Thresholds for Safety Inspection Measurements Based on Control Charts

Abstract

The rapid growth of air traffic density has long demanded the Federal Aviation Administration (FAA) to design an effective safety inspection system. Well defined thresholds are essential to such an inspection system since they provide standards for both monitoring and regulating purposes. In this paper, we use control chart techniques to derive thresholds and standards for inspection measures, and to provide charts for monitoring them continuously. These thresholds charts play the same role as control charts do in statistical quality control for the monitoring of manufacturing processes. In quality control, the centerline of the chart indicates the target value and the control limits determine whether the process is out of control. In safety inspections, we view the centerline as the safety standard, and justify some properly chosen levels of control limits as meaningful thresholds. For FAA safety inspection surveillance results concerning air carriers, these thresholds are termed alert, advisory, expected, and informational. They provide a concrete measure of the inspection results in terms of the severity of potential flaws, and serve as a guideline for the general rating of the safety performance of each carrier. Furthermore, we can now use the so-called average run length to measure the effectiveness of the inspection system with the proposed thresholds. This approach is implemented on a dataset of an operational performance measure collected from ten air carriers over a period of 6 months. The results are very supportive.