Aviation Safety: Information on FAA's Data on Operational Errors 
At Air Traffic Control Towers (23-SEP-03, GAO-03-1175R).	 
                                                                 
A fundamental principle of aviation safety is the need to	 
maintain adequate separation between aircraft and to ensure that 
aircraft maintain a safe distance from terrain, obstructions, and
airspace that is not designated for routine air travel. Air	 
traffic controllers employ separation rules and procedures that  
define safe separation in the air and on the ground.1 An	 
operational error occurs when the separation rules and procedures
are not followed due to equipment or human error. Data maintained
by the Federal Aviation Administration (FAA) indicate that a very
small number of operational errors occur in any given year--on	 
average about three operational errors per day occurred in fiscal
year 2002. However, some of these occurrences can pose safety	 
risks by directing aircraft onto converging courses and,	 
potentially, midair collisions. Congress asked us to provide	 
information on FAA's data on operational errors and whether this 
data can be used to identify types of air traffic control	 
facilities with greater safety risks. Specifically, we were asked
to (1) determine what is known about the reliability and validity
of the data that FAA maintains on operational errors and (2)	 
identify whether comparisons of operational errors among air	 
traffic control facilities can be used to determine the 	 
facilities' relative safety record.				 
-------------------------Indexing Terms------------------------- 
REPORTNUM:   GAO-03-1175R					        
    ACCNO:   A08561						        
  TITLE:     Aviation Safety: Information on FAA's Data on Operational
Errors At Air Traffic Control Towers				 
     DATE:   09/23/2003 
  SUBJECT:   Air traffic control systems			 
	     Data collection					 
	     Data integrity					 
	     Strategic planning 				 
	     Transportation safety				 
	     Errors						 

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GAO-03-1175R

GAO- 03- 1175R Operational Errors Data United States General Accounting
Office

Washington, DC 20548

September 23, 2003 The Honorable James L. Oberstar Ranking Democratic
Member Committee on Transportation and Infrastructure U. S. House of
Representatives

Subject: Aviation Safety: Information on FAA*s Data on Operational Errors
at Air Traffic Control Towers

A fundamental principle of aviation safety is the need to maintain
adequate separation between aircraft and to ensure that aircraft maintain
a safe distance from terrain, obstructions, and airspace that is not
designated for routine air travel. Air traffic controllers employ
separation rules and procedures that define safe separation in the air and
on the ground. 1 An operational error occurs when the separation rules and
procedures are not followed due to equipment or human error. Data
maintained by the Federal Aviation Administration (FAA) indicate that a
very small number of operational errors occur in any given year* on
average about three operational errors per day occurred in fiscal year
2002. However, some of these occurrences can pose safety risks by
directing aircraft onto converging courses and, potentially, midair
collisions.

You asked us to provide information on FAA*s data on operational errors
and whether this data can be used to identify types of air traffic control
facilities with greater safety risks. Specifically, you asked us to (1)
determine what is known about the reliability and validity 2 of the data
that FAA maintains on operational errors and (2) identify whether
comparisons of operational errors among air traffic control facilities can
be used to determine the facilities* relative safety record.

1 The Federal Aviation Administration (FAA) has established a separation
standard in the en route environment of 5 nautical miles horizontally and
either 1,000 or 2,000 feet vertically depending on altitude. In the
terminal environment, horizontal separation is generally between 3 and 5
nautical miles

depending on the type of aircraft. 2 Data reliability refers to the
accuracy and completeness of data. We define data as reliable when they
are (1) complete and (2) accurate. Reliability does not mean that data are
error free, but that the data is sufficient for the intended purposes.
Validity refers to whether the data actually represent what one thinks is
being measured. See U. S. General Accounting Office, Assessing the
Reliability of ComputerProcessed

Data, GAO- 02- 15G (Washington, D. C.: Sept. 2002).

2 GAO- 03- 1175R Operational Errors Data To answer these objectives, we
reviewed past GAO studies 3 and reports by the Department of
Transportation (DOT) and DOT*s Inspector General (IG) that pertain to
FAA*s data on operational errors and applied standard methodological
practices for data reliability, validity, and analysis. 4 Data Has
Reliability and Validity Limitations We identified several potential
limitations with FAA*s data on operational errors based on our

