Dormant failures

Dormant failures relate to nonactive failures, where the failure is not detectable. In this case, the probability for a flight cycle is the probability per flight hour multiplied by the check interval rather than the average flight time. The accepted equation actually uses half of the check interval, on the basis that the furthest you can be from knowing the state of the item is either half the check interval from the last checked state, or half the check interval from the future checked state. 

Finally, an annex of the SSA (or a separate FTA report) should include a summary of the fault tree results, and if they are compfiant to the numerical targets and, for top events with a catastrophic severity, whether any single point failures have been identified. Additionally, the report should detail any maintenance actions that are necessary to alleviate the effects of dormant failures. 

CMRs are failure finding tasks (not preventative maintenance tasks) and exist solely to limit the exposure to otherwise hidden/dormant failures. 

The failure rate of non-tin doped mercury wetted relays was found increasing significantly. An event happened at Embalse NPP, consisting on the opening of the main steam safety valves due to dormant failures of these relays. Besides, these relays are included on the safety systems of the plant. The corrective actions were to replace such relays by tin-doped relays, which prevents the jumping. Other issues related to l C reliability components are evaluated and considered in the PSAs. 

For revealed failures the MDT consists of the active mean time to repair (MTTR) PLUS any logistic delays (e.g., travel, site access, spares procurement, administration). For unrevealed failures the MDT is related to the proof-test interval (T), PLUS the active MTTR, PLUS any logistic delays. The way in which failure is defined determines, to some extent, what is included in the down time. If the unavailability of a process is confined to failures while production is in progress then outage due to scheduled preventive maintenance is not included in the definition of failure. However, the definition of dormant failures of redundant units affects the overall unavailability (as calculated by the equations in the next Section). 

FMECA is a good tool to assess the testability parameters of a system. Dormant failures list with severity information is provided to testability engineers via engineering coordination memos as well as FMECA reports. Testability engineers use failure detection method and failure rate columns of FMECA in order to calculate Fault Detection Rate (FDR) and Fault Isolation Rate (FIR) of a system. And also perform necessary revisions in order to lessen the number of dormant failures by taking into account the failure severities. 

After the dormant failure of Diesel2, nothing happens because the power supply is still performed by the first line. 

A dormant failure refers to a component that is not checked for operability before the start of a mission thus, it could unknowingly be failed when required... 

A latent failure refers to a component that is not checked for operability before the start of a mission thus, it could unknowingly be failed when required for use. When failure of the component occurs, it is not detected or annunciated. Certain latent faults can be of system safety concern because they are involved in system designs where operation is critical. Latency can significantly increase the potential safety risk because this situation effectively increases the component exposure time. The latent time period is the time between maintenance checks, which can often be significantly greater than the mission time. This large exposure time can make a large impact on the probability. Latency is also sometimes referred to as dormancy or dormant failure. 

The performances of a safety system are maintained throughout its operation only if the maintenance range and periodic inspections required during the safety study are met, particularly considering the detection of dormant failures those that appear only when the system is solicited. The frequency of systematic verification comes from the collection of safety data and is undertaken about once or twice a year for each sensor and each actuator. 

Independent failures, which separately do not degrade safety significantly, may combine to produce a hazardous situation. There may be a combination of active failures and passive failures, such as dormant failure of a standby system before the main system fails or an undetected leak of flammable vapour followed by a spark caused by an electrical failure. 

Passive (or dormant) failures could result in degraded performance without giving a definitive indication to the aew. This could go undetected until discovery during maintenance checks, or until discovered too late at a time when functionality is required. An example of the latter would be a dormant failure in an auto-land system where accuracy of the ILS centre line may be compromised. 

The second solution above has its limitations. For instance, the series system would also mean that if one detector fails to function, then the system may fail to operate when required to do so. Practical judgement will be required to balance the probability of failure to operate when wanted against the probability of operating when not required. It may be more desirable to add a comparator , which compares the results of the two detectors, monitors correct functionality and then prompts the stick shaker to respond. However, bear in mind that the comparator also has failure modes (which could include a dormant failure). 

Note how the above example raises a point about fail-safe design and dormant failures. 

Bl S in already inoperative when the pilot switches over to it. This case is exactly equivalent to the dormant failure of the warning. 

These risks have nothing to do with the duration of the flight. The fact that the system is used only for a short period, particularly towards the end of a flight (e.g. brakes, flaps, etc.) does make the system s integrity vulnerable to dormant failures which become apparent only when the system is needed. In these cases, if the probability per use = p, then the probability of failure in a flight = np, where n is the number of uses per flight. 

Case A loss of main with dormant failure of warning. 

Dormant failure of S/B before main system failure. 

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