Failure Mode Effects and Criticality Analysis (FMECA)/Fault ... · Web view2012/12/12 · LOG 211...
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LOG 211 Supportability Analysis
Student Guide
Log 211 Supportability Analysis
Student Guide
Failure Mode Effects and Criticality Analysis (FMECA)/Fault Tree Analysis (FTA)
Content
Slide 61. Failure Mode Effects and Criticality Analysis (FMECA)/Fault Tree Analysis (FTA)
Welcome to Lesson 6 on Failure Mode Effects and Criticality Analysis (FMECA) and Fault Tree Analysis (FTA).
Introduction
Content
Slide 62. Topic 1: Introduction
Technology Maturation & Risk Reduction
Slide 63. Life Cycle Management Framework: Where Are You? What Influence Do You Have?
Failure Mode Effects and Criticality Analysis (FMECA) and Fault Tree Analysis (FTA) are critical for effective system design that meets Reliability, Maintainability, and performance requirements. Both analyses identify system failures and causes and recommended mitigation strategies to reduce the risk of failure.
The FMECA and FTA are fundamental in validating the design. Failures, their consequences, and their mitigation are essential to influencing the design for Supportability. The maximum benefit of completing FMECA and FTA is realized when the investigation of failures is conducted during the Technology Maturation and Risk Reduction (TMRR) and Engineering and Manufacturing Development (EMD) phases of a system’s life cycle rather that after the system’s design is finalized.
Failure modes and their mitigation are validated through the following reviews:
Alternative Systems Review (ASR)
System Functional Review (SFR)
Preliminary Design Review (PDR)
Critical Design Review (CDR)
Developmental Test and Evaluation (DT&E)
Functional Configuration Audit (FCA)
Production Readiness Review (PRR)
Physical configuration Audit (PCA)
Operational Test and Evaluation (OT&E)
Where Are You?
FMECA/FTA analyses occur continuously as a system’s design matures and operational data is gathered from the field.
For competitive prototypes, the initial analysis of system failures, failure mechanisms, and criticality begins in the Technology Development Phase. The earlier these analyses are conducted, the more opportunity to eliminate or mitigate failures through design.
FMECA/FTA are then conducted again during Engineering & Manufacturing Development, as more data become available with system maturity.
Finally, FMECA/FTA are revisited, when required, during Operations & Support, when additional fault data is collected or critical incidents occur which require further investigation into root causes.
What Influence Do You Have?
The Reliability Engineers conduct FMECA and FTA. The Life Cycle Logistician (LCL) plays a prominent role in reviewing the maintenance planning recommendations and modifications that result from these analyses for effectiveness and suitability. The LCL understands each analysis and how they are interrelated, the more impact the LCL will have on achieving an effective and affordable Product Support Strategy.
This role is detailed in Lesson 9: The Maintenance Task Analysis (MTA).
Content
Slide 64. FMECA/FTA Lesson Approach
The Set Up, Analyze, and Report Findings approach, as shown on this slide, will frame the discussion on FMECA/FTA. This lesson will provide a detailed description of each of these three process steps.
FMECA Key Questions
How can the system fail?
What are the consequences of failure?
FTA Key Questions
Given a single, undesirable event (usually a failure with serious or catastrophic consequences), what is the cause or combination of causes?
What is the probability of that critical event?
What design or maintenance changes will increase system Reliability and prevent the critical failure?
Content
Slide 65. Topics and Objectives
Overview of FMECA and FTA
Content
Slide 66. Topic 2: Overview of FMECA and FTA
Slide 67. What Are FMECA and FTA?
The Failure Mode and Effects Analysis (FMEA) is a Reliability evaluation and design review technique that examines the potential failure modes within a system to determine the effects of failures on equipment or system performance. Each hardware and software failure mode is classified according to its impact on system operating success and personnel safety. The FMECA’s ‘C’ is for Criticality, which assigns a criticality rating based on severity of impact and frequency. Some level of expert judgment is required to assign criticality rankings.