review of issued GAO and DOT reports and application of best
methodological practices. First, it is very difficult to determine the
completeness of the data. FAA collects data on operational errors from two
sources* self- reporting by air traffic controllers and automatic reports
of errors detected on the en route portion of a flight. The possibility
exists for underreporting by air traffic controllers, since some errors
are self- reported and some air traffic controllers may not self- report
every incident. Second, due to the way the data are recorded, the severity
of many errors cannot be determined or is misleading. Prior to 2001, minor
errors, such as establishing a 4.5- mile rather than a 5- mile separation,
were counted in the same way as more serious errors, according to DOT. 5
In 2001, DOT began to address this issue by establishing a rating system
to identify the severity of, or collision hazard posed by, operational
errors. The system uses a 100- point scale to rate and categorize
operational errors as high, moderate, or low severity. However, in 2003,
DOT*s IG reported continuing concerns with FAA*s data on operational
errors. 6 The IG noted that the new rating system provides misleading
information and that FAA needs to modify the system to more accurately
identify the most serious operational errors. The DOT IG found that in one
instance FAA

rated an operational error as moderate that was less than 12 seconds from
becoming a midair collision. The IG believed that this operational error
should have been rated as high severity. The IG also reported that FAA
cannot be sure that air traffic controllers report all operational errors.

Comparison of Operational Errors Alone Does Not Provide Valid Conclusions
About Safety of Air Traffic Control Facilities

Comparisons of operational errors among types of air traffic control
facilities, such as FAAstaffed facilities versus contractor- staffed
facilities, cannot be used alone to provide valid conclusions about safety
due to three factors that we identified based on standard methodological
practices and our understanding of FAA*s data. First, such problems as the
completeness and specificity of data on operational errors are likely to
affect the validity of comparisons among air traffic control facilities
because operational errors may not be comparably reported at the types of
facilities being compared. For example, as we mentioned above, FAA cannot
be sure that all operational errors at either FAA- staffed or

3 See, for example, U. S. General Accounting Office, Air Traffic Control:
FAA Enhanced the Controllerin- Charge Program, but More Comprehensive
Evaluation Is Needed, GAO- 02- 55 (Washington, D. C.: Oct. 31, 2001). 4
See GAO- 02- 15G; U. S. General Accounting Office, Government Auditing
Standards, GAO- 03- 673G (Washington, D. C.: June 2003); and GAO Policy
and Procedures Manual, Factors Affecting a Design*s Credibility. 5 U. S.
Department of Transportation, Performance Report Fiscal Year 2000,
Performance Plan, Fiscal

Year 2002 (Washington, D. C.: April 2001). 6 U. S. Department of
Transportation, Office of Inspector General, Top Management Challenges,

Department of Transportation, PT- 2003- 012 (Washington, D. C.: Jan. 21,
2003) and Safety, Cost, and Operational Metrics of the Federal Aviation
Administration*s Visual Flight Rule Towers, AV- 2003- 057 (Washington, D.
C.: Sept. 4, 2003).

3 GAO- 03- 1175R Operational Errors Data

contractor- staffed towers were reported. When such a situation exists, it
is difficult, if not impossible, to determine whether the comparative
results are valid or are an artifact of under- reporting at one or both
types of air traffic control facilities. Second, in order to make valid
comparisons a number of factors that might affect the rate of operational
errors would need to be accounted for in an analysis. For example, air
traffic density, other operating conditions such as the number of flights,
age and experience of air traffic controllers, and weather conditions at
the time the error occurred all might influence operational errors. These
factors would have to be accounted for in any analysis comparing
operational errors among different types of facilities in order to
determine if the errors are associated with something other than the type
of air traffic control facility. Finally, as previously mentioned, a very
small number of operational errors occur in any given year (6.7
operational errors per million operations, on average, across all FAA
towers in fiscal year 2002), which may make it difficult to detect any
real differences in the error rates among facilities. Because of these
factors, the determination of real differences in the rate of operational

errors between different types of air traffic control facilities is
difficult, and comparisons of operational error rates alone are not
sufficient to draw conclusions about the relative safety records of air
traffic control facilities. At a minimum, the additional factors mentioned
above would need to be considered and analyzed with a technique that
models the occurrence of rare events and looks at these events over time.
This approach, however, is not without risk and would depend upon the
existence of proper and reliable data on operational error rates,
operating conditions at the towers at the time the error occurred, and
other factors that may be associated with operational errors. Such an
approach would allow for a more meaningful comparison of facilities*
operational errors through ascertaining and accounting for the multiple
factors that may be associated with such errors.

As arranged with your office, unless you publicly announce its contents
earlier, we plan no further distribution of this report until seven days
after the date of this report. At that time, we will send copies of this
report to interested congressional committees. The report will also be
available on GAO*s home page at http:// www. gao. gov. If you have any
questions about this report, please contact me at (202) 512- 2834 or by e-
mail at dillinghamg@ gao. gov. Key contributors to this assignment are
Isidro Gomez, Brandon Haller, Teresa Spisak, and Alwynne Wilbur.

Sincerely yours, Gerald L. Dillingham Director, Civil Aviation Issues

(540075)

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