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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.

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