Aircraft repair damage tolerance analysis

Abstract

Modern air travel has become a perpetual evolution both from a practical and scientific point of view. However, it is also becoming increasingly common to fly in an airplane with little or no regard for the immense engineering involvement that goes into making air travel as safe and efficient as possible. This report considers the problems of aircraft fatigue and how it translates to inspectability for safety in order to predict problems and solve them before they actually occur. The most common aircraft repair is a crack in a pressurized skin panel. This report evaluates the structural integrity of a particular panel that is assumed to have failed in service and thus been repaired by the addition of a doubler. Damage tolerance analysis is used to evaluate a conservative crack growth scenario for a typical business jet with a structural economic life of 15,000 flight hours. The step shown follow the guidelines approved by the regulating aviation bodies of both Canada and the United States (Transport Canada and the FAA respectively). Structural inspections are a common practice for aircraft at their half lives; in this case it would be 7,500 flights. The report determines that this particular scenario defines a threshold inspection interval of 8,414 flights and a repeat of 2,944 flights thereafter. In comparison with an actual test aircraft, having experienced an almost identical failure and repair program, the test aircraft experienced failure at 9,963 flights. Therefore, the intervals presented herein provide adequate clearance for the detection and repair of such damage. The purpose of this report is to introduce the underlying principals of damage tolerance analysis to the reader and illustrate the analytical process with a real world example. Such is the job of an aerospace stress engineer.