Presents a method, and the data used in its development, for estimating USAF aircraft recoverable spares investment cost at the major subsystem level. The investment estimated is in peacetime operating stock (POS) used to support specific mission/design/series aircraft. Estimating relationships are also presented for the replacement of condemnations. The estimating equations implicitly assume that recoverable spares for future USAF aircraft will be procured, managed, and applied as they were in the recent past. They were developed by regressing subsystem POS investment on candidate explanatory variables from sets of data relating to aircraft or subsystem characteristics, activity, deployment/support/supply, and cost. A logarithmic form was chosen for the regression models. The equations are designed for use at the earliest stages of USAF aircraft development when detailed statements of individual spares requirements are not yet available. They were derived from data on 16 USAF aircraft for which production and deployment were nearly complete.