A computer-implemented method for safely extending maintenance intervals for structural components of aircraft within an aircraft fleet, the components having respective component life limits, the method comprising:. identifying a first aircraft sub-fleet based upon a first usage profile and a second aircraft sub-fleet based upon a second usage profile, each aircraft sub-fleet having a sub-fleet population comprising a plurality of aircraft, wherein the second usage profile comprises a less extreme usage profile than the first usage profile;. determining a sub-fleet reliability value for each aircraft sub-fleet (A, B) based upon a component life limit;. determining a baseline fleet reliability for the aircraft fleet by multiplying together the sub-fleet reliability values raised to the sub-fleet population for each aircraft sub-fleet;. applying at least one credit to the component life limit of the second aircraft sub-fleet to produce an extended component life limit;. determining a post-credit sub-fleet reliability value for each aircraft sub-fleet based upon the component life limit for the first aircraft sub-fleet and the extended component life limit for the second aircraft sub-fleet;. determining a post-credit fleet reliability for the aircraft fleet by multiplying together the post-credit sub-fleet reliability values raised to the sub-fleet population for each aircraft sub-fleet (A, B);. comparing the baseline fleet reliability with the post-credit fleet reliability to identify a change in fleet reliability;. determining whether the change in fleet reliability is within a predetermined threshold to validate the at least one credit; and. if the credit is validated, extending the component life limit for the second aircraft sub-fleet to the extended component life limit such that maintenance is carried out on the structural components of the second aircraft sub-fleet according to the extended component life limit.. identifying a first aircraft sub-fleet based upon a first usage profile and a second aircraft sub-fleet based upon a second usage profile, each aircraft sub-fleet having a sub-fleet population comprising a plurality of aircraft, wherein the second usage profile comprises a less extreme usage profile than the first usage profile;. determining a sub-fleet reliability value for each aircraft sub-fleet (A, B) based upon a component life limit;. determining a baseline fleet reliability for the aircraft fleet by multiplying together the sub-fleet reliability values raised to the sub-fleet population for each aircraft sub-fleet;. applying at least one credit to the component life limit of the second aircraft sub-fleet to produce an extended component life limit;. determining a post-credit sub-fleet reliability value for each aircraft sub-fleet based upon the component life limit for the first aircraft sub-fleet and the extended component life limit for the second aircraft sub-fleet;. determining a post-credit fleet reliability for the aircraft fleet by multiplying together the post-credit sub-fleet reliability values raised to the sub-fleet population for each aircraft sub-fleet (A, B);. comparing the baseline fleet reliability with the post-credit fleet reliability to identify a change in fleet reliability;. determining whether the change in fleet reliability is within a predetermined threshold to validate the at least one credit; and. if the credit is validated, extending the component life limit for the second aircraft sub-fleet to the extended component life limit such that maintenance is carried out on the structural components of the second aircraft sub-fleet according to the extended component life limit.