d. Ram Pressure Ratio. Defined as the engine inlet total pressure divided by the engine exhaust nozzle exit static pressure, it is the result of airflow through the test cell causing pressure differences between the front and rear of the engine. Positive ram pressure ratio values are equivalent to flying the engine at a low forward velocity that affects the engine thrust. Negative ram can be deleterious to the operation of the engine. Since the main objective of engine test cell tests is to determine static engine performance corrected to sea level, static, standard day conditions, the effects of ram pressure ratio must be measured, documented, and accounted for in thrust determination either separately or in a correlation factor.
e. Cell Depression. The difference between ambient barometric pressure and the static pressure in the test cell test chamber referenced to the engine inlet is known as cell depression. The difference is caused by pressure loss due to the velocity of air through the cell. It is usually measured in inches of water and denoted by Pcd. High levels of cell depression are usually a result of cell inlet or exhaust pressure losses. Most test cell designs result in cell depression of 2-4 inches of water, but higher or lower pressures may be acceptable. However, very low cell depression may result in vortices in the test chamber or engine exhaust gas re-ingestion into the engine inlet, causing unacceptable performance. A very high cell depression requires a large correlation factor, may indicate inadequate airflow to the engine, and can cause excessive loads on the test cell walls.
f. Cell Bypass Airflow Interactions. When a gas turbine engine is operated in an indoor test cell, its performance is altered because of the aerodynamic interference between the engine and the cell. In order to establish the thrust of the engine, the interaction of the engine with the test cell environment must be evaluated. Aerodynamic thrust corrections result from flow-induced forces within the test cell, which can be divided into three components:
(1) Inlet momentum drag;
(2) Structural drag on the engine and thrust stand; and
(3) Static pressure drag along the engine.
g. Inlet Momentum Drag. The most significant aerodynamic component of the thrust measurement is the inlet momentum, also known as the intrinsic inlet momentum. As a result of drawing air into the test cell, a force is produced on the engine. For static engine testing (i.e., non-ram facility), the magnitude of this force may be substantial; values from 1 to 10 percent of the measured thrust are typical. Since this force is, in effect, a drag term, it must be added to the measured thrust of the engine. The inlet momentum is a function of the engine inlet airflow and the approach airflow velocity in front of the engine. The approach velocity is significantly affected by the amount of cell airflow and the geometry of the test cell.
h. Structural Drag. Structural drag is generated by the cell bypass airflow scrubbing the exposed surface area of the engine casing and pushing against the exposed structure that supports the engine on the thrust measurement stand.
i. Static Pressure Drag. Local acceleration of cell bypass airflow results in static pressure gradients along projected surfaces of the engine, particularly the bellmouth and exhaust nozzle. These pressure gradients generate horizontal forces that act in a direction opposite to the measured thrust of the engine.
Static pressure drag is sometimes broken down into the bellmouth drag and boat-tail drag. The magnitude of the static pressure drag is very sensitive to the cell exhaust geometry and the spacing between the engine exhaust and the cell exhaust system.
Par 305 Page 17
j. Dress Kit Hardware.
(1) Engine performance is influenced by the airflow in the engine test section. Airflow characteristics and pressure losses in the test section are affected by protrusions in close proximity to the engine. Dress kit hardware, such as external engine cowling, bellmouth, exhaust nozzle, accessories, and test instrumentations, are examples of typical protrusions. Therefore, dress kit hardware may influence engine performance and the resulting correlation factor(s).
(2) For accuracy and consistency, the part number and serial number of each dress kit hardware item, where appropriate, and the bellmouth, fan exhaust nozzle, and primary exhaust nozzle area measurements should be documented during the correlation. Engine testing at the reference and the applicant’s facilities are generally performed using each of their respective dress kit hardware. The dress kit hardware that was used during the applicant’s correlation test should be used for all subsequent tests of the engine model in the applicant’s test cell, unless the following applies;
(a) If the applicant owns several sets of engine dress kit hardware, it may be necessary to establish correlation factors for each dress kit configuration. In this case, immediately following correlation of the applicant’s test cell, back-to-back testing using different dress kit hardware should be performed to determine if any change to the correlation factor is necessary. Test configurations need to be documented by dress kit part number, serial number, and area measurements and matched to the appropriate correlation factor.
(b) If an applicant purchases a new piece of engine dress kit hardware, a similar back-to-back test should be performed at that time to establish if the correlation factor needs to be changed.
