  Aircraft structures experience various kinds of loads over their entire
lifetime, typically leading to fatigue and ultimately structural failure. In
order to avoid structural failure during operation, the maximum number of
flight cycles and flight hours is regulated by laws ensuring continued
airworthiness. However, since every flight impacts the aircraft differently,
not all airframes are equally stressed at the time of decommissioning. This
paper proposes a new retirement criterion based on the so-called fatigue damage
index (FDI). The criterion takes into account that aircraft are differently
operated and thus enables an individual decommissioning of aircraft without
compromising its safety. Based on aircraft sample data covering 95% of the
Airbus A320 fleet over two years, the enhanced decommissioning criterion is
estimated to significantly extend the average aircraft service life. The impact
varies within the fleet, depending on the experienced seat load factors, cruise
altitudes, and taxi times considered for the individual aircraft during
operation. While seat load factors and flight altitudes significantly affect
the defined FDI, the influence of taxi times is only minor. Based on the
estimated increase in aircraft service life, the paper argues that for service
life extensions, the FDI shall be considered as the limit of validity in the
regulatory framework governing the decommissioning of aircraft.
