CHAPTER 1. INTRODUCTION
100. GENERAL. This AC describes an enclosed test cell or facility, with fixed instrumentation, that is used for in-service acceptance testing of turbofan and turbojet engines. The AC prescribes:
/g183/g32 General test cell design guidelines /g183/g32 Initial correlation procedures for these test cells /g183/g32 Procedures that may be necessary following repairs and/or modifications to the test cell and/or engine test hardware /g183/g32 General procedures for maintaining a satisfactory correlation status
101. BACKGROUND. Turbine original engine manufacturers (OEM) require that their turbine engines be evaluated for performance and operational characteristics by engine testing following overhaul and certain other maintenance. The OEM’s overhaul manual specifies engine acceptance test procedures, parameters to be measured, and “operational” limits. The FAA requires engine maintenance and testing in accordance with 14 CFR part 43, section 43.2(a)(2).
a. The design and construction of turbofan/jet engine test facilities will influence engine operation and performance. Hardware installed on engines, such as bellmouths, thrust reversers, exhaust nozzles, noise suppressers, cowling, and quick engine change (QEC) components, can alter engine performance and influence test results. Therefore, accurate turbofan/jet engine performance evaluation requires that the influences of both the test facility and the engine, including its installed equipment, be known. Test cell correlation testing is performed to determine the effect of these items on the performance of an engine relative to the baseline performance of that engine. The Federal Aviation Administration (FAA) intends for the incorporation of the information described in this AC to provide a controlled, accurate, consistent, and repeatable engine acceptance test of turbofan and turbojet engines.
b. Test cell correlation is the recommended and most common method used to demonstrate that a test cell is operating correctly and producing engine performance data that matches the reference or baseline test cell data within an acceptable small tolerance. Therefore, the correlated test cell provides a means to ensure that accepted engines meet minimum rated requirements. Test cell correlation factors, which may be determined to be necessary based on the results of correlation testing, would then be applied in all subsequent engine acceptance testing. When a test cell is correlated for a specific engine model, the correlation may also be valid for derivative models of engines having lower thrust but using the same design installed hardware. Test cell correlation procedures, pertinent to specific engine models, may be provided in a document published by the OEM. These correlation procedures should be specific enough to ensure that future in-service acceptance testing will consistently result in obtaining the required engine performance parameters within the manufacturer’s prescribed accuracy. When an OEM’s correlation documents are nonexistent or nonspecific, this AC provides information for satisfactory test cell correlation test procedures.
c. Unless an acceptable alternative method is performed, test cell correlation or recorrelation should be accomplished under the following circumstances:
(1) Following construction of a new test cell;
(2) When either a new engine model or derivative model of higher (or sometimes lower) thrust is introduced;
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(3) When repairs or structural modifications have been made to an existing test cell that significantly affect engine performance;
(4) When repairs or modifications that significantly affect engine performance are made to the engine test hardware, as well as to the entire engine configuration (e.g., QEC, thrust reverser, bare engine);
(5) 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;
(6) If and when recommended by the OEM; and
(7) Where the FAA has determined that there is valid reason to perform a test cell correlation.
d. Reference engine testing, which is the first stage of a correlation, is usually performed at the OEM’s baseline test facility. This baseline test cell is correlated or referenced to an open-air condition through testing using an outdoor test facility or through analysis. For various reasons, the baseline test facility is sometimes not available for reference testing. In this case, another facility may be used for reference testing. This facility must be FAA-approved and traceable to the OEM’s baseline facility. The use of the OEM’s test cell, which is FAA-approved for production acceptance testing of engines of the same type and model, is permissible for correlation reference testing.
102. CALIBRATION AND CORRELATION STANDARDS AND METHODS. Test cell correlation is a separate requirement beyond the calibration requirements for the test cell instrumentation. During the performance of instrument calibration, each instrument must be tested for conformity to an accepted known standard (e.g., National Institute of Standards and Technology (NIST)). In addition, a check of each instrument system should be performed by comparing a particular readout device and its associated components from the engine interface point with either a primary or secondary standard or a known input source.
a. Test cell correlation testing should be accomplished by operating a designated correlation engine that exhibits stable performance and produces at least minimum thrust. Also, it should be of the same type and model that is to be tested in the test cell. In some cases, correlation testing may also be accomplished by using an engine model of a higher thrust rating to substantiate test cell utilization for derivative models of lower thrust.
b. The performance of the designated correlation engine should be established by conducting performance tests at an open-air test site or in the OEM’s baseline test cell. However, an alternative accepted practice is to determine the performance of the correlation engine at another FAA-approved facility, provided the facility is traceable to the OEM’s baseline cell. This FAA-approved facility is designated a “Reference Facility.”
