On the application of a capital replacement model for a mixed fleet

A replacement model is described for a fleet which is inhomogeneous, comprising a number of subfleets classified by, for example, (manufacturer’s) model, age, and/or condition. It is assumed that replacements are made to single subfleets rather than to the whole fleet sumultaneously. As well as considering the timescale for replacement, a method is proposed to consider also the optimum size of new subfleets. This is done through the concept of a penalty cost of unavailability. Optimal values for the decision variables are based on minimizing an equivalent rent. The model is considered for a fleet operated under simple conditions, and applied to that operated by a large Malaysian intercity bus company.