SUBSTANTIATION OF DAMAGE TOLERANT DESIGNS IN CIVIL HELICOPTERS

The application of damage tolerant design features is gaining in importance in civil helicopter designs but, paradoxically, substantiation of damage tolerance features has not kept pace. For instance, safe-life substantiation of civil helicopter structure is commonly used even when fail-safe or damage tolerance design features are provided. An FAA rulemaking program began in 1983 to require both the provision of damage tolerance design features, where practical, and substantiation that the design features provide damage tolerance (ref. 1). Although the application of damage tolerance design features and damage tolerance substantiation techniques has been successfully applied to airplanes for years, the application of damage tolerance concepts to helicopter design has proven to be a difficult process. The reasons are threefold: 1. Helicopter structural components have more complex structural shapes than most airplane components. 2. Helicopters have many more potentially damaging load cycles. 3. Airplanes tended to be designed with multielement, fail-safe structure even before damage tolerance was mandated; helicopters historically have not been so designed until recently. Although the application of damage tolerance to helicopters is difficult, now is an appropriate time to start. More extensive use of damage or flaw resistant materials such as composite construction or multiple load path constructions is being incorporated into new helicopter designs. Innovative substantiation methods will be required to supplement more conventional substantiation methods in demonstrating that a given design is, in fact, damage tolerant.