Major failure conditions

Major failure condition must be no more frequent than Remote. 

B Hardware functions whose failure or anomalous behavior, as shown by the hardware safety assessment, would cause a failure of system function resulting in a hazardous/ severe-major failure condition for the aircraft. 

Hazardous/severe-major Failure conditions which would reduce the capability of the aircraft or the ability of the crew to cope with adverse operating conditions to the extent that there would be... 

Level D Software whose failure would cause or contribute to a major failure condition... 

An important application of this equation is to distinguish between two extreme failure conditions, known as the active and passive failures. First, the active and passive states of stress may be explained as follows Consider a cohesionless Coulomb powder. If the powder is assembled in a large container in successive horizontal layers without disturbance, there will be no shear stresses along the horizontal and vertical planes inside the powder because of the symmetry of the problem. Thus, at any point, the horizontal and vertical normal stresses are the principal stresses at that point. In this case, if the major principal stress is the horizontal stress, passive state of stress. On the other hand, if the major principal stress is the vertical stress, active state of stress. Thus, Eq. (8.9) can be written for each state as... 

Several Surlyn-sealed cells were exposed to outdoor conditions over a period of one year. Major failures only occurred due to imperfect sealing and ageing effects on the electrical contacts. The results of the best performing cells are shown in Fig. 7.9 and it can be seen that the efficiency remains remarkably constant after one year of outdoor exposure. 

This topic is of course very important in engineering practice. Many geotechnical professionals are uncertain how to interpret readings from piezometers in non-hydrostatic conditions. This becomes very important when interpreting conditions in an unstable slope or comparing assumed or predicted conditions in finite element analyses (e.g. using Plaxis) with the field conditions they are supposed to represent. Misconceptions about the water regime often contribute to major failures in slopes, excavations and tunnels. 

This information provides a major benefit to owners/operators, especially for PE logic solvers and PE field devices, where the FMEA can be very complicated, and embedded software could include potential systematic dangerous failure conditions. 

Therefore, the safety failure cycle model suggests that managers may compromise safety goals, particularly under conditions of scarce resources. Furthermore, they may fail to correct this bias in resource allocation because the nature of safety feedback makes learning and control difficult. Facing uncertainty and imperfect feedback about the outcomes of their resource allocation, managers may find it difficult to maintain or restore balance. The consequent resource allocation leads to a gradual drift and eventually to a major failure. 

Fatigue has been recognized as a major failure mode in pressure vessels, and specific rules for its prevention appear in design codes. Stated simply, fatigue failure is caused by the cyclic action of loads and thermal conditions. In many design situations, the expected number of cycles is in millions and for all practical purposes can be considered as infinite. Accordingly, the concept of endurance limit has been employed in a number of design rules. 

Various studies have focused on the degradation mechanisms of either the fuel cell system or its components under steady or accelerated operational conditions. The major failure modes of different components of PEM fuel cells are listed in Table 1.2. 

Bell Labs technicians identified two major failure modes that they described as causing catastrophic loss of insulation resistance due to the formation of conductive bridges between conductors. The first failure mode—through-substrate shorts—only occurred above 75°C and 85 percent RH and thus was not considered to be a problem at use conditions (see Fig. 56.2). 

The Corrective Maintenance (CM) is widely used but as a reactive approach does not prevent costly repairs or replacements. This is a general problem (Guedes Soares et al. 2010) but in the offshore context failures often occur during seasons of the year when the accessibility to the turbine is limited due to the weather conditions leading to production losses (Nielsen Sorensen 2011). In contrast. Preventive Maintenance (PM) can be planned in time so that the risk of major failures and downtimes is minimized and, the logistics and maintenance costs along with the system operability are optimized. 

In technical conditions, errors that stand out include mistakes in design and specifying standards and in selecting inappropriate materials and forms of ventilation. Furthermore, there can be serious operational and maintenance errors that can cause major failures and consequently undesired events. 

A major objective of the regulation (expressed in paragraph 25.1309 of the [FAR/CS 25]) is that a catastrophic event must be extremely improbable . A first level of interpretation is given by that same regulation a catastrophic event is a Failure Condition which would result in multiple fatalities, usually with the loss of the airplane . This is partly explained for example, if the available roll rate is higher than 3 /second, then the situation is not catastrophic if the aircraft is below the threshold, the criticality is evaluated by pilots and accepted, or not, by official services. 

If similarity cannot be justified, but the system is conventional in its relevant attributes, then compliance may be shown by means of a qualitative assessment. This also applies to systems of high complexity, provided that there is reasonable confidence that the failure condition is not worse than major. 

The old maxim if it ain t broke don t fix it is very applicable in today s machinery. A study conducted at a major nuclear power facility found that 35% of the failures occurred after a major turnaround. This is why total condition monitoring is necessary in any performance based total productive maintenance system and leads to overhauls being planned on proper data evaluation of the machinery rather than on a fixed interval. 

---