Changes in items' failure pattern during maintenance: An investigation of the perfect repair assumption

It is clear that the simple dasiaperfect repairpsila assumption is not immediately applicable to any of the studied types of hardware, i.e. cooling turbine., high-voltage generator, hydraulic accumulator, and radar transmitter. Hence, the assumption of 'perfect repair' needs to be validated for each specific type of item. Assumptions based on the type of physical hardware (e.g. mechanical item or avionics item) are not always trustworthy. Strangely enough the 'perfect repair' assumption fits best for the cooling turbine, which is a highly stressed mechanical item, while the fit is much poorer for the radar transmitter, which is an avionics item and for the hydraulic accumulator. For the radar transmitter the trend seems to be very scattered. For items with a large number of failures early in their life-cycle, repair is 'better than perfect', i.e. the items become more reliable after repair, presumably by elimination of less reliable subcomponents. However, this effect is not seen for items with few failures early in their life-cycle. For these items 'perfect repair' initially seems to be a valid model. However, in many casesrepair becomes 'less than perfect' later in the life-cycle. For the hydraulic accumulator this trend is even more accentuated and individual items seem to fall into two distinct subpopulations with opposite reliability trends.