NOTE: Paragraph 403 describes back-to-back testing.
306. DATA ACQUISITION.
a. A primary consideration of engine performance measurements should be data acquisition. When computers are not utilized in a test facility, data must be collected by visually reading and hand recording all measured parameters on a log sheet. The use of an automatic computer-operated digital data acquisition system improves the accuracy of measurements and recordkeeping.
NOTE: Technology advancement in automated data acquisition systems has given rise to potential problem areas. Close attention should be paid to potential problems in the planning stages of developing this type of system. Special attention should also be paid to cell-to-cell engine performance differences during correlation when a difference exists between data acquisition systems.
b. The following paragraphs contain items that may have a significant effect on test cell correlation.
(1) Data Scan Characteristics. Computer-operated digital data acquisition systems scan, within a short period of time, hundreds of channels of data (e.g., temperatures, pressures, vibrations, resolver angles) in short, controlled bursts of repetitive signal collection. These bursts of measured data collection may be averaged by a computer. The following additional information is offered to provide a better understanding of scan characteristics.
(a) Recommended scan characteristics may be prescribed by the OEM.
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(b) Taking multiple scans or measurements of the same parameter will obtain a better average value. Due to the total interaction of the engine control system and hardware, the performance parameters of even a well stabilized engine slowly oscillate around a steady state average. This parameter oscillation may resemble a sine wave characteristic with a period possibly lasting up to several minutes.
(c) Whether data is manually or automatically recorded, this recording process or scan takes a finite amount of time. With a rapid scan, the engine operating conditions change negligibly during a single scan period. However, with a slow scan, the sequence in which the parameters are recorded can influence the data quality due to time skew. Therefore, thought should be given to the recording sequence and parameter groupings.
(d) The period (i.e., engine cycle) of each measured parameter should be determined either analytically or empirically. This information should be used in designing a scan rate for each parameter.
Data scan techniques, including the rates, may be different for digital versus analog data acquisition systems. An ideal technique would be to record several points per engine cycle over a large number of engine cycles, so that a true average value will be calculated from the data recorded.
(e) The oscillating engine performance discussed in paragraph 306b(1)(b) is further complicated by engine thermal soak-in. Soak-in is a result of the turbine mass (particularly the disks) slowly heating up and causing reduced turbine blade-to-shroud clearances. Thermal soak-in initiates immediately upon reaching a given power point and results in a real and measurable shift in engine performance during thermal stabilization. Soak-in can take from a few minutes to as much as 15 minutes for complete thermal stabilization to occur. Data acquisition should not be initiated before full thermal stabilization. If data acquisition is initiated before full thermal stabilization, some parameters will incur a real migration in the average value (i.e., a performance shift). This can be accommodated by selectively organizing the data scan. However, as previously stated, waiting for the engine to fully soak in is more desirable.
NOTE: A selectively organized data scan should read the stable parameters that are unaffected by thermal soak-in (i.e., most pressures and cell temperature) toward the beginning of the scan after a minimal soak period, but before full thermal stabilization occurs. Soak-affected parameters (e.g., turbine temperature, core speed, fuel flow, variable vane angles) should be read toward the end of the scan.
(f) Parameters such as fuel flow, which are measured as frequency, could have significant errors in the calculated average value that are solely attributable to scan rate and sequencing. Therefore, special attention should be given to cell-to-cell engine performance differences during a correlation when a mismatch exists between the data acquisition systems. This could become particularly difficult when trying to correlate a computer-driven acquisition system against a manual or hand-logged system.
(2) Data Averaging and Editing. Once a data acquisition methodology has been selected, an averaging technique should be determined. Also, a procedure pertinent to editing the data should be considered. These parameters are then used in the performance calculations.
(a) Several averaging techniques can be employed. A straight average of all measured data is commonly performed. Alternatively, more selective techniques requiring an automatic screening of all data can be applied, with some data being classified as possibly erroneous.
(b) Averaging techniques can have a significant effect on the calculated parameter average and, thus, the resulting performance, particularly if the engine oscillates significantly or the data were acquired before full thermal stabilization. Therefore, any differences between averaging methodologies Par 306 Page 19
should be carefully considered when correlating a test cell in which different averaging techniques have been used between the test cells.
