Traditionally the maintenance onboard is carried out as per a schedule (preventive) that could be either calendar time or running time based. This is based on the assumption that every piece of machinery has a predictable useful life. However modern research investigating the way components fail has questioned this paradigm. Three such analyses have been carried out by United Airlines in 1968, Bromberg in 1973 & the US Navy in 1982 (NASA, 2000). While there is admittedly considerable variation in the results, the inferences drawn remain indisputable. We could divide the life cycle of a component into three stages.