Levels of criticality are assigned to the consequences of failure. Some functions are not critical and are left to "run to failure" while other functions must be preserved at all cost. Maintenance tasks are selected that address the dominant failure causes. This process directly addresses maintenance preventable failures. Failures caused by unlikely events, non-predictable acts of nature, etc. will usually receive no action provided their risk (combination of severity and frequency) is trivial (or at least tolerable). When the risk of such failures is very high, RCM encourages (and sometimes mandates) the user to consider changing something which will reduce the risk to a tolerable level. The result is a maintenance program that focuses scarce economic resources on those items that would cause the most disruption if they were to fail. RCM emphasizes the use of predictive maintenance (PdM) techniques in addition to traditional preventive measures. The term "reliability-centered maintenance" was first used in public papers authored by Tom Matteson, Stanley Nowlan, Howard Heap, and other senior executives and engineers at United Airlines (UAL) to describe a process used to determine the optimum maintenance requirements for aircraft. Having left United Airlines to pursue a consulting career a few months before the publication of the final Nowlan-Heap report, Matteson received no authorial credit for the work. However, his contributions were substantial and perhaps indispensable to the document as a whole. The US Department of Defense (DOD) sponsored the authoring of both a textbook (by UAL) and an evaluation report (by Rand Corporation) on Reliability-Centered Maintenance, both published in 1978. They brought RCM concepts to the attention of a wider audience. The first generation of jet aircraft had a crash rate that would be considered highly alarming today, and both the Federal Aviation Administration (FAA) and the airlines' senior management felt strong pressure to improve matters. In the early 1960s, with FAA approval the airlines began to conduct a series of intensive engineering studies on in-service aircraft. The studies proved that the fundamental assumption of design engineers and maintenance planners—that every airplane and every major component in the airplane (such as its engines) had a specific "lifetime" of reliable service, after which it had to be replaced (or overhauled) in order to prevent failures—was wrong in nearly every specific example in a complex modern jet airliner. This was one of many astounding discoveries that have revolutionized the managerial discipline of physical asset management and have been at the base of many developments since this seminal work was published.