FMECA analysis is a “bottom up” system analysis. This approach begins looking at the effects of failure at the lowest level of the system hierarchy, and tracing upwards to determine the end effect of each failure on system performance.
Fault Tree Analysis (FTA) is a systematic methodology for defining a single undesirable event and determining all possible reasons (combination of failures) that could cause the event to occur in a “top down” analysis. The FTA focuses on a select subset of failures, specifically those that can cause a catastrophic “top event”, while the FMECA progresses sequentially through all possible system failure modes regardless of severity.
Content
Slide 68. FMECA/FTA: Process Map
FMECA and FTA promote greater understanding of the system design, from identifying design deficiencies to improving maintenance process effectiveness.
Content
Slide 69. What Are FMECA/FTA? Influencing Design
FMECA and FTA provide uniform methods for analyzing failures and their effects before finalizing the design. The goal is to improve the system to achieve Reliability and safety requirements effectively and affordably.
Specifically, FMECA and FTA evaluate the system against:
Design requirements
Design criteria
Performance requirements
FMECA/FTA Reliability, safety, and design analyses assess the validity of design enhancements to assure Reliability and critical safety issues are appropriately mitigated or eliminated.
FMECA/FTA are conducted continuously as part of the closed loop Systems Engineering process defined in Lesson 5: R&M.
Content
Slide 610. What Are FMECA/FTA? Promoting Supportability & Process Efficiency
In addition to recommending design changes to eliminate or mitigate failure modes, FMECA/FTA map failures to corrective and preventive maintenance strategies that reduce the likelihood and mitigate the impact of system failures.
FMECA/FTA provide data for:
Reliability and Maintainability Analyses (e.g., reliability block diagrams)
Reliability Centered Maintenance (RCM) Analysis
Maintenance Task Analysis (MTA)
Level of Repair Analysis (LORA)
Additional FMECA/FTA refinements
Root failure analysis (diagnostic routines for fault detection and fault isolation)
Determining useful life of a system
Developing built-in test, troubleshooting, and quality assurance methods
Developing maintenance manuals and troubleshooting guides
Content
Slide 611. What Are FMECA/FTA? Inputs and Outputs
This diagram provides a high-level view of the inputs, process, and outputs of both FMECA and FTA.
Content
Slide 612. FMECA/FTA and the ASOE Model
FMECA and FTA are the foundation of the Affordable System Operational Effectiveness (ASOE) Model, performing the following functions:
Determining what drives system failures
Assessing failure criticality/impact on system Availability and safety
Recommending remediating action
These FMECA and FTA attributes contribute to ASOE by exposing and prioritizing design flaws early to assure design optimization and mission effectiveness, while reducing Life Cycle Cost/Total Ownership Cost.
Content
Slide 613. ASOE Trade-off: Capability vs. Maintenance
FMECA and FTA serve to balance design effectiveness and process efficiency by mitigating failures early in the design process to achieve an affordable solution:
Does the design meet all requirements in the CDD? Does the design meet the KPPs?
What redesign efforts should be undertaken to mitigate failure modes that prohibit achieving technical performance and mission requirements? Note that reliance on a Maintainability-focused maintenance strategy may not mitigate failure modes.
Trade-off considerations:
The cost of redesign vs. the risk/probability of mission failure
The cost of proactive maintenance vs. the probability of system failure or safety hazard to personnel
Set Up – Preparing for FMECA and FTA
Content
Slide 614. Topic 3: Set Up – Preparing for FMECA and FTA
Slide 615. Set Up – FMECA & FTA
Set Up is similar for both FMECA and FTA: each requires up-front planning and selection of an appropriate tool to conduct the analyses. Additionally, FMECA and FTA draw from similar data inputs.
Content
Slide 616. Build a Plan: Process and Data Management
SAE GEIA-STD-0007
Planning for FMECA/FTA should include the phases of Set Up, Analysis, and Report Findings, and should consider initial and iterative analyses based on design updates and field data.
