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Reliability Programs
FMEA/ FMECA
Reliability Predictions
Reliability Modeling
FRACAS
Reliability in Design
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Failure Mode Effects and Criticality
Analysis (FMECA)
This the Failure Modes and Effect Analysis,
commonly referred to a FMEA, is one of the most utilized methods for
conducting reliability analyses. The Failure Modes and Effects Criticality
Analysis (FMECA) is really an extension of the FMEA, focusing on the
quantitative parameters for a criticality assigned to each probable failure
mode, and is discussed below. A widely accepted military standard for
conducting FMEAs is Mil-Std-1629. This military
standard details the specifics in conducting a FMEA.
Like any analytical tool, if used and
implemented correctly the FMEA is a powerful design engineering
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aid, and is used in the aerospace, military,
automotive and space sectors. These industries have there own variance on how
to and why conduct a FMEA, however their intent is the same. For instance
NASA focuses on the qualitative aspect of failure modes and their effect on a
system, rather than a quantitative approach, which would not be the case in conducting
a FMECA as opposed solely to a FMEA. Supportting
the NASA FMEA process is a Critical Items List (CIL). This list contains all
the failure modes that would have a catastrophic effects
on a system or mission.
Failure Modes and Effects Criticality Analysis
(FMECA)
The Failure Modes and Effect (Criticality) Analysis is termed as a bottoms
up analysis. The FMEA is based on an qualitative approach, whilst the FMECA
takes a Quantitative approach and is an extension of the FMEA, assign a criticality
and probability of occurrence for each given failure mode.
To identify potential design weaknesses through systematic analysis of the
probable ways (Failure Mode) that a component or equipment could fail. This
would include the identification of the cause of the failure and its effect
on the operational capabilities (functions) of an end item, be it an
equipment or system. Each mission phase of the equipment or system would
normally be taken into consideration.
The FMEA/ FMECA is generally viewed as an
analysis, which should be implemented during the design phase, to have
maximum influence and impact on the final design. The FMEA/ FMECA serves to
input and support other engineering design activities for example:
Safety
Engineering: The FMECA
would support the Safety Engineering efforts in analysis such as the Fault
Tree Analysis. The failure modes with their assigned criticality would be
seen as basic events.
Testability Engineering: In
the development of the FMECA, a column is reserved to annotate the method of
failure mode detection/ isolation. This information can be used to support a
fault diagnostics procedure or validate the effectiveness of an equipment built in test capability. Additionally,
associated with safety, critical failure modes maybe identified that would
otherwise go undetected , presenting themselves as
potential hazards.
Maintainability
Engineering: As part of the maintainability analysis, critical to
its undertaking, is the importance that detection and isolation is accurately
reflected in the overall Mean Time To Repair calculations.
Logistics Engineering: For
each failure mode occurrence a resulting corrective maintenance task would be
implemented. Of equal importance, in the development of Preventative
Maintenance tasks through a Reliability Centre Maintenance approach the FMEA/
FMECA plays a significant supporting role. Therefore the occurrence of
failure modes, which are caused by wearout characteristics would be
identified and used to supplement the RCM effort.
Availability Engineering:
If a complex system architecture is developed, such
as a high availability system employing the use of redundant elements, the
FMECA is paramount in ensuring that there are no failure modes in the
architecture that would degrade the final availability. This could be most
beneficial in sensitive areas such as redundant cross over points (potential
single point failures) etc.
Design Engineering: The
FMECA would support the design engineering effort to ensure that program
design requirements are addressed. These could be in the support of
requirements such as no single points of failure etc.
The FMEA/ FMECA can be implemented as a functional and or physical
analysis. Earlier in a design process a functional analysis approach would be
taken. With better definition of the design and as more details are firmed up
then this will permit a physical analysis to be implemented. The FMECA is
most effective in providing a contribution to the final system configuration,
with respect to reliability performance characteristics, DURING the actual
design phase.
