Several widely publicized accidents claimed human error in maintenance as a contributing factor. However, when human error in maintenance is detected through system malfunction, only the resulting aircraft discrepancy is known. What is rarely known is the cause of error. If an accident or incident occurs, the pilot is always “on the scene” at the time of the accident or incident. In contrast to the “real-time” nature of error in ATC and the flight deck, maintenance errors are often not identified when the error is made. In some cases the maintenance technician who erred may never know of the mistake, since error detection could occur days, months or even years after the error was made.