formally defined as ‘‘the maintenance carried out at predetermined intervals or corresponding to prescribed criteria and intended to reduce the probability of failure or performance degradation of an item’’ (British Standard, 1984). These predetermined intervals can be either time-based (calendar time) or use-based (total operating time, number of operations, mileage, Waeyenbergh and Pintelon, 2002; Kumar, 1996). These intervals are determined using statistical-based models and optimisation, which is the case when implementing preventive maintenance (PM) policies such as age and block replacements (Sherwin, 2000).