Aircarrier C 1582 responded to Aircarrier D 978. Possibly, the most
potent factor enticing a pilot to respond to an instruction for other
aircraft is its similarity to the instructions the pilot is waiting for,
rather than just a similar callsign. Although none of the three
responses to wrong aircraft that remained undetected appears to have had
consequences, the potential for an incident is evident.

5.5 CONCEPTUAL ERRORS

The 29 conceptual errors *identified*[^7] in this analysis are not
*communication* errors per se, but rather errors in cognition, such as
when a controller sends a pilot to the wrong runway or a pilot dials in
the ground control frequency when he or she wants to talk to the ramp.
They are included in this discussion because they also affect the
efficiency of the ATC system.

5.5.1 [Conceptual Errors By Controllers]{.underline}

Figure 8 shows the distribution of the nine conceptual errors made by
controllers. Three taxi instructions sent the aircraft to the wrong
destination, e.g., a gate instead of the cargo ramp, the wrong ramp
spot, or, more seriously, to runway \"32 left\" instead of \"right.\"
Two instructions gave the wrong sequence (\"after Aircarrier A 75\"
instead of \"Aircarrier B 737\"), and two more the wrong taxi direction
(east instead of west, or left instead of right). For the last two
errors, controllers pointed out traffic from the wrong direction and
issued the wrong ATIS as current. Except for the ATIS, all errors were
corrected during readback or hearback and acknowledged by the other
party.

![](media/image3.emf)

**FIGURE 8. NUMBER OF CONCEPTUAL ERRORS BY CONTROLLERS FOR EACH TYPE OF
INFORMATION.**

5.5.2 [Conceptual Errors By Pilots]{.underline}

Figure 9 shows the distribution of the 20 conceptual errors made by
pilots. Twelve of these involved dialing in the wrong frequency, i.e.,
talking to ground control when trying to reach the ramp (seven cases),
arrival ground (three cases), or clearance delivery (two cases). Nine of
these erroneous contacts were pointed out by the controller, but for two
ground control did respond with an instruction. One remained without
response. In five instances, a pilot gave a wrong location (the wrong
bridge, ramp, taxiway or runway). All of these were corrected, two by
the controller, three by the pilot after the controller's response. In
two cases, pilots inadvertently keyed the mike while speaking to each
other. One of these transmissions may have been mistaken for an
acknowledgment by the controller, the other was resolved with a question
from the controller. In one last case, a pilot indicated the current
ATIS as "Yankee" instead of "Sierra" and was corrected by the
controller.

**FIGURE 9. NUMBER OF CONCEPTUAL ERRORS BY PILOTS FOR EACH TYPE OF
INFORMATION.**

5.6 COMMUNICATION PROBLEMS AND TRANSMISSION DENSITY

The incidence of communication problems seems to be unaffected by the
frequency congestion during the period sampled. No or negligible
correlations were found between conceptual and callsign errors and
transmission density or between readback errors and instruction density
at a facility (Pearson r~xy~\<\|.26\|). Interestingly, when the readback
error rate was calculated as a percentage of the overall transmission
rate instead of only the instruction rate and correlated to the
transmission density, the correlation was somewhat higher (Pearson
r~xy~=.40, that is, 16 percent of the increase in readback errors was
associated with an increase in transmission density).

6\. **SUMMARY AND COMPARISON WITH PREVIOUS STUDIES**

A first finding was that while the majority of instructions in the
ground control environment contained three or fewer pieces of
information, over a third of the instructions contained four or more
pieces of information to remember. This finding is important considering
the effect of complexity of the instructions on how pilots respond to
the instructions and on the incidence of readback errors and requests to
hear the instructions again (see below).

Ideally, a pilot would read the instructions back in full, so that the
controller can verify whether the instructions have been understood
correctly. The data show that the incidence of full readbacks dropped
significantly with increasing complexity, while the incidence of partial
readbacks increased. Overall, fewer than a third of the instructions
were read back in full. This needs to be kept in mind when looking at
the nevertheless impressively low readback error rate of fewer than one
percent.

One-word acknowledgments also decreased with increasing complexity.
Requests to repeat all or part of the information, although overall low
(fewer than one percent), increased from half a percent at complexity
level one to over three and a half percent at complexity level eight.