The level or detail to which the FMECA should be performed would be based
upon the purpose and objectives of the analysis. This may mean that certain
elements in a system architecture are analyzed to no
lower than a higher functional level, or in the case of safety critical
elements the FMECA maybe required to be developed to include the failure
modes of peace part or discrete components.
Example higher functional
level failure modes: A power
distribution network, consisting of redundant elements. The purpose of the
FMECA maybe to ensure that there are no single point failures in the network
architecture which would otherwise have a profound impact upon the system
availability
Example failure modes of peace part: Used
in a pyrotechnic firing circuit employed in safety equipment, deployed on an
off-shore oil and gas rig. (e.g. firing mechanism to
inflate a life raft). It maybe be necessary to analyse all of the components associated with this circuit
and their individual failure modes, giving full consideration to the possible
effects due to its operating environment and the dormancy. These components
could include the pyrotechnic device itself, wiring harnesses, electronic
sensor and firing mechanism.
Failure Modes
The following is a general list of the failure modes of various
components.
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Component
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Failure Mode
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Failure Cause
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Relay
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Contacts Fail Shorted
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Contacts Welded
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Contacts Fail Open
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Contacts Dirty
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Coil Fails Open
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Open Circuit Coil
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Transformer
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Coil Shorts
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Insulation Breakdown
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Coil Fails Open
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Open Circuit Coil
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Motor
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Bearings Fail
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Worn Bearing, Lubrication Problem
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Brushes Fail Open
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Dirty Worn Brushes
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Coil Fails Open
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Open Circuit Coil
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Coil Fails Short
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Insulation Breakdown
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Actuator (Hydraulic)
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Leaks
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Worn Seals
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Fails to Return
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Lines Blocked
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Switch (SP,DT)
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Contacts Fail Shorted
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Contacts Welded
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Contacts Fail Open
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Contacts Dirty
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Fails to active
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Mechanism Failure
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Cathode Ray Tube
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Performance Degradation (Illuminance)
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Chemical Coating Degradation
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Blurred Image
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Focusing Screen Misalignment
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Incorrect Colour
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Failure of Colour Gun
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Brake Mechanism
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Failures to Activate
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Mechanism, Corrosion, wear, blockage of fluid line
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Failures to Disengage
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Mechanism, Corrosion, wear, blockage of fluid line
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Power Supply
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Loss of Output
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Internal Component failure
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Unregulated Output
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Internal Rectifier, Condenser Failure
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Incorrect Voltage Level
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Internal Regulator Failure
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Excessive Noise
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EMI Filter Failure
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FMECA Column Definitions
The following provides a typical definition for a sample of columns used
in a FMECA. MIL-STD-1629. This military
specification should be referred to understand each column used and their
full definition.
SEQUENCE NUMBER
A serial number or other reference designation identification number for
each failure
ITEM NAME AND FUNCTION
This can be a type of hardware (Electronic Module, Relay, Mechanical Valve
etc.) or a function (e.g. Electrical Distribution Function)
FAILURE MODES
All probable failure modes for each item/ function under analysis.
FAILURE EFFECTS
The consequences of each assumed failure mode on item operation, function
or status are identified in the FMECA sheets. Failure effects can focus on
the specific block diagram element which is affected by the failure under
consideration. The failure under consideration may impact the several
indenture levels, from Local Effects, Next Higher Level Effects and End
Effects, as described below.
LOCAL EFFECTS
This is normally limited to the effects on the item exhibiting the
specific failure mode, for example, an Electrical relay failure mode maybe
"Coil fails open circuit", Local effect would be "Unable to
Energize Relay"
NEXT HIGHER LEVEL EFFECTS
The effects of the failure as it would been seen at the
Next Higher Level (within the system/ equipment structure) would be
noted in this column. In the example of the failure mode of the relay, had it
been unable to energize, the effect at the Next Higher Level maybe "Loss
of 28Vdc to the Motor Starter Circuit Breaker"
END EFFECTS
Evaluate and define the total effect an assumed failure has on the
operation, function, availability or status of the system (or equipment).
