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Definitions
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DEFINITIONS - FAILURE MODES AND EFFECTS ANALYSIS
Compensating Provisions
Provides details of any means by which the effects of a failure can be reduced e.g.
- Redundancy (hot and cold standby).
- System Monitoring / Testing.
- Preventative Maintenance.
Detection Methods
Identifies any means by which a failure can be detected. This may for instance include such methods as:
- Visual detection - This may refer to visual detection by the operator, the maintainer or possibly during inspections.
- Built In Test (BIT) - This refers to detection methods which are designed into the system for self diagnosis.
- Test Set - This may refer to a piece of test equipment which is connected to the equipment with a view to detecting faults / failures in a test environment.
Failure Effects
For each identified Failure Mode, there will be at least one Failure Effect. Failure Effects provide details of how the system will behave as the
result of a Failure Mode. These effects are often analysed at multiple levels within the system hierarchy. Effects are often categorised as ‘Local
Effects’ (low / component level effects), ‘Next Effects’ and ‘End Effects’ (system level effects).
Failure Modes
Every component / item within a FMEA will have at least one Failure Mode. These Failure Modes detail the methods by which failures can occur. Guidance
for the application of Failure Modes for any given component can be found in a number of different standards and publications. One of the most common
of these is FMD 95. Using FMD 95, most electronic components can be easily categorised and applied to a pre-defined set of Failure Modes and
apportionments. For some custom and mechanical devices however, such information may not be available and it may therefore be necessary to apply
engineering judgement.
Mission Phases
The system mission is divided into discrete phases, each of which is likely to have different environmental profiles and hence different
impacts on component behaviour. As an example, the mission phases for an aircraft might be as follows:
- Taxi - Duration 30 minutes, Temperature 60 degrees.
- Takeoff - Duration 5 minutes, Temperature 60 degrees.
- Cruise - Duration 1 hour, Temperature 30 degrees.
- Landing - Duration 5 minutes, Temperature 30 degrees.
Severity
Defines the Logistic or Safety consequences of a Failure Effect. Severity is often divided into 4 categories e.g.
- Catastrophic - death or system loss.
- Critical - Severe injury, major property damage.
- Marginal - Minor injury, minor property damage.
- Negligible - Unscheduled maintenance or repair.
A number of the defense / military standards describe severity categories such as MIL-STD-1629A and DEF STAN 00-56.
DEFINITIONS - CRITICALITY ANALYSIS
Failure Effect Probability (ß)
This parameter represents a probability (based upon engineering
judgement) that a failure mode will result in the identified criticality
/ severity. In many cases, this is simply set to ‘1’ by the engineer which
represents a ‘worst case’ scenario.
Failure Mode Criticality Number (Cm)
This parameter associates failure probabilities to each failure mode and is of the form:
Failure Mode Ratio ( )
This parameter represents the fraction of the overall item failure rate which relates to the failure mode under consideration.
Failure Rate ( )
This is the probability of failure for a part (often expressed in failures per million hours) derived from an appropriate Reliability Prediction.
Time (t)
This parameter is the operating time or operating cycles per mission.
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