An aircraft is a complex operating system subject to the aging and degeneration processes. It is also maintained and repaired to keep operational condition high. Declining condition increases the chance of failures. Since repair is typically variable and incomplete, the question of the reliability of aircraft as they age is significant.In this paper the predictability of failures of aircraft as they age is considered. The methodology is to consider moderate mechanical failures which result in unscheduled landings. The records of a single aircraft model:B737 for a carrier whose fleet has large numbers of that model are analyzed. A Poisson regression model is fitted to the number of unscheduled landings over a 3 year period, with the rate depending on age and periodic maintenance. A clear age pattern emerges. Although the rate of decline depends on the model, and the rate of improvement through repair depends on the carrier, the aging is real since these factors are held constant.