Failure Mode Effects and Criticality Analysis Planning
FMECA planning includes:
Ground rules & assumptions
FMECA approach (hardware, software, functional, combination)
Lowest indenture level for analysis. Guidelines:
Lowest level specified in LSA candidate list
Lowest level assigned Level I (Catastrophic) and Level II (Critical) severity category
Specified/intended maintenance and repair level for items assigned Level III (Marginal) or Level IV (Minor) severity
Contractor’s procedures for implementing requirements
General statements on what constitutes a failure (performance parameters and allowable limits)
Use of analysis to provide design guidance
Contractor’s procedures for updating FMECA with design changes
FMECA worksheet formats (organization and documentation of FMECA methods)
Coordination of effort (FMECA results are inputs into other analyses)
Failure rate data sources
Coding system (identification of system functions/equipment for tracking failure modes)
Fault Tree Analysis Planning
FTA uses a similar planning methodology to FMECA. However, the FTA is geared toward the most significant or catastrophic failure events. Planning should incorporate provisions of DoD RAM Guide and MIL-STD-882D Standard Practice for System Safety, with particular emphasis on Appendix A (Guidance for Implementation of a System Safety Effort).
By keeping the safety program in view, the FTA will naturally link to the safety performance requirements, to include:
Quantitative requirements
Mishap risk requirements
Safety design requirements—interlocks, redundancy, fail safe and fire suppression
Unacceptable condition elimination
Reduction of mishap risk to acceptable level
FTA planning should include considerations for:
Functional analysis of highly complex systems
Observation of combined effects on the top event
Evaluation of safety requirements and specifications
Evaluation of system Reliability, human and software interfaces
Evaluation of potential corrective actions
Simplification of maintenance and troubleshooting
Logical elimination of causes for an observed failure
Role of the Integrated Product Team (IPT)
Members of the IPT team include engineering, design, logistics, and maintenance professionals, who contribute their expertise for FMECA/FTA analysis. During Set Up, the IPT:
Identifies roles: Who is doing what?
Defines analysis goal
Defines schedule/timeline
Establishes Working-level Integrated Product Team (WIPT) expectations, roles and objectives
Establishes report processes: FMECA/FTA worksheets, preliminary updates and final reports.
Coordinates SAE GEIA-STD-0007 Logistics Product Database update process
Content
SAE GEIA-STD-0007
Slide 617. Determine Data Inputs and Analysis Tools
Analysis Inputs for FMECA and FTA:
1. System configuration and design characteristics
Identify system functions down to lowest indenture identified
Identify each item/configuration and its performance requirements
Types of data:
Engineering data, studies, drawings
Technical specifications/development plans
Design reports, data
Functional block diagrams/schematics
Commercial off-the-shelf (COTS)/Government Furnished Equipment (GFE): Vendor information
COTS/GFE: Original equipment manufacturer (OEM)
Developmental Testing results
Test result reports
Engineering investigation reports
Failure investigation reports
Modeling and simulation data
Reliability inputs—Reliability Analyses
Reliability characteristics of system
Mean Time Between Failure (MTBF)
Failure characteristics: PF curve, wear out, random
Time to Failure (calculated or estimated) for non-reparable items
Failure mode occurring within service life of equipment
Reliability Block Diagrams (RBDs)
Reliability data
MIL-HDBK-217 prediction
Operational data/test data (given similar conditions/ items)
Safety and Hazard Analysis (MIL-STD-882D) (Human Systems Integration)
Troubleshooting guides/charts for existing equipment
Subject Matter Experts with knowledge of equipment and operating context
Operator
Maintainer
In-service engineering agent – The activity that performs sustaining engineering requirements
Technical representative – Called a ‘Tech Rep,’ Normally a master level technician from the OEM or In service engineering organization that troubleshoots complex faults and updates troubleshooting procedures for the entire agency.
