The Art of Helicopter Usage Spectrum Development

In the past 15 years, the Navy has conducted a number of different usage surveys for helicopter models, such as AH-1W, H-46, H-60, and H-3, in an effort to better define fleet usage and develop accurate usage spectra for each model. The surveys were conducted with various Structural Usage Monitoring Systems (SUMS's), such as on-board regime recognition, flight parameter recording, and limited flight parameter recording with a low cost usage monitoring system. The number of aircraft involved in the survey was 50, 20, 10, and 7 for the AH-1W, H-46, H-60, and H-3, respectively. The surveys ranged in duration from months to years. This paper presents an innovative technique to use in data recording, sampling, RAM, and storage memory for various monitoring systems and the systematic approach of usage spectrum development using statistical cumulative frequency distribution of usages. The sensitivity of usage spectrum on rotating component fatigue life is studied and used in the usage spectrum refinement. Further optimization of the usage spectrum is carried out with respect to cycle counting and prorating of angle of bank (AOB), altitude, velocity, and gross weight. In addition, the contribution of usage spectrum to reliability of computed retirement life is studied. The usage spectra developed using SUMS's are compared with design, operational, and classical AR-56 spectra for various classes of helicopters. Present analysis leads to the conclusion that additional usage monitoring should be conducted to enhance the database, aid in refinement of AR-56 spectra to reflect the actual modern fleet utilization of Navy helicopters, and update associated requirements. Individual helicopter usage monitoring is essential to enhance flight safety and identify possible logistics cost savings.