(c) Along with the averaging techniques employed, an appropriate editing procedure should be considered. Different types of editing methods can be used in conjunction with the averaging technique chosen to produce the most representative data. Editing methods include data tolerances, minimum and maximum limits, and an outlier test. Editing is especially useful on multiple pressure readings (e.g., bellmouth static or total pressures), where line leaks can affect the validity of the data.
Caution should be used when applying editing to parameters, such as fuel flow and engine exhaust gas temperatures, where editing might mask engine stability problems. When using any editing method, all measured data should be displayed and the edited data flagged so mismeasurements can be corrected.
(d) The goal of data editing is to discard the erroneous data points and use only the correct
(i.e., true) data measurements. One goal of correlation testing is to find and remedy problems in the test cell measurement systems. When data editing is used, there is risk that data showing a real mismeasurement problem may be discarded as erroneous data. If this happens, the opportunity to find and correct a measurement problem may be missed.
(3) Software Verification. Computer software may be used in data acquisition and reduction.
The FAA recommends that steps be taken to ensure that:
(a) The appropriate software program, including version (i.e., revision), is used.
(b) The software reflects the test requirements and performance calculations of the engine manual.
(c) For a given input, the reference and applicant’s facilities produce the same output.
(d) Data known to be valid can be used to verify the software functionality.
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CHAPTER 4. TEST CELL CORRELATION
400. CORRELATION ACCURACY ASSURANCE. Correlation relates an engine’s performance to a known standard so that the performance level of the engine can be compared with established limits.
These limits are derived from a performance baseline for a particular engine model. This baseline performance standard is usually the average of a number of engine tests, as established by the OEM, and is defined for a specific engine test hardware configuration and test facility combination. This combination is thereafter considered the baseline test hardware configuration and test facility for that particular engine model. To conduct a successful correlation program, identifying a suitable reference facility is necessary. It is desirable to use the OEM’s baseline test facility as the reference test facility.
However, access to the baseline facility is not always possible. In this event, another facility may be designated as the reference facility. This is discussed in paragraphs 101 and 102 of this AC.
a. Uncertainty Stackup. Additional information on determining correlation factors for the applicant’s facility is provided in paragraph 401e(3)(d).
(1) If a facility not designated as the OEM’s baseline facility is used for reference testing, an analysis of the stackup of uncertainty of the acquired data should be performed for all correlation steps between the baseline facility and the applicant’s facility. The applicant’s facility correlation should be deemed satisfactory if this uncertainty is shown to be smaller than the OEM’s recommended acceptance tolerances.
(2) As the correlation of the applicant’s facility is further removed from the baseline facility, the uncertainty of the applicant’s test facility accuracy tends to increase. There are measures that may be used to increase confidence in the correlation and reduce uncertainty to the acceptable level. The following examples may reduce uncertainty:
(a) Requiring in-service acceptance limits of engines tested in the applicant’s facility to be less than those tolerances specified by the OEM.
(b) Applying correlation factors to replicate the characteristics of the OEM’s baseline facility.
(c) Performing multiple cross-cell tests, which is also called round-robin testing. This involves running one or more engines in more than one previously correlated test cell. Confidence grows when more engines are used and more test cells compared.
(d) Performing multiple correlation tests between the reference and the applicant’s facilities to develop sufficient confidence in the correlation. This could include conducting multiple consecutive runs at each facility or retesting at the reference facility following testing at the applicant’s facility (which is referred to as A-B-A testing).
NOTE: All of these techniques assume that an unbroken sequence of correlation tests exists between the baseline and the applicant’s facilities. If this sequence does not exist, then the correlation method described in this AC is not possible. In this event, efforts to substantiate that a test cell is relating engine performance back to the known standard will have to be found acceptable by the FAA. This is discussed in paragraph 103.
b. Engine Test Hardware Configuration For Reference Testing. As previously stated, correlation relates an engine’s performance to a known standard for a specific engine model. This standard is defined Par 400 Page 21
not only for testing an engine in a specific facility (i.e., the OEM’s baseline test facility), but also for testing a specific engine test hardware configuration (i.e., the OEM’s baseline test hardware configuration).
(1) For testing at the reference facility, the FAA recommends that the engine test hardware configuration be the same as:
(a) The hardware to be used during correlation testing at the applicant’s facility;
(b) The hardware to be used for future in-service acceptance testing at the applicant’s facility; and
(c) The OEM’s baseline test hardware configuration.
