TS Newsletter Archive

7.04.2011

FAA recommendation on minimum instrumentation and calibration for Engine Test Cell Correlation

The main function of the engine test cell is to obtain a proper performance evaluation of a Turbofan or Turbojet engine. The test facility must provide a traceable and reliable data measurement and acquisition function and a stable test environment. These conditions can be affected by several factors, such as:
  • Instrumentation (including the calibration, location, measurement accuracy, and quantity) 
  • Configuration of the test cell (particularly the test cell inlet, engine test section, and augmentor tube and exhaust stack configurations) 
  • Hardware used for indoor test 
  • Engine position in the test cell 
  • Ambient conditions and surrounding area configurations 
  • Test cell thrust measurement system 
  • Test procedures 
  • Data acquisition system 
  • Fuel properties 
  • Cell airflow 

In this article, we will review the minimum instrumentation required for an engine test and the calibration requirements recommended by the FAA in their AC43-207.

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.

a. Pressure Measurement Systems.
  • Usually use simple pressure gauges, individual dedicated pressure transducers, or a scanning-valve system

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.

c. Engine Speed Measurement System.
  • 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

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. 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
e. Vibration Measurement System.
  • Even though vibration is not typically evaluated during test cell correlation testing, turbine engines cannot be properly evaluated during in-service acceptance testing without employing a system to provide vibration measurement and its related components

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.

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 according to the maximum rated thrust of the engine to test.


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. The calibration exercise should be properly documented per OEM requirements and for traceability purposes.

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.

Typical thrust mount design
The test cell can be considered a component of an engine’s test equipment, as such it requires proper calibration, being the correlation exercise the procedure that complies with this requirement, thus traceability, comparison with a know standard and exhaustive documentation are required.

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

In future issues of TeraSigma newsletter, we will cover the requirement of appropriate cell correlation documentation, and the actions needed to maintain it.

Source: FAA AC43-207

1 comment:

  1. That’s essentially what we can get from load cell devices. The application varies on different kinds of heavy measurement in a more accurate digital form; which is why these devices are commonly used in different industries.

    ReplyDelete