The 28 readback errors were highly correlated to the complexity of the
instructions, ranging from fewer than one percent up to complexity level
three to over two percent at complexity level seven. Besides complexity,
contributing factors to erroneous readbacks were interference from other
information in the same instructions and, in one case each, fluency in
English and controller speech rate. Twelve of the readback errors
remained unresolved, with consequences unknown. With eight errors each,
the most frequently misunderstood information was on taxi limits and
routes.

Compared to the en-route environment with fewer than five percent of all
instructions containing more than four pieces of information (Cardosi
1993), instructions from tower-ground (over 35%) and tower-local control
(31%, Cardosi 1994) were considerably more complex. Also, the
transmission density in the three environments varied considerably,
going from 1.8 transmissions per minute in the en-route environment to
3.9 and 8 transmissions per minute at local and ground control,
respectively. Pilots' perception of how busy a controller and how
congested a communication frequency is may thus be a factor in the fact
that instructions were most likely read back in the en-route environment
and least likely in the ground environment.

Similar, however, was the incidence of readback error rates and of
requests to repeat the information in the three environments, all below
one percent. And for all three environments, the errors and requests for
repetition increased with increasing complexity of the instructions.
This finding has also been confirmed experimentally in Bürki-Cohen, 1995
a and b, and Morrow and Rodvold, 1993.

Morrow, Lee, and Rodvold (1993) documented a similarly low readback
error rate for the TRACON environment, analyzing 42 hours of
communications with over 6 transmissions per minute, although only half
of these errors were corrected by controllers. In the en-route
environment (Cardosi, 1993), only 11 percent of readback errors were not
corrected by the controller (hearback error). In the tower environment,
however, the hearback-error rate rose to 37 for local and 40 percent for
ground control. It appears that hearback errors increase with increasing
transmission density, given the considerably higher transmission
densities in the tower and TRACON compared to the en-route environment
with fewest hearback errors.

Other communication problems examined in this study include aircraft
callsign discrepancies and conceptual errors. Both types of problems may
occur in any type of transmission, not only instructions and responses
to instructions. Again, the incidence was lower than one percent,
although conceptual errors may not always be apparent from the
audiotapes. The few conceptual errors apparent from the audiotapes were
made mainly by pilots; most involved dialing in the wrong frequency.

Callsign discrepancies appeared to be initiated mainly by controllers.
Most of these were either explicitly or implicitly resolved, or the
instructions remained unanswered. Of the callsign confusions in pilot
transmissions, half were corrected, usually by controllers. Most
confusions involved only one number or one letter, rarely the aircarrier
name. Some were induced by a similar callsign on the same frequency.
Although none of the discrepant callsign uses appeared to represent a
safety problem *per se*, many resulted in lengthy exchanges involving
multiple callsign confusions, tying up the communication frequency. More
problematic, from a safety standpoint, are the fortunately only four
responses to a wrong callsign by pilots, of which three remained
undetected\--with unknown outcome. Interestingly, similarity between the
*callsigns* of the intended and the responding aircraft appeared to be a
factor in only one of these cases, suggesting that it was the match
between the expected and the issued *instructions* that enticed the
wrong pilots to respond. Cardosi (1993, 1994) reported similar
incidences of callsign discrepancies for the en-route and local control
environments.

**7. RECOMMENDATIONS**

A readback error rate of fewer than one percent is a tribute to the
pilots and controllers operating in the National Airspace System.
Nevertheless, the readback errors encountered were serious and had the
potential for grave consequences, especially the 50 percent errors that
were not corrected or for which the correction was not acknowledged.
Similarly, callsign confusions, responses to wrong callsign, and
conceptual errors were relatively few compared to opportunity, but, even
in the best of cases, impose a load on the communication frequency. The
following recommendations to further improve ATC communications and thus
the margin of safety in the ground-control environment arise from this
investigation:

1.  Controllers should keep instructions short. Lengthy instructions
    also increase the possibility of interference from similar pieces of
    information within the same transmission, such as when a controller
    designates one side of a runway for departure while asking to hold
    short of the other side of the same runway. Furthermore, the shorter
    an instruction, the more likely will it be read back by the pilot.

2.  Controllers should listen to what a pilot reads back, especially
    regarding hold-short and taxi instructions and frequencies. More
    emphasis should be given to hearback during controller training
    (ASRS Callback, 1992).

3.  Controllers should try to speak slowly *especially* when they are
    under pressure and don't have time to repeat information.

