TS Newsletter Archive

6.05.2011

FAA Recommendations for Turbofan/Turbojet engine test cell correlation

Turbine original engine manufacturers (OEM) require their turbine engines to be tested for performance and operability capabilities on different circumstances, such as new engine delivery, after overhaul and other maintenance activities. Each OEM operation manuals specifies test procedures, operational limits and test restrictions for each model, that should be in accordance with FAA 14 CFR part 43, section 43.2.

The construction of turbofan/turbojet test facilities (either L, U or folded inlet type) along with the hardware used to enable ground test, such as bellmouths, thrust reversers,  noise control devices, cowling and quick engine change (QEC) components, will have an impact on engine operation and performance, influencing test results. Therefore, accurate engine test facilities and measurement instruments impact have to be known, in order to be able to perform accurate engine performance evaluation.

Engine Test cell L type

Engine Test cell U type

Engine Test cell Folded inlet type

Test cell correlation is the most common and recommended process to obtain test facility + hardware equipment impact on test data, thus demonstrate the test cell facility operates correctly and performance data matches the reference test cell within an acceptable tolerance.

FAA AC 43-207 provides guidance regarding test cell correlation procedures for test cells used for in-service  acceptance testing of turbofan and turbojet engines, addressing the effects of the design, operation and modifications on the test cell and engine test hardware on engine performance test results; among other recommendations.

This FAA document, in section 401.a, summarizes the correlation procedure into the following steps:


1) Identify a correlation engine. The engine must:

     a. Develop minimum rated thrust.
     b. Exhibit stable performance.

2) Calibrate the applicant’s test cell instruments and perform an end-to-end check of systems.

3) Notify the respective Flight Standards District Office (FSDO) and verify FAA-approval and the correlation status of the reference facility with respect to the baseline facility.

4) Perform a shakedown engine run at applicant’s test cell. The run will:
     a. Verify instrumentation accuracy.
     b. Check out all other systems.

5) Perform an engine test at the reference facility using the correlation engine.

6) Notify the respective FSDO.

NOTE: The FAA recommends that reference testing be performed before testing at the applicant’s facility. However, this process may be performed in reverse, if necessary.

7) Perform the applicant’s facility test cell correlation.
     a. Notify the respective FSDO.
     b. Perform an 8- to 15-data point engine run.
     c. Perform engine run in the same sequence and direction of approach to the test points (i.e., acceleration versus deceleration) as that performed at the reference facility.
     d. Shut down the engine in preparation for an engine and facility repeatability check.
     e. Repeat the engine test using the same procedure.

8) Perform a retest at the reference facility (recommended, but optional).

9) Perform data analysis.
     a. Apply corrections to measured data for common reference (e.g., standard day). Graphically plot the data and draw a smooth characteristic line through the data for each plot. Regression analysis of the data is also an acceptable means of determining the characteristic line.
     b. Analyze the data for measurement error (e.g., perform data validation).
     c. Analyze the data for performance shift.
     d. Determine the cell correlation factor(s) and apply to (usually) corrected common reference data. Plot the adjusted, corrected data.
     e. Perform an acceptance test in accordance with the OEM’s overhaul manual to ensure the applicant’s test cell is capable of being utilized for satisfactory in-service acceptance testing.

10) Develop documentation.
     a. Write the Correlation Report to document details for the entire correlation exercise.
     b. Develop and document procedures for operating and maintaining the test cell and ensuring satisfactory maintenance of the test cell correlation status (i.e., develop a facility operations manual).
A detailed process for each step and correlation phase (pre correlation, during and post correlation) is also described in the document.

Once the Cell Correlation factors have been defined and characterized, usually using advanced regression techniques, would then be applied in all subsequent engine acceptance tests. The factors are valid for the engine model at the highest rating tested and derivative models having lower thrust using the same installed hardware.


A correlation exercise is necessary in any of the following cases:
  • Following a construction of a new test cell
  • When either a new engine model o derivative engine model of higher rated thrust is introduced
  • When repairs or modifications have been made to an existing and correlated test cell and/or engine test hardware, that significantly affect engine performance.
  • When modifications have been made to existing data acquisition or data reduction systems, and these modifications are introduced in the test cell in a manner that could affect engine performance evaluation;
  • If and when recommended by the OEM; and
  • Where the FAA has determined that there is valid reason to perform a test cell correlation.



Other related publications and materials, besides this FAA AC, can be obtained from the Society of Automotive Engineers (SAE), under the issues:

(1)  SAE Aerospace Recommended Practice (ARP) 741B, Turbofan and Turbojet Gas Turbine Engine Test Cell Correlation, Revision B. Revised 2000-12-07 (or latest revision).
(2)  SAE Aerospace Information Report (AIR) No. 4869, Design Considerations for Enclosed Turbofan/Turbojet Engine Test Cells. Issued October, 1995.

Our consultants at TeraSigma have wide experience on Test Cell Correlation activities, following this process in addition with Six Sigma tools experience, making our service a complete set of solutions for the MRO engineering team on their troubleshooting, analysis and monitoring tasks.  

In following issues of TeraSigma’s newsletter, The power of Collaboration, we will cover more topics related to Engine test cell correlation.

Next issues:
  • Restrictions for engine test cell correlations
  • Measurement system calibration
  • Correlation accuracy and data quality assurance procedures
  • Cell correlation monitoring and maintaining



Should you have any comments or questions regarding Engine Test Cell Correlation and monitoring, get in contact with TeraSigma Consulting, The Power of Collaboration.

Source FAA Advisory Circular AC43-207

3 comments:

  1. Can the correlation be carried out ,
    1. if the test cell geometry is altered
    2. when the detuner is reconstructed
    3 . once in a year , to know the condition of the correlation engine , or ant adjustment of box factor

    ReplyDelete
  2. If a correlated cell is modified in any way that the airflow path is affected, a verification run can be performed in order to validate the current factors. If the results of this verification run show a deviation of the cell factor, a new correlation must be done. FAA AC43-207 describes the procedures to maintain properly a Cell Correlation as well.

    ReplyDelete
  3. What is the recommended course of action if the OEM can no longer provide "master" correlation run data (master test cell no longer exists and master correlation engine no longer exists). Other 2nd or 3rd generation cells for this engine model have not been correlated back to the OEM for 20+ years.

    ReplyDelete