Budget estimating relationships BERs for flying depot-level reparables DLRs explain the direct effect of specified variables on obligated funds associated with spare parts that directly support the U.S. Air Force USAF Flying Hour Program. In FY02, net sales of DLRs to Air Force commands hit historic highs. To provide the Air Force Cost Analysis Improvement Group with a tool to better understand the commands-- budgetary submissions, we develop several explanatory BERs to understand why flying DLRs are at their particular levels. Using longitudinal regression statistical methods, we explain the historical net sales of flying DLRs using estimating models that relate net sales to the contemporaneous values of aircraft characteristics, operational tempo, and time-related variables. This is but one part of a larger project to develop better estimating methods for use by the acquisition community and to examine the impact of Air Force and DoD policies on weapon system costs. The findings will also be of interest to those in the national security community who are involved in analyzing alternative military postures, and to members of the aircraft industry's analytical community.