103. ALTERNATIVE CORRELATION METHODS. There may be test cell correlation methods alternative to those methods described in paragraph 102. When a facility proposes to correlate its test cell by procedures that deviate from the OEM’s recommendations or this AC, the facility should refer the proposed correlation process to its respective FAA Flight Standards District Office (FSDO). Although the Regulations permit an alternative means of compliance, an FAA aviation safety inspector (ASI) should not make this determination alone because the proper correlation of a test cell is generally an engineering function. Facilities correlating test cells by alternative means may receive assistance from one or more of the following: the OEM, respective FSDO, FAA regional engineering organization, and Page 2 Par 101
FAA Designated Engineering Representatives (DER) with knowledge and experience in engine test cell correlation.
104. DEFINITIONS OF TERMS. The following are clarifications of the terms used within this AC.
They reflect the definitions generally accepted in the industry.
a. Facility. The facility under evaluation, which is to be correlated against the reference facility.
Also termed Applicant’s Facility.
b. Back-to-Back Test. A test comparing performance parameters measured on the same engine in the same facility, typically before and after a repair or modification to a test facility and/or engine test hardware.
c. Baseline Facility. A facility designated by the OEM as the standard for certification of an engine model.
d. Cell Depression (Pcd). The difference between ambient barometric pressure and the static pressure in the test cell chamber referenced to the engine inlet.
e. Correlation. The comparison of performance parameters measured on the same engine (i.e., correlation engine) tested in two individual test facilities, where one facility is the reference/baseline.
f. Correlation Engine. An engine of known and repeatable performance, possibly equipped with extra instrumentation, designated to be used for test cell correlation.
g. Correlation Factor. A multiplier or an algebraic difference that is normally used to adjust corrected performance parameters measured at the facility to be correlated. This adjustment accounts for the difference in engine performance between the facility to be correlated and the reference, or baseline, facility. Correlation factors are also known as correction factors or facility modifiers.
h. Engine Dress Kit. Hardware, consisting of external engine cowling, bellmouth, exhaust nozzle, accessories, and test instrumentation, to facilitate engine operation in a test cell. The terms engine dress kit and engine test hardware are used interchangeably throughout this AC. They are intended to have the same meaning and may include QEC package hardware.
i. Engine Test Hardware. See Engine Dress Kit.
j. Instruments. Measurement and readout devices (e.g., gauges and sensors such as probes and pickups) used for obtaining test cell and engine operation or performance measurements.
k. Instrument Calibration. The comparison of a particular instrument with either a primary or secondary (transfer) standard. The standard should be traceable to a standard maintained by NIST or another national standards organization.
l. Instrument System. A system comprising a readout device and its associated components (e.g., sensors, interconnecting cables, tubing, fiber optics) from the engine interface point to the respective readout device.
m. One-away Facility. An FAA-approved facility that is used for reference testing and has been correlated directly to the OEM’s baseline facility.
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n. Outdoor Test Facility. An open-air facility without any enclosure for testing engines in an environment not influenced by man-made surroundings (e.g., walls, silencers). Not considered a test cell within this AC.
o. Quick Engine Change (QEC) Kit. Engine accessory packages that permit QECs.
p. Reference Facility. A designated test facility of known performance that is traceable to the OEM’s baseline facility. The reference facility is the test facility against which the facility to be correlated is compared. The reference facility may be the OEM’s baseline facility.
q. Test Cell. An enclosed facility that provides a controlled environment of fixed instrumentation designed to evaluate turbine engine performance following overhaul or other maintenance (when specified by the OEM). Test cells, as defined in this AC, do not include open-air facilities, outdoor test facilities, or on-wing test devices.