4.  When talking to foreign pilots, controllers should take into account
    the potential for phraseology differences and reduced English
    language proficiency. An example of a phraseology difference is that
    a foreign pilot who is instructed to "taxi into position" may *hold
    short of* the runway, instead of taxiing *onto* the runway. The FAA
    should compile a list of such differences to be distributed to
    controllers *and* pilots, especially those flying internationally.
    Foreign pilots may also be used to a different definition of flight
    levels as well as encounter conversion problems (e.g., millibars
    into inches of mercury). Moreover, repeating numbers in grouped
    format, i.e. "seven-teen," instead of sequential format, "one
    seven," as recently authorized for emphasis of altitudes (FAA,
    1992), may backfire with foreign pilots who group numbers
    differently in their own language (e.g., in French, "seven-teen" is
    said as "ten-seven").[^8] A last recommendation is to speak
    "*staccato,*" that is, to break the instruction up into its
    component words by inserting tiny pauses. Recognizing where one word
    ends and the next begins is notoriously difficult for any
    inexperienced listener of a foreign language.

5.  Pilots should ask when they are not sure about a piece of
    information. Simply reading the instructions back may not be enough,
    because controllers are often busy with other tasks during readback.
    But even if pilots are sure that they have heard and remembered
    correctly, they should at least read back hold-short instructions
    and frequency changes.

6.  Whenever possible, controllers should point out similar callsigns on
    the same communication frequency. All instructions and readbacks
    should include the full callsign.

7.  Both controllers, when listening to readbacks, and pilots, when
    taking instructions, should be aware of how their expectations may
    affect what they hear. Pilots expecting certain instructions must
    wait for complete aircraft identification before taking action on
    the instructions.

**8. REFERENCES**

ASRS Callback Number 163, 1992. Readback/hearback - Some fresh ideas.

Bürki-Cohen, J. 1995a. Say again? How complexity and format of air
traffic control instructions affect pilot recall. *40^th^ Annual Air
Traffic Control Association Conference Proceedings*.

Bürki-Cohen, J. 1995b. How to say it and how much: The effect of format
and complexity on pilot recall of air traffic control clearances.

Cardosi, K. 1993. An analysis of en route controller-pilot voice
communications. DOT/FAA/RD-93/11.

Cardosi, K. 1994. An analysis of tower (local) controller-pilot voice
communications. DOT/FAA/RD-94/15.

Danaher, 1993. Close encounters of the wrong kind; accidents/incidents
involving communications. Paper presented at *"Communicating for
Safety." A Seminar for Pilots and Air Traffic Controllers.* Phoenix,
Arizona, May 18-20.

Federal Aviation Administration 1991. Runway Incursion Plan. Associate
Administrator for System Engineering and Development (ARD-100).

Federal Aviation Administration 1992. Air Traffic Control (7110.65G).
Air Traffic Rules and Procedures Service.

Federal Aviation Administration 1993. Statistical Handbook of Aviation.
Office of Aviation Policy, Plans and Management Analysis, FAA APO-95-5.

Monan, W.P. 1988. Human factors in aviation operations: The hearback
problem. NASA Contractor Report 177398.

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

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

[^1]: 1\. For example, Digital Automatic Terminal Information Service
    (ATIS) and pre-departure clearances (ACARS, AIRINC Communications
    Addressing and Reporting Systems).

[^2]: . For example, Digital Automatic Terminal Information Service
    (ATIS) and pre-departure clearances (ACARS, AIRINC Communications
    Addressing and Reporting Systems).

[^3]: . Please note that the transmission density rank order of these
    facilities correlates only very weakly (Pearson r~xy~=.40) with the
    rank order based on total annual operations, which is led by Chicago
    O'Hare, followed by Dallas Ft. Worth, Los Angeles, Atlanta, and,
    with other airports in between, Miami and Boston (FAA, 1993).

[^4]: . This and all other statistical analyses using Pearson r show to
    what degree the incidence of the different response types changed
    together with the complexity of the instructions, but do not inform
    on the causality of the relationship.

[^5]: . As determined by the subject matter experts (one controller and
    three pilots) who analyzed the audiotapes. Decisions as to who was
    using the incorrect callsign were a judgment call, because the
    analysts did not have the flight strips.

[^6]: . An alternative explanation is that the controller may have been
    right after all.

[^7]: . Only errors that were either corrected in a subsequent
    communication or apparent from the context could be identified.

[^8]: . We are currently investigating the effects of grouping numbers
    on pilot recall and have so far found that the costs of grouping may
    outweigh the benefits also for pilots whose first language is
    English

    (Bürki-Cohen, 1995 a, b).