Program Manager
COTS/GFE Only: Maintenance history
Existing/previous maintenance plans/tasks
Existing/previous maintainer/operator manuals
In-service performance data
Age exploration data
Item repair histories
Failure reporting/corrective action system reports
Computerized Maintenance Management System (CMMS) data
Previous FMECA, FTA, RCM analyses
Failure Reporting, Analysis, and Corrective Action System (FRACAS)
FRACAS is system of reporting and analyzing failures, recommending corrective action
Developed from Test & Evaluation (T&E) events and field failure/repairs
Common data captured in FRACAS include field MTTR, MTBF, Reliability growth, failure analysis (incident, type, location, root cause, etc.)
Production inspection records after the system is fielded
FMEA/FMECA/FTA Tool Sets
Spreadsheet template (FMEA/FMECA)
LSAR (SAE GEIA-STD-0007 compliant tools): SLICwave, powerLOG-J, EAGLE, Omega (FMEA/FMECA)
Data management and reporting
Item analysis and failure criticality calculation
Windchill Quality Solutions—(FMEA/FMECA/FTA)
Data management and reporting
FMECA functionality to identify failures and plan for mitigation
RCM++ (FMECA/FMECA/RCM)
Data management and reporting for RCM Analysis
Full-featured FMEA/FMECA functionality
Maintenance task selection
Optimal interval calculation for preventive repairs/replacement
Cost comparison
Supports industry standards for RCM (e.g., ATA, MSG-3, SAE JA1011 and SAE JA1012)
MPC: Maintenance Program Creation Software (FMEA/FMECA/RCM)
MSG-3-compliant maintenance creator tool for aircraft/aerospace industry
Analyses included for significant items, functions, failure modes, effects, causes, and tasks
Analysis – FMECA
Content
Slide 618. Topic 4: Analysis – FMECA
Content
Slide 619. Analysis – FMECA
FMECA primarily examines hardware failures, both critical and non-critical. Analysis candidates include components (parts), systems/subsystems, processes, and functions.
A person knowledgeable of the application and operation of the system, such as a design or Reliability Engineer, typically conducts the analysis, because experience-based judgment is required to assign effectively the criticality factors.
Content
Slide 620. FMECA Analysis: Process Map
FMECA consists of two analyses:
Failure Mode Effects Analysis (FMEA)
Analytical Process
Functions: Defines the intended purpose of the system under analysis
Functional Failure: Defines what constitutes a failure of the system to perform its function
Failure Modes: Identifies potential ways that functional failure may occur (failure modes) and the root causes for the failure modes (failure mechanisms)
Effect: Assesses impact (effects) of each failure mode on equipment and entire system performance (higher-level systems)
Analysis begins at lowest level of indenture, then works up to successively higher system levels
Examines single-point failures (versus impact of multiple/simultaneous/combined failures)
Criticality Analysis (CA)
Analyzes severity of effects of the failure mode
Analyzes probability of occurrence of the failure mode
Ranks failure modes by severity and probability
FMECA may approach analysis in two ways:
Hardware analysis: The FMECA evaluates individual hardware items and their failure modes.
Functional analysis: In this approach, the function and outputs of each item are evaluated. Often, this approach is used when individual hardware items cannot be uniquely identified.
Note: Complex systems may use both hardware and functional analyses.
Content
Slide 621. Define System to Analyze: FMECA
In order to conduct FMECA, clearly and thoroughly define the system under analysis, including:
Mission functions (tasks and outputs) and operational mode
Environment, mission, times, equipment utilization, functions and outputs of each item
System restraints
Internal and interface functions for each item
Lowest indenture level to be analyzed
Performance requirements down to lowest indenture level to be analyzed
Failure definitions (in general vs. specific failures)
System definition also includes constructing functional block diagrams, which illustrate the operation, interrelationships, and interdependencies between functions of a system. In short, they illustrate the functional flow of a system, which is then used to determine failure impact on the various levels of indenture. Diagrams may be functional or reliability block diagrams.