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CHAPTER 2. CONFIGURATION AND CONSTRUCTION
200. GENERAL DESIGN CONSIDERATIONS. The primary function of the engine test cell (facility) is to provide a controlled environment for engine testing that is compatible with the engine under test and, thus, will not hinder engine operation. It is therefore necessary to conduct tests in a facility that can accurately and consistently provide a measurement of engine performance relative to the performance that would have been obtained if the engine had been tested at the OEM’s baseline facility. All test facilities affect the testing environment and influence the data obtained during testing. This is particularly true of indoor test cells. An OEM’s document, such as a test cell facility planning manual, may be used as a design guide. If a document is not available or if the facility elects not to use it, the information contained in this AC may be used as a general guideline in planning and operating a test cell facility. This AC can be useful in evaluating the design of a test cell, troubleshooting correlation results, and in maintaining a satisfactory correlation status of the facility.
a. Generally, the cell construction is of reinforced concrete for primary support structures with control room construction that consists of either reinforced concrete or concrete block. Secondary cell support areas, such as preparation rooms, may be of any industrial construction. Additional information on design can be found in reference 1-2(a)(2) of Appendix 1.
b. The following system elements are common to most cell configurations. The design of these components may affect engine performance and data accuracy and, thus, the correlation results.
/g183/g32 Intake section
/g183/g32 Engine test and mount system section
/g183/g32 Augmentor/diffuser section
/g183/g32 Exhaust section
/g183/g32 Data acquisition and reduction system
/g183/g32 Instrumentation system and control room
/g183/g32 Engine fuel supply system
/g183/g32 Auxiliary system (may be required by a particular powerplant being evaluated)
201. TEST CELL CONFIGURATIONS. The presence of vortices, turbulence, and non-uniform temperatures and pressures in the area surrounding the engine under test can drastically affect engine performance and test repeatability. Therefore, all test cell configurations should be designed to provide stable testing conditions and minimize turbulent flow by minimizing pressure loss and temperature and pressure variations. Under most environmental conditions, a test cell configuration should not allow recirculation of engine exhaust gases from the cell exhaust stack into the cell inlet. It should also prevent reingestion of engine exhaust gases at the rear of the engine back into the engine inlet. Test cell designs are typically of the following three general configurations:
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a. “L” Type. This design has a horizontal intake and a vertical exhaust. Airflow treatment may include at least one turning vane to turn the intake air uniformly. A grid may be installed on the horizontal inlet to assist with airflow straightening and provide increased noise reduction. The type “L” is the simplest design and the least costly to construct. A horizontal inlet will generally have good flow distribution and a reduced Pcd when the external wind is directly entering the inlet. This configuration is sensitive to prevailing wind conditions and loses efficiency and repeatability when the wind changes direction. This configuration requires a relatively large, unobstructed test area at the engine inlet station for maximum performance (see Figure 2-1 for details).
FIGURE 2-1. TEST CELL CONFIGURATION: “L” TYPE
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b. “U” Type. This design incorporates a vertical stack for both inlet and exhaust. The vertical intake tends to have a more uniform inlet air velocity. This design is less subject to wind disturbances.
Airflow treatment may include at least one turning vane to turn the intake air uniformly. The inlet may be designed to produce a uniform airflow within the test area at the engine inlet station, as well as a uniform engine inlet temperature. A grid may be installed on the vertical inlet to assist with airflow straightening and provide increased noise reduction (see Figure 2-2 for details).
ExhaustAugment orPrimary Test Chamber Screen Test St and w it h Thrust Frame Inlet Acoust ic Baffl es
FIGURE 2-2. TEST CELL CONFIGURATION: “U” TYPE
c. Folded Inlet Type. This design usually incorporates vertical inlets on the building sides, resulting in a more accessible test section because the engine entry door can be located directly in front of the thrust stand. Another variant of this design includes a horizontal inlet on the building top. In both cases, the exhaust is configured vertically. Using a series of turning vanes, the inlet air is uniformly drawn through a winding flow path to a large plenum, which then supplies air to the test section. This configuration is also not significantly affected by wind direction (see Figure 2-3 for details).
