Downloaded from https://www.everyspec.com ADS-79D-HDBK ―damaged‖ component ( pitting damage on two or more pinion teeth) are significantly different than the CI values of an ―undamaged‖ (no pinion teeth damage) component. A test statistic is used to make a decision about the sample data set. The test statistic selected is dependent on the hypothesis and statistical characteristics of the data. The t test is the test statistic selected for this analysis. It will be used to compare the CI mean values from the damaged and undamaged gears to decide if they are statistically different.
The test statistic is used to determine if there is enough evidence to reject or fail to reject the null hypothesis ( H o ). The alternative is referred to as H 1 . The test statistic selected for this analysis will determine if the CI values from the damaged component are significantly different from the component with no damage: H o : µ 1 = µ 2 (CI values for the damaged and undamaged gear respond the same) H 1 : µ 1 ≠ µ 2 (CI values for the damaged and undamaged gear respond differently) Where:
µ 1 = population mean of undamaged gear teeth µ 2 = population mean of gear teeth with pitting damage on two or more pinion teeth If we reject the null hypothesis ( H o ) the means are not equal and CI values differ significantly. These differences will enable differentiation of a damaged and undamaged gear based on the response of the CI to the damaged and undamaged gear states. Since the test statistic selected is dependent on statistical characteristics of the data, the CI data and a histogram of the eleven tails in the time interval prior to removal and after replacement are plotted in Figures I-2 through I-12.
The time that the gear damage occurred within the replacement interval was unknown. This means that the gears could have been undamaged during collection of the CI data within the replacement interval. For this reason, a threshold of 2 was defined to indicate the start of damage to a gear tooth. Once the CI was found to be equal to or greater than 2, from that time forward the data was used for the damage state of the gear. Figure I-13 is a plot of the CI and histogram for all eleven helicopters with this method applied to the damage dataset.
Review of Figures I-2 through I-13 found the CI, Sideband index increased when the gear was in the damaged state in the time period prior to replacement. Several observations can be made after reviewing Figures I-2 through I-13. In Figure I-7 the CI did not appear to respond to the damaged gear as well as CI values measured on the other helicopters. This could have been due to the type or level of damage that occurred on these pinion teeth was less than the damage observed on the other pinion teeth prior to NGB replacement. The tear down analyses of these eleven helicopters are currently under review and a damage level factor will be defined prior to the next revision of this handbook.
Another observation is that in Figure I-12, only two data points were available after replacement. Due to this limited data, the tail plotted in Figure I-12 was not used for further analysis bringing our dataset down to 10 helicopters. As additional data points are collected for these helicopters they will be added to future analyses. . 157