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.