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containment range which dominates the risk. The nature of these large errors has to be
assumed in the absence of any actual data.
For existing systems, equipment performance data can be fairly easy to obtain. For
instance, radar and radio telephony (RT) failures are routinely recorded by ATC
authorities. Also, quite detailed aircraft performance data are available for the aircraft of
some manufacturers. Some types of data, however, may require specific data collection
exercises. An example of this is the collection of aircraft height-keeping performance
being undertaken for the application of a Reduced Vertical Separation Minimum (RVSM)
in the North Atlantic. This required a major international effort to develop and operate a
complex monitoring program.
The collection of human performance data is the most difficult area. A significant amount
of work has been done in other industries, such as nuclear power generation and the
chemical process industry, and also on aircrew performance, but little has been done in air
traffic control. The frequency with which human error is cited as a major cause of
DATA NEEDS AND MODEL CALIBRATION
8-3
aviation accidents highlights the importance of this factor. Some data on human error can
be obtained from records for existing systems. For instance, the frequency with which
pilots fail to follow clearances in the current system can be obtained. There are, however,
serious problems in terms of under-reporting these types of events and this must be taken
into account in any analysis. That is, these reports probably cannot be used to obtain
statistical estimates of the frequencies of such events. The reports and, in particular, their
narratives can provide a rich source of qualitative information on the types of human
errors that can occur and the circumstances surrounding their occurrences.
Accident and incident reports often contain data on two or all three of the categories listed
at the beginning of this section. Accidents, for example, usually result from a combination
of failures or inadequacies, human and system, and the environment in which these failures
occur may also be relevant.
8.4 DATA REPOSITORIES
There is a virtually unlimited number of aviation data repositories. Many developed
nations track and retain a wide variety of aviation data. Difficulties arise in the use of
these data as a result of data idiosyncrasies, incompleteness, and inconsistencies, both
within a data base and among data bases. As a result, if one is not totally familiar with all
aspects of the data generation process, one may use “rational” assumptions about the data
that are, in fact, incorrect. Coded data may, for example, reflect the state of knowledge of
the coder or the politics of the organization at the time of coding. Accident investigators
may honestly differ in the interpretation of physical data, with consequent differences in
reported causal factors. Data may be incomplete in that they are missing, or data values
may reflect incomplete records, such as a “shortened” flight time resulting from a flight
track being “timed out” by a computer. It is thus incumbent on the analyst to become as
familiar as possible with the data being used and with the processes that were used to
collect and refine/process the data.
It is impossible to list any substantial fraction of the aviation data bases. To give some
idea of the wide variety of data available, provided below are two examples of FAA data
repositories, each of which includes several data bases that originate with a variety of FAA
organizations.
8.4.1 The National Aviation Safety Data Analysis Center (NASDAC)
NASDAC is a repository of several safety-related data bases. It includes several software
tools to assist analysts in extracting data relevant to their interests. As described above,
care must be taken in using these tools because of the idiosyncratic nature of data and,
therefore, it is recommended that the analyst seek the assistance of the Center’s staff in
the use of the repository. The Center is located in the headquarters building of the FAA in
Washington DC. Much of the Center’s data are also available on the Center’s Web site at
http://nasdac.faa.gov/safety_data/. Among the data bases included in this repository are the
following:
SEPARATION SAFETY MODELING
8-4
· The National Transportation Safety Board (NTSB) Aviation Accident/Incident
Database is the official repository of aviation accident data and causal factors. In the
database, an event is classified as an accident or an incident. "Aircraft accident" means
an occurrence associated with the operation of an aircraft which takes place between
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a concept paper for separation safety modeling(55)