Once again using the relay failure mode example this loss of the Motor
Starter Capability at the system level would, cause the loss of an electric
motor. HOW CRITICAL IS THE ELECTRIC MOTOR OPERATIONS.
If the motor was be used for a Lathe in a factory, it maybe not so critical.
If the Motor was be used in the guidance (steering) function of a remote
robotic manipulating arm then it could be more critical or severe.
SEVERITY CLASSIFICATION
A severity classification category assigned to each failure mode depending
upon its effects of an equipment and/or system operation. The severity
classification are consistent between MIL-STD-1629 and MIL-STD-882, and are
listed below
· Category I - Catastrophic - A failure
which may cause death or weapon system loss (i.e. aircraft, tank, missile,
ship, etc.)
· Category II - Critical - A failure which may cause severe
injury, major property damage, or major system damage which will result in a
mission loss.
· Category III - Marginal - A failure which may cause minor
injury, minor property damage, and minor system damage which will result in a
delay or loss of availability or mission degradation.
· Category IV - Minor - A failure not serious enough to
cause injury, property damage, or system damage, but which will result in
unscheduled maintenance or repair.
It should be noted that although the definitions of each severity
classification category tend to be heavily biased to a military application, they
could also be adapted to address commercial concerns, for example:
· the loss of an commercial airliner;
· the failure of an railway signaling system, resulting in a train
collision or even; or
· the failure resulting in the severe damage to a production plant,
resulting in a prolonged plant shutdown and loss of production revenues can
all have there own profound impact
FAILURE DETECTION METHOD
A description of the methods by which occurrence of the failure mode is
detected by the operator. The failure detection means, such as visual or
audible warning devices, automatic sensing devices, sensing instrumentation
or none will be identified.
FAILURE ISOLATION
A description of the procedure that would allow the operator (and
maintainer) to isolate the malfunction or failure. The operator may be
interested to isolate to a functional level, so that he/she may implement
Operational Actions, such as shutting down the offended component within a
system. This action may include the switching to a system's redundant
element. On the other hand the maintainer would be interested in isolating
the failure to the element that would allow for a corrective maintenance
action to be implemented. This may be to the offending Line Replacement Unit
(or assembly).
REMARKS
Any pertinent remarks pertaining to and clarifying any other column in the
worksheet line shall be noted. Notes regarding recommendations for design
improvements shall be recorded and further amplified in the FMECA report
Criticality Analysis
The purpose of the Criticality Analysis is to rank each failure mode as
identified in the FMEA, according to each failure mode's severity
classification and its probability of occurrence. MIL-STD-1629 is an
excellent data source for the implementation of a Criticality Analysis. The
result of the Criticality Analysis will leads itself to the development of a
Criticality Matrix. The failure mode criticality number for each specific
failure mode (Cm) is calculated as follows:
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Where:
 e.g. 20 hours
The criticality number of each assembly (or system) is calculated per
each severity category. This criticality number is the sum of the specific
failure mode criticality numbers related to the particular severity
category:
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Where: m - number of failure modes at the particular severity category
The resulting FMECA analysis will enable a criticality matrix to be
constructed. The criticality matrix displays the distribution of all the
failure mode criticality numbers according to the severity category and
referring to the criticality scale. According to Mil-Std-1629 the scale is
divided into five levels:
Level A -
Frequent. The high probability is defined as a probability which is equal or
bigger than 0.2 of the overall system probability of failure during the
defined mission period.
Level B - Reasonable probable. The
reasonable (moderate) probability is defined as probability which is more
than 0.1 but less than 0.2 of the overall system probability of failure
during the defined mission period.
Level C - Occasional probability.
The occasional probability is defined as a probability, which is more than 0.01
but less than 0.1 of the overall system probability of failure during the
defined mission period.
Level D - Remote probability. The
remote probability is defined as a probability, which is more than 0.001 but
less than 0.01 of the overall system probability of failure during the
defined mission period.
Level E - Extremely unlikely
probability. The extremely unlikely probability is defined as probability
which is less than 0.001 of the overall system probability of failure during
the defined mission period.
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