(2) However, the configurations that were used during testing at the baseline, reference, and applicant’s facilities may be different. These differences should be evaluated for their impact on correlation.
401. CORRELATION PROCEDURES.
a. Procedure Overview. The following activities are a general overview of the step-by-step correlation process. Following this overview, details of these activities are discussed in paragraphs 401b through 401d.
(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 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.
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(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, as described in paragraph 402, 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).
b. Pre-correlation Procedure. The following activities should be performed before correlation testing:
(1) Determine correlation test procedures in accordance with the OEM’s correlation test requirements or recommendations.
(2) Instrument the engine in accordance with the OEM’s recommendations.
(3) Obtain fuel samples at both facilities, before or during correlation testing, and determine specific gravity and LHV.
(4) Identify and record appropriate engine test hardware configuration(s) to be utilized at the reference and applicant’s facilities (see paragraphs 305j and 400b). Obtain measurements of the Par 401 Page 23
bellmouth, fan exhaust nozzle, and primary exhaust nozzle areas to be used for reference and applicant facility testing. These measurements should be compared and may be useful when evaluating the correlation. Additionally, when specified by the OEM, bellmouth area measurement may be required for calculating airflow.
(5) Identify an acceptable correlation engine. A new, used, or rebuilt engine can be used as long as it demonstrates at least minimum takeoff thrust and maintains stable performance across the corrected thrust range from idle to takeoff. The engine’s ability to reach rated takeoff thrust without overtemperaturing or overspeeding is important. It allows engine testing to be performed without safety or performance deterioration concerns. The use of an engine experiencing a gradual run-to-run variation in performance, such as a gradual performance deterioration, could result in a nonstandard and needlessly complex correlation evaluation.
(6) Immediately before the correlation run, calibrate the applicant’s test cell instrumentation and engine instrumentation to the selected standard. Also, perform an end-to-end check of the applicant’s facility measurement systems (see paragraph 301).
(7) Notify the respective FSDO:
(a) Early in the planning process so that all requirements are identified, and
(b) Prior to testing at both the reference and the applicant’s facility, should the FAA elect to witness the test.
NOTE: If applicable, obtain proof that the reference facility is FAA-approved and traceable to the OEM’s baseline facility.
(8) Perform a full power shakedown run prior to the correlation run at the applicant’s facility.
The entire facility, including the test cell, data system, and calculation procedures, should be thoroughly tested. This may be performed with an engine other than the correlation engine. This is also another opportunity to investigate and eliminate facility measurement system problems, since a check of the entire system operating together may not have been previously performed.
c. Correlation Procedure. The test procedure used at the applicant’s facility should be the same in all aspects as the procedure conducted at the reference facility. This includes:
/g183/g32 The number of data points acquired /g183/g32 Thrust levels /g183/g32 Stabilization time /g183/g32 Sequence /g183/g32 Direction of approach to the test points /g183/g32 The parameters recorded
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(1) Once the facility to be correlated (i.e., applicant’s facility) is thoroughly tested via the shakedown run, the formal correlation process can begin. In most cases, the correlation engine is first tested in the reference test facility. A correlation engine test usually includes 8 to 15 data points distributed across the thrust range.
NOTE: Under certain conditions, fewer data points could be acceptable.
(2) Care should be taken to fully stabilize the engine thermally and dynamically before data acquisition is initiated. The stabilization time should be recorded so that it can be duplicated during testing at the applicant’s facility.
(3) Weather conditions should be recorded at both facilities at the time of test, as well as any weather changes occurring during the test. It is desirable to minimize the ambient temperature and humidity differences between the two facilities. Any preestablished climatic envelopes are to be observed for satisfactory testing.
(4) To ensure consistent and repeatable engine testing, the proposed test procedure should include a listing of parameters to be measured and the measurement precision and range. The following parameters should be recorded:
/g183/g32 Engine fuel flow
/g183/g32 Engine thrust
/g183/g32 Engine rotor speeds
/g183/g32 Engine airflows
/g183/g32 Engine pressures and temperatures
/g183/g32 Engine inlet conditions
/g183/g32 Test cell temperatures and pressures
/g183/g32 Test cell inlet airflow
/g183/g32 Engine control systems and variable geometry
/g183/g32 Barometric pressure
/g183/g32 Ambient humidity
NOTE: Sufficient data must be obtained to meet acceptance test procedures specified by the OEM. Secondary parameters used in the derivation of any of the above items must also be measured and recorded. In addition, redundant instrumentation can be useful for data verification and troubleshooting purposes.