ExhaustAugmentorPrimary Test Chamber Test Stand with Thrust Frame Reverse Flow Inlet Acoustic Baffles Retractable Turning Vanes
FIGURE 2-3. TEST CELL CONFIGURATION: FOLDED INLET TYPE Par 201 Page 7
202. ENGINE TEST CELL MAJOR COMPONENTS/SYSTEMS.
a. Engine Test Section. The engine test section is the area immediately approaching the engine under test. Generally, this area will be of a sufficient cross section so that the air velocity approaching the engine inlet will not exceed approximately 50 feet per second (fps). In this section of a well-designed test cell, the airflow tends to have uniform pressure distribution. Test section design and construction may incorporate tapered or concave corners at the rear section where the air flows into the augmentor.
b. Interior Treatment.
(1) Steep floor grades should be avoided
(2) All drains, fittings, and other floor access areas should be flush with the floor
(3) Floor surfaces should be clean, dry, and sealed to prevent foreign object ingestion into the engine and degradation of the test cell surfaces due to leakage of engine oils, jet fuel, and other engine-related substances.
(4) Interior walls and ceilings should be smooth and free of protrusions.
(5) Moveable mechanical equipment should be removed from the engine test area during testing.
c. Engine Mounts. The engine mounts support the engine during testing and permit engine thrust to be accurately measured. Thrust of the engine is usually produced at the engine centerline and transmitted through the mounts to a thrust frame. The thrust frame then pushes against or pulls on a load cell. Thus, it enables the reaction to be measured.
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NOTE: The most common method of engine mounting is overhead suspension.
However, at some engine test facilities, the engine is mounted on a pedestal supported by the test cell floor. The overhead mount more closely simulates the mounting in many aircraft, and more easily accommodates the cleaning of the engine test section and the accessibility of bottom-mounted engine accessories. The engine mount should be designed to prevent transverse motion, fishtailing, or any type of lateral instability. With turbofans, poor lateral stability due to mount flexibility can result in severe engine oscillations during testing. Thrust mount designs should also ensure that engine axial alignment is maintained during testing.
See Figure 2-4 for a schematic representation of a typical overhead mount installation.
FIGURE 2-4. TYPICAL THRUST MOUNT DESIGN
d. Test Cell Inlet System. The test cell inlet system conditions incoming air to reduce the effects of wind speed, direction, and extreme temperatures. This system consists of flow straighteners, heaters, screens, and noise suppressers. These components tend to create a pressure loss or Pcd. The following test cell inlet design features may have an influence on engine performance:
/g183/g32 Area
/g183/g32 Blockage
/g183/g32 Turning angle
/g183/g32 Axial distance between engine inlet and exit of the cell inlet
/g183/g32 The angle of inlet turning vanes
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e. Test Cell Exhaust System. The test cell exhaust system augmentor removes engine exhaust gases from the engine test section, induces the flow of secondary air for cooling, and provides some noise abatement. The mix of exhaust gases and the cooling secondary airflow that goes through the augmentor is then directed through an exhaust stack prior to exiting the facility. The following test cell exhaust design features may have an influence on engine performance:
/g183/g32 Augmentor configuration (i.e., convergent or divergent)
/g183/g32 Augmentor tube length and diameter
/g183/g32 Exhaust inlet tube diameter
/g183/g32 Axial distance between engine exhaust and augmentor inlet
/g183/g32 Area ratio of the engine exhaust to the augmentor area Page 10 Par 202
CHAPTER 3. MEASUREMENT SYSTEMS, CALIBRATION, AND CORRELATION
300. MEASUREMENT SYSTEMS. The following paragraphs provide a general description of typical measurement systems. Instrumentation quantity, position, range, and accuracy are usually specified in an OEM’s document, such as a facility planning manual or overhaul manual.
NOTE: End-to-end checks of each of the systems addressed in this section are extremely important to assure the validity and accuracy of the data being collected.
These checks involve the comparison of data from a particular readout with its associated primary or secondary standard or with a known source. During these system checks, many physical, electrical, or flow discrepancies can be detected and corrected before correlation testing begins.
a. Pressure Measurement Systems. Pressure measurement systems usually use simple pressure gauges, individual dedicated pressure transducers, or a scanning-valve system. Scanning-valve systems are typically a single pressure transducer that alternately measures many pressure lines of similar pressure range.
NOTE: High humidity or rain drawn into the test cell can result in water getting into the pressure lines and producing erroneous pressure measurements for both types of pressure management systems. (See section 305a(2) of this AC.)
b. Temperature Measurement Systems. Temperature measurement systems should be capable of independently evaluating “low and medium” and “high” temperatures. Commonly, low and medium temperature measurement systems utilize E- or J-type thermocouples to measure engine cold section and facility temperatures. High temperature systems typically use K-type thermocouples to measure exhaust gas temperatures. The parameters to be measured will depend on the OEM’s requirements for testing.
