"General This paper describes a recommended practice and procedure for the correlation of test cells that are used for the performance testing of APU (auxiliary power unit) engines. Test cell correlation is performed to determine the effect of any given test cell enclosure and equipment on the performance of an engine relative to the baseline performance of that engine. The baseline performance is generally determined at the OEM designated test facility. Beneficiaries This recommended practice will benefit the original equipment manufacturer (OEM)@ commercial users@ repair stations and military depots as well as intermediate level maintenance activities. Specific cases in which the information contained herein will be beneficial are: a. As an aid for providing correlation of test cell data between engine and airframe companies supporting commercial and military requirements. b. As an aid for providing military maintenance facilities and commercial repair stations a method by which to correlate test cells. c. As an aid in establishing correlation practices for new test cells@ for updating@ and maintaining existing test cells. d. As an aid to an engine manufacturer's facility in correlation of test cells used for engine development and acceptance in accordance with the applicable engine model specification. Limitations Known methods of determining test cell correlation factors include@ but are not limited to@ the following: a. Back-to-back b. Cross-cell c. Correlation engine The ""correlation engine"" procedure is the recommended and most common method for the correlation of an engine test cell. This paper is limited to the discussion of this one method. "