Content
Slide 622. Define Functions: What Should the System Do?
The first step in FMEA portion of FMECA is to define the functions of the system or component under review.
What is the desired capability of the system (task)?
How well must the system perform, based on user needs (upper and lower limits)?
Under what circumstances must the system perform?
When describing functions, identify primary and secondary functions:
Primary function: Main reason the item exists
Secondary function: Additional functions the item is required to perform, such as:
Warning or status indicators
Safety functions
Fluid containment
Comfort and aesthetics
Environmental protections
Controlling features
“Do not combine” functions
When describing functions:
Define operating context/scenarios
Use clear, concise language
Use verb, direct object, and specific limits
Description of functions are found in:
Performance specifications
Operating and Maintenance manuals
Engineering Drawings and Lists
Reliability Block diagrams
Content
Slide 623. Define Functional Failures: How Does the System Fail to Perform?
Functional failure is performance that falls outside specified parameters. This failure may be total or partial.
When describing functional failures:
Restate defined function
Define all possible functional failures for each system function
Give upper and lower limits of failure, if different from functional criteria
Include compensating provisions for failure, which are used to determine failure effects, severity, and consequences:
Redundant systems
Safety devices
Operator actions to mitigate failure
Content
Slide 624. Define Failure Modes & Causes: Why Does the Failure Occur?
Failure modes are all the causes for a functional failure that may occur. Failure mechanisms identify all possible root causes for each failure mode.
Failure Modes (Failure Conditions)
Typical failure conditions, or modes, include:
Failure to operate at required time
Failure to stop operating
Operating before or after required time
Inconsistent operation
Degraded capability
Keep the following in mind when identifying failure modes:
Be descriptive and specific (e.g., failure, part, location, event, timing, mission/operational phase, etc.)
List failure modes separately when they vary by effects, rates, detection methods, possible failure management strategies
When combining similar failure modes, design preventive maintenance around the most severe consequence and combined rates
Failure Mechanisms (Root Causes of Failure Modes)
List all possible causes of failure mode:
Why does the component fail to operate at required time?
What causes the component to stop operating?
What causes the component to operate before the required time, or after the required time?
Why is operation inconsistent?
What may cause degraded capability?
Note: Diagram displayed on slide is an Ishikawa, or fishbone, diagram. Its purpose is to show causes of a specific event.
Content
Slide 625. Analyze Failure Effects: What Are Impacts on the System?
Failure effects describe the impact of the effects of a failure mode on the functional capability of the system under analysis. In other words, what happens when a component or system fails to function and how serious are those consequences?
The impact of primary failures, and their secondary effects, are assessed at three levels of indenture:
Local
Effect of failure mode on the item under analysis
This item is the focus of compensating provisions and other corrective and preventive maintenance actions
Next Higher
Effect on next higher level of indenture
Effect on system/subsystem
End Item
Effect on the system/asset, or the ‘”System of Systems”
Keep in mind the following when describing failure effects:
Include description of effect severity
Include detail to accurately assess the consequences of the failure
Describe effects on personnel safety, environment, mission, assets, economics
Describe operating context (e.g., mission usage/profiles)
List different effects based on usage scenarios
Describe operator/maintainer methods to detect failure occurrence, including means (e.g., visual/audible warnings, sensors, Built-In-Test)
Describe operator/maintainer actions to restore function (assuming no existing preventive maintenance tasks)
Describe existing compensating provisions, if applicable
Content
Slide 626. Failure Impact: Strike Talon RDB Example
This slide presents indenture levels B and C of the Strike Talon UAV. Using these reliability block diagrams, what is the impact of a failure of one Card Crypto on the UAV systems?
Content
Slide 627. Determine System Effects: powerLOG-J
FMECA results are documented directly in the SAE GEIA-STD-0007 Logistics Product Database, powerLOG-J in the Strike Talon case study.