(5) During the engine test at the reference test facility, the sequence and direction of approach to the test points performed (i.e., acceleration or deceleration) should be recorded. The data points taken at Par 401 Page 25
the facility to be correlated should also be collected in the same sequence and approach to eliminate performance variability due to control system hysteresis.
(6) Once the engine test at the reference facility is complete, the correlation engine should be immediately preserved and shipped to the applicant’s facility for correlation testing. This will minimize engine performance deterioration due to additional operation.
(7) Following the initial correlation engine run at the applicant’s facility, the engine should be shut down for a minimum of several hours to reach a cool engine state. Time periods for cooling can vary; refer to the OEM’s instructions. After the shutdown period, the engine test should be repeated for an engine and facility repeatability check. The repeatability check is intended to demonstrate stable engine performance and engine run-to-run repeatability of the data measurement, acquisition, and reduction systems.
(8) Ideally, the test cell correlation should include engine runs at the reference facility, at the applicant’s facility, and again at the reference facility (i.e., A-B-A). If the engine is routinely used for correlation testing, a retest at the reference facility may not be necessary if correlation engine data from several previous runs are consistent.
(9) Following the performance correlation runs, the correlation engine or another suitable engine should be tested at the applicant’s facility using the overhaul manual in-service acceptance procedures.
This will ensure that the applicant’s facility is capable of being utilized for satisfactory engine acceptance testing. This should not be performed until after all correlation testing is complete and the correlation data has been reduced, analyzed, and accepted.
(10) Data obtained during testing at the reference and applicant’s facilities must be recorded in a manual or automatic log sheet at the time of test. The FAA recommends that copies of these records be provided to the FAA representative witnessing the testing (if applicable) immediately upon conclusion of testing.
d. Post-correlation Procedure. The following paragraphs identify the post-correlation steps. The FAA recommends that the correlation engine be carefully stored until the correlation analysis is complete, in case further testing is necessary.
(1) Following completion of correlation testing, the data must be reduced (i.e., calculations must be performed). Paragraph 305b describes the process of correcting or normalizing the measured data to sea level, static, standard day conditions. The corrected data is typically graphically displayed, and a smooth characteristic curve is drawn through the data.
(2) Paragraph 401e below gives a detailed description of the post-test correlation data analysis of the plotted data. The OEM’s manuals and reports should also be referenced for a more thorough analysis description relative to a specific engine model.
e. Correlation Data Analysis. Once engine tests at both facilities are complete (all data has been edited, averaged, corrected, and plotted graphically) and a smooth characteristic line has been drawn through the data for each performance plot, the data must be analyzed. Data analysis is an engineering function and requires specialized knowledge. It is typically performed by the applicant, OEM, or other qualified personnel. It should include the following three steps: data validation with instrumentation error isolation, performance shift determination, and correlation factor determination. Each of these three elements should be isolated independently in the order given. If data errors or real performance shifts are not recognized, their respective effects on performance might be erroneously lumped into the third item, Page 26 Par 401
the correlation factor. This could result in possible inconsistencies during future in-service acceptance testing of engines at the applicant’s facility.
(1) Data Validation. Data validation analysis is the process of determining the acceptability of the measured, corrected data. The process of data validation requires specialized knowledge of standardized methods (e.g., cross-plotting of various parameters, error and cycle derivatives, influence coefficients) and general knowledge (preferably engine model specific) of aerodynamic/thermodynamic cycle effects. The following paragraphs provide some general methods of determining data validity by analyzing the characteristic plots.
(a) Plots should be constructed with the reference and applicant’s facility data plotted together for comparative analysis. Sample correlation plots are given in Appendix 3. Generally, each corrected parameter is plotted separately on the y-axis, while either the corrected thrust, engine pressure ratio (EPR), or corrected rotor speed is plotted on the x-axis of each plot. Utilizing a practice of plotting the same ordinate (y-axis) values against another parameter, such as corrected rotor speed, is a useful way of independently judging data validity.
(b) Plotting each pressure ratio against corrected rotor speed can be useful in determining possible pressure mismeasurements, though care should be taken to verify that there is not a real performance shift.
