General

This document describes a recommended practice and procedure for the trending of parameters to maintain the test cell correlation status. Trending is performed to monitor test cells for changes that can affect engine performance or the data acquired from engine tests.

Benefits

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 a recommended and less expensive method to maintain test cell correlation status.

b. As a method for maintaining correlation of test cell data between engine, airframe and third party overhaul centers supporting:

1. Commercial requirements

2. Military requirements

3. As an early indicator for changes in performance such as:

(a). Performance changes driven by the test cell

(b). Performance changes driven by the engine

(c). Performance changes driven by the measurement system

By trending engine and test cell parameters it can be determined whether changes have occurred to the test stand, instrumentation, or engine that will affect the performance data collected in that specific test stand. The trended data allows the test cell owner to be confident that the data taken from tests in the test cell is valid and accurate. If a change to the test cell, instrumentation, or the engine has occurred, trending allows the change to be identified early.

If the test cell is subjected to abnormal test conditions (i.e. stalls, surges, blade-outs, engine failure) either intentional or otherwise which can damage instrumentation or the test facility hardware, then data trending will show a shift in the trend indicating a resulting effect. Trending will also capture instrumentation drift.

After a test facility is correlated and the correlation factor has been determined, trending is an effective way to maintain confidence in the correlation. The trending of the data needs to commence immediately following the correlation test. Therefore the test cell does not need to be re-correlated as often. Indeed, referring to the reference documents, trending is a recommended alternative to a physical re-correlation of the test cell. If trending is performed on a consistent basis then re-correlation need only be performed when deemed necessary based on the trended data, engineering judgment, or if modifications have been made to the test cell.

The trend data can be taken a step further and the operator can compare the trend data not only to the correlation factors, but they can also use the trend data to monitor the shop performance, engine build standards and overall fleet health for that engine model.

Limitations

This recommended practice is meant as a general guide to trending; therefore specific practices and details may be instituted by the OEM or government agencies. Although this document describes the practice of trending gas turbine engine parameters, these trending practices can be used for trending any type of measurement.