RESULTS
A total of 1359 questionnaires were returned, representing a response
rate of ∼28%. Of the returned surveys, 945 were from airline maintenance
personnel. The remaining surveys were from non-airline technicians
who worked on general aviation aircraft, typically small private planes
that are usually maintained during standard business hours. Because of
the small number of shiftworkers among the non-airline respondents,
their responses were excluded from further analysis. Some 402 of the
airline maintenance personnel reported maintenance incidents, providing
369 descriptions of errors in which the time of the incident was
reported and sufficient detail was available to classify the error. Of these,
177 were classified as skill-based errors, 68 as rule-based mistakes, 55 as
knowledge-based mistakes, and 69 as procedure violations. In 58% of
cases, the reporter indicated that they had been involved in the incident;
in the remaining cases, the reporters indicated that they had witnessed
the incident. Fifty-six percent of the cases had occurred within the last
year, the remaining cases had occurred prior to this.
Typical skill-based errors were forgetting to remove a rig pin at the
end of a task, bumping a work-stand into an aircraft, or failing to notice
an otherwise evident defect while performing a visual inspection. A
typical rule-based error was a technician wrongly assuming that a coworker
had performed a critical task step. An example of a knowledgebased
mistake was a case in which a technician did not place the necessary
chocks under the wheels of an aircraft after it had been towed, because
he misunderstood the operation of the aircraft brake system and reasoned
(incorrectly) that the aircraft could not move once the park brake was set.
An example of a procedure violation involved a senior technician who
was required to certify that he had personally checked the work of a
junior technician, but who relied instead on a verbal assurance that the
task had been performed.
Nine hundred fifteen of the airline respondents reported their pattern
of work attendance in the demographic section of the questionnaire.