Monitoring load experience of individual aircraft

Abstract

The actual service load experience of aircraft may differ appreciably from design assumptions. The necessity to monitor service loads is generally recognized now for military aircraft. This article starts with a general review of the overall life management procedure commonly used today. Specific elements in this procedure are discussed in some detail. Specific attention is paid to the amount of scatter in severity between different flights and the required sample sizes of flight load measurements for obtaining reliable average load spectrum data. Possible causes for variation in load experience between different aircraft flying the same duty are analyzed. It is concluded that individual aircraft tracking (IAT), if necessary at all, can usually be adequately accomplished by administrativ e means, indicated as monitoring. IRCRAFT structures have a limited fatigue life. During design, a sufficient fatigue performance under the anticipated loading environment is certified. However, the actual usage and consequent loading may (and usually does), differ considerably from the design assumptions. Therefore, monitoring of the actual loading in service and an associated reassessment of fatigue performance is required. The general introduction of damage tolerance (DT) concepts in structural design has not relaxed this requirement. On the contrary, the DT inspection periods, necessary to maintain airworthiness, are directly related to load experience, whereas for the calculations of durability lives, considerably smaller than formerly used safety margins are applied when calculating so-called safe service lives. Hence, service load monitoring is even more desirable than before. This article starts with a review of the general life management procedures applied by main Air Forces today. Specific aspects of this procedure will be discussed in more detail, using experience obtained with Royal Netherlands Air Force tactical aircraft. Finally, special attention will be paid to the desirability of so-called individual airplane tracking (IAT), and the most effective means of such IAT.