From the studies conducted for ten starter-generators, the dependence of change of the brush's length in operating time function was determined. It was established that the wear process of brushes is of a random character. It results in the necessity – in the case of the modelling of the accrual process of failures – applying probabilistic methods. The developed deterministic model of abrasive wear of brushes became the basis for the probabilistic model. Building upon the difference equation, a Fokker-Planck partial differential equation was derived, which describes the accrual process of wearing the brushes in a probabilistic way. The solution of this equation is a normal distribution of the degree of wear. The way of estimating the distribution parameters was given. Based on the distribution of the degree of wear with the known parameters, the method for predicting the durability of investigated devices was developed. The presented method enables to predict a residual durability of selected aircraft commutator devices due to the character of changes of diagnostic parameter's values – wear of commutator brushes – collected during the operation.