Thermocouple junction locations should be minimized within the test cell environment to reduce system noise and error.
c. Engine Speed Measurement System. The engine speed measurement system must be capable of measuring all engine speeds. A typical system may consist of a digital electronic frequency counter device and a readout device, or an analog type with a graduated display dial readout. Both systems provide engine speeds in revolutions per minute or percentages.
d. Fuel Flow Measurement Systems. A typical fuel flow measurement system may consist of one or more turbine flow meters (or comparable flow measuring device(s)), each connected to a digital frequency counter and display gauge. This total measurement system, from the flow meter through the frequency counter to the display gauge, must be checked end-to-end for the entire operating range of the system. For example, a defective frequency counter could provide an acceptable error in the low flow range, while generating a divergently increasing unacceptable error in the high range.
NOTE: Fuel flow measurement becomes increasingly important when it is used in turbine gas temperature calculations. This calculation is sometimes required by OEMs for acceptance testing.
e. Vibration Measurement System. Vibration is not typically evaluated during test cell correlation testing because it does not normally affect engine performance. However, turbine engines cannot be properly evaluated during in-service acceptance testing, as required by the overhaul manual, without employing a system to provide vibration measurement. System components can include vibration pickups, amplifiers, tracking filters, spectrum analyzers, band pass filters, and audio oscillators. Engine Par 300 Page 11
pickup locations can include the fan case, intermediate case, diffuser case, and turbine exhaust case. Hot section vibration pickups may require insulation to prevent high-temperature damage.
f. Trim Balance System. Although trim balancing of the low speed compressor is not required for test cell correlation, performing such balancing before correlation testing is acceptable. In addition, the OEM may specify a trim balance during in-service engine acceptance testing to determine imbalance moment values and, thus, the location and weights of balance counterweights. Generally, trim balancing is accomplished with the aid of a trim balance analyzer. However, procedures are also available for trim balancing with trial balance weights.
g. Thrust Measurement Systems. Thrust measurement is typically accomplished by either an electronic strain gauge or a hydraulic measurement system. The required system accuracy should be specified by the OEM.
301. INSTRUMENT CALIBRATION.
a. Engine Test Facility. The test cell can be considered a component of an engine’s test equipment.
As such, it requires instrumentation calibration and an end-to-end check of the measurement systems.
Instrument calibration plus system checks ensure acceptable measurement accuracy within engine acceptance testing limits specified in the OEM’s overhaul manual. The OEM’s overhaul manual and other documents, such as a facility planning manual, reflect the procedures and test equipment that are required to test engines and the parameters to be measured. Test cell instrumentation must be calibrated and traceable to an accepted, known national standard.
b. Standards. In the United States, the NIST represents the United States at the International Conference on Weights and Measurements and has the primary responsibility for maintaining the national and international standard units of length, mass, time, temperature, and electrical quantities. Other nations have comparable standards bodies. An acceptable secondary or transfer standard has been generally regarded as any standard that is at least four times more accurate than the accuracy requirement for the instrument being calibrated and is traceable to a national standard, such as those maintained by NIST.
c. Calibration of Instruments and a Check of Associated Systems. Calibration is used to check an instrument’s measurements against an accepted known standard and to eliminate unacceptable measurement errors. Instruments should be calibrated over the expected range of operation. Periodic calibration of individual instruments is necessary and can be performed in place or in the instrumentation shop.
d. Traceability. Traceability of each instrument calibration is important to show where all possible error contributions occur between the instrument and the standard. Traceability should be well documented. The calibration hierarchy, with inherent accuracies at each point, must be documented.
Traceability back to a specific standard does not reduce the uncertainty of a measurement; instead, it provides clear documentation of the process of its determination.
302. THRUST CALIBRATION. To calibrate the load cell, a common procedure is to apply known centerline forces using a centerline pull rig. This calibration procedure results in the development of a calibration curve of the known force and the measured load cell reaction. This curve is used during the correlation to obtain a true force from the load cell measurement. Another method is the use of a calibration device built into the thrust stand.
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