(c) Once a parameter is determined to have an unacceptable error, every effort should be made to eliminate that error. Extensive troubleshooting of the measurement system should be performed.
Not only should all gauges, signal conditioners, instrumentation lines, pressure probes, thermocouples, and rakes be carefully examined in an end-to-end check, but the data acquisition system and correction methodology should also be checked. Once the measurement or calculation error has been found and corrective action taken, a determination must be made as to whether another correlation run is necessary.
Some errors, such as a calculation error or gauge scalar error, may be corrected without having to conduct another engine run.
(2) Performance Shift Determination.
(a) Once unacceptable measurement errors have been eliminated or compensated for, an assessment must be made as to whether the engine’s performance remained unchanged between the reference and applicant’s facilities. This can be done by cross-plotting the corrected data. If a shift in performance is not recognized, an incorrect correlation factor could be developed and applied to all future engine test data, resulting in repeated errors in assessing subsequent engine test data.
(b) If there is an apparent performance difference between the two test runs (i.e., reference facility and applicant’s facility) that cannot be accounted for by expected facility and/or engine test configuration influences, further testing may be necessary. Follow-on actions will be determined from the results obtained. Paragraph 303 describes factors that influence correlation.
NOTE: Facility configuration and engine or facility instrumentation differences can be difficult to uncover and quantify.
(3) Correlation Factor Determination.
(a) Correlation factor determination should be done only after the first two parts of the correlation analysis (data validation and performance shift determination) have been satisfactorily completed.
Par 401 Page 27
NOTE: Some OEMs specify run-to-run parameter tolerances for engine correlation testing. If the OEM’s baseline facility was used for reference testing and the data obtained at the two facilities was within the specified tolerances, then the applicant’s facility should be deemed correlated and would not require a correlation factor. See paragraphs 400a and 401e(3)(d) for facilities other than the baseline facility that are used for reference testing.
(b) Turbofan and turbojet engines with a choked core exhaust nozzle generally require that a correlation factor be applied only to thrust. However, engines with unchoked core exhaust nozzles may require additional correlation factors (e.g., exhaust gas temperature (EGT), fuel flow, rotor speeds).
(c) OEM’s manuals should be referenced for correlation factor determination and application methodology. Generally, correlation factors are applied as a curve input (algebraic difference) or a scalar (multiplier, usually greater than unity). They also usually represent the differences in corrected data obtained during testing at the reference and applicant’s facilities. Correlation factors are usually applied to corrected, not raw, data. For example, when a correlation factor is developed for a choked engine, the x-axis may represent corrected thrust, corrected rotor speed, or EPR, and the y-axis would be used for plotting the appropriate correlation factor.
(d) When using a one-away facility for reference testing, caution should be used in determining correlation factors for the applicant’s facility. The one-away facility may not have exactly repeated the OEM’s baseline facility performance curve characteristics. As long as the one-away’s characteristics were within the manufacturer’s acceptable run-to-run tolerance, then a correlation factor applied to the one-away facility would not have been necessary. When a third facility’s (or applicant’s) characteristics are compared to the one-away’s performance characteristics and found to be within the same manufacturer’s acceptable run-to-run tolerance, there exists a potential for compounding the acceptable tolerance. This results in an error greater than the allowed tolerance relative to the baseline facility. The potential compounded tolerance error can be averted by taking into account the characteristic differences between the one-away and baseline facilities when evaluating the correlation factor for the applicant’s facility. The possibility of compounding tolerance errors becomes greater the further the reference facility is from the baseline facility, with respect to correlation. Paragraph 400a provides additional information on this.
NOTE: As previously discussed, every effort must be made to completely eliminate or compensate for all measurement errors during data analysis. Once this is accomplished, attributing the remaining known minor, but elusive, differences to the correlation factor is generally acceptable. For example, there might be a 5-degree measured and corrected EGT difference between the applicant’s facility and the reference facility. The applicant’s facility would then apply the 5 degree difference to all future measured and corrected EGT data. This methodology should be applied only when the parameter has a constant error over the entire engine operating range and is constant from test to test.
NOTE: If the characteristic curve shapes for a particular parameter measured at the two facilities are divergent (not parallel), then this error should not be applied as a correlation factor. The error would result in the cell producing erroneously variable data during future acceptance testing of engines at the applicant’s facility.
In this case, a continued effort should be made to determine the cause and eliminate the error.
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