Assessing Sustained Fatigue Damage in Traditionally-Benign Steady-State Helicopter Flight Conditions

Helicopter fatigue reliability inherent to traditional safe-life fatigue methodologies depends on conservatively accounting for strength, load, and usage variability. These methodologies call for use of a "working curve" with appropriate strength reductions, use of "top of scatter" flight test loads to account for load variability, and use of a "composite worst case spectrum" to account for usage variability. Use of a composite worst case spectrum in a traditional safe-life fatigue methodology is not consistent with allowing flight at gross weight and altitude combinations where sustained fatigue damage accrues in traditionally benign steady-state flight conditions, such as level flight or low-angle-of-bank turns. In fact, these traditionally benign regimes are often under-represented in a composite worst case usage spectrum. Operation in such damaging flight conditions without redesign may never be practical from a life-cycle cost standpoint. However, there continues to be increasing programmatic pressure to push the limits of existing aircraft designs. In this paper, the authors propose specific modifications to traditional helicopter safe-life fatigue methodologies in order to allow for appropriate technical metrics to be established and to foster the ability of materiel developers to properly consider life-cycle costs. The result of the paper will be the ability for materiel developers to evaluate trades between flight restrictions (performance) and fatigue retirement intervals (availability and life-cycle costs) while maintaining baseline reliability (risk). Although wise materiel developers would reserve acceptance of reduced reliability (increased risk) for meeting only the most critical mission needs, such considerations would also be possible. Impacts of the proposed modifications are illustrated using numerical methods based on analytical cumulative-damage reliability tools.