Downloaded from https://www.everyspec.com ADS-79D-HDBK M.5.4 Limits: Maximum allowable level and trending limits for safe operation should be set for each parameter tested. Level limits provide the maximum value for the various parameters measured, for example, 1000 PPM of water in gearbox oil or 45 PPM of iron (Fe) in a gearbox. Trending limits provide the maximum trend (rate) at which a parameter should increase (or decrease). Limits are statistically derived from the large population data sets that essentially cover all oil-wetted faults.
(See Appendix I, Sample Size) A basic statistical process control technique is used for evaluation of the data generating a normal frequency distribution. Level and trend limits should be tracked by component (i.e. sub-assembly level) serial number, not tail number. M.5.4.1 Level Limits: Historically, AOAP warning and alarm level limits were statistically set at the mean plus 2 and 3 standard deviations, respectively, for oil condition and wear debris. 76 Analysis of historical data provides the limits utilized for sampled oil analysis as shown in Figure M-2. Alert level limits are set at two standard deviations and reportable level limits are set at three standard deviations.
At-line instrument limits are statistically set in a similar fashion. On-line sensors tend to have pre-established level limit guidelines. 77 FIGURE M-2: Example standard bell shape curve. Long term statistical analysis supports routine sample analysis M.5.4.2 Trending Limits: To diagnose and predict machinery and fluid condition, the rate of change should be trended. Level limits only state how much damage has occurred. The predictive or forecasting nature of condition monitoring is based on rate trending to determine the degree of damage and remaining useful life of the component or fluid.
There are numerous 76 ASTM D7720-11, Standard Guide for Statistically Evaluating Measure and Alarm Limits when Using Oil Analysis to Monitor Equipment and Oil for Fitness and Contamination. 77 ASTM D7685-11, Standard Practice for In-Line, Full Flow, Inductive Sensor for Ferromagnetic and Non- ferromagnetic Wear Debris Determination and Diagnostics for Aero Derivative and Aircraft Gas Turbine Engine Bearings. 244