Content
Slide 628. Qualitative Criticality Analysis: How Severe Are the Failure Effects?
Criticality of a failure mode is based on the severity of the effect of that mode on the end item and the probability, or frequency, of that failure’s occurrence (Mean Time between Failure).
The purpose of criticality analysis is twofold:
Measure worst case effect of a failure or design error
Determine priority for correcting issues (design changes or corrective/preventive maintenance to mitigate critical failures)
While criticality is defined by your specific organization’s policy and contract terms, general categories of severity are:
Category I – Catastrophic
Death, destruction, significant breach of environmental regulation, damage over $1 million, downtime > 2 days
Category II – Critical
Severe personal injury, major property/system damage >$100K, inability to perform critical mission (mission loss), downtime 24 hours < 2 days
Category III – Marginal
Minor injury, minor property/system damage $1K < $100K, degraded ability to perform a critical mission, downtime 8 < 24 hours
Category IV – Minor
No personal injury, property/system damage <$1K, unscheduled maintenance/repair, downtime <8 hours
Notes:
Categorize the same failure mode differently, based on operating context/phase/scenario.
Involve Human Systems Integration Safety representative (where applicable) to assist in recognizing/classifying events having harmful consequences to people, to equipment, and to the mission.
Criticality Matrix: Severity vs. Frequency
Frequent
> 1 per 1,000 miles
Probable
> 1 per 20,000 miles
Occasional
> 1 per 50,000 miles
Remote
> 1 per 80,000 miles
Improbable
< 1 per 100,000 miles
Catastrophic
High
(red)
High
(red)
High
(red)
Medium
(yellow)
Acceptable
(green)
Critical
High
(red)
High
(red)
Medium
(yellow)
Low
(light green)
Acceptable
(green)
Marginal
Medium
(yellow)
Medium
(yellow)
Low
(light green)
Acceptable
(green)
Acceptable
(green)
Minor
Acceptable
(green)
Acceptable
(green)
Acceptable
(green)
Acceptable
(green)
Acceptable
(green)
Content
Slide 629. Quantitative Criticality Analysis: What is the Risk Priority Number?
The Risk Priority Number (RPN) is a quantitative ranking approach used in many FMECA and FTA tool sets. The RPN is useful in determining the most significant failure events that are most appropriate for further modeling in the Fault Tree Analysis.
Car Cooling System Risk Priority Number Matrix
Item / Functional Description
Potential Failure Mode
Mode %
Potential Local Effect(s)
Potential End Effect
Severity (S)
Potential Cause(s) of Failure
Occurrence (O)
Current Controls Prevention
Current Controls Prevention
Detection(D)
Risk Priority Number(S*O*D)
Car Cooling System
(Provides Fluid around Engine, Maintains Fluid Temperature within Operating Parameters)
Water Pump Degraded Operation
15.00
Reduced Coolant Fluid Flow
Engine Over Heats
9
Failed Water Pump Belt
5
Check Belts for Proper Tension
Replace Water Pump 60k Miles
8
360
Car Cooling System
Radiator Degraded Operation
15.00
Reduced Coolant Flow; Hot Coolant
Engine Over Heats
6
Clogged Radiator
5
Clean Radiator Every 5 years
Change Fluid Periodically
9
270
Car Cooling System
Fluid Temperature Loss of Control
30.00
Hot Coolant
Engine Over Heats
7
Stuck Thermostat
6
7
294
Car Cooling System
Cooling Fan Does not Spin
10.00
Hot Coolant
Engine Over Heats
7
Defective Cooling Fan
4
4
112
Car Cooling System
Leaking Radiator Fluid
30.00
Radiator Fluid Low
Engine Over Heats
8
Radiator Corrosion
6
Change Radiator Fluid Periodically
Clean Radiator Every 5 years
1
48
Content
Slide 630. Determine Criticality: powerLOG-J
Content
SAE GEIA-STD-0007
Slide 631. Analyze & Allocate Failure Modes: powerLOG-J
The Analyze and Allocate task links faults to their maintenance strategies.
A failure mode may have several different root causes, each with varying probabilities. The SAE GEIA-STD-0007 tool allocates the likelihood of each failure mechanism. As a result, a single failure mode may have different triggers, corrective actions, and preventive maintenance tasks, depending on the individual cause.
An individual maintenance task, such as remove and replace a tire, may have several failure modes that would trigger that task. These triggers may be corrective (flat tire) or preventive (replace every 50,000 miles).
Content
Slide 632. Failure Modes Map to Maintenance Tasks
Analysis - FTA
Content
Slide 633. Topic 5: Analysis – FTA
Content
Slide 634. Analysis – FTA
Unlike FMECA, which examines an entire system, FTA focuses on a specific part of the design or a single undesirable or catastrophic event in order to determine the lower level contributors.
FTA:
Is useful with complex functional paths
Is used with software, hardware, and human interface systems
Considers mission profile/operational mode/environment, which impact hardware configuration, functional paths, application stresses, and critical interfaces
Results may include design change or redundancy to mitigate or prevent failure
Content
Slide 635. FTA Analysis: Process Map
Content
Slide 636. Define Undesirable Event
The first step in an FTA is to identify the undesired or catastrophic event to undergo analysis. The undesired event is determined by:
Critical Event
Safety, such as loss of life or aircraft
Operations, such as loss of production or mission
FMECA Results
FMEA unable to identify all effects of a failure mode and, therefore, unable to determine criticality.
FMECA determines that a failure mode is serious, but further analysis is required to determine if the failure is caused by multiple failures, or to determine what combinations of lower level events lead to top event.
Maintenance
Troubleshooting is complex
Engineers with knowledge of the system, or systems analysts with engineering backgrounds, define the event. Examples are:
Design: Flight safety, munitions handling safety, safety of operating/maintenance personnel
Event: Crash of commercial airliner with no survivors
Event: Loss of spacecraft and astronauts on space exploration mission
Event: Vehicle does not start when ignition key is turned
Event: No spray when demanded from containment spray injection system in a nuclear reactor
Content
Slide 637. Define Undesirable Event: Family Car: Critical Failures
This slide presents the criticality of several failure modes of the family car, identified through FMECA.
Content
Slide 638. Construct Fault Tree
Unlike the tabular approach of FMECA, Fault Tree Analysis is graphical. FTA builds a logic diagram depicting parallel and sequential failure events (causes) and their probabilities that result in the top level event.
The top level event is the single undesired or critical event under analysis. Consider the scope of that event when building the diagram:
If the event is too broad, the tree becomes unmanageable
If the event is too narrow, the tree fails to provide managers/engineers with sufficient data to make cost-effective decisions
Describe level of risk or circumstances where event becomes intolerable
Next, identify first level, second level, and third level contributors (causes) to that top event. System analysts/system designers with full knowledge of the system complete a list of causes (faults) to study through the fault tree, numbering and sequencing the faults in order of occurrence.
Faults are the state of the system or component, and can be hardware, human, or other faults. Fault descriptions include what occurs, when, and how.
Primary fault: fails within qualified environment
Secondary fault: fails outside qualified environment
Command fault: human operation of component
Note: Only causes with a probability of 0 or higher of affecting the top event are included in the FTA. Exact probabilities are impossible (due to cost/time); therefore, computer software is often used to conduct analysis.
Logic gates and event symbols represent the relationship between events, linking branches together.
Event Symbols
Illustrate the different types of events (e.g., no fault scenarios)
Symbols include: Rectangle, circle, diamond, triangle, house, oval
Gate symbols
Illustrate the relationship between lower events that lead to the higher event in the sequence
AND Gate: Both input events must occur for event to happen
OR Gate: At least one input event must occur for event to happen
Gate inputs are the lower level fault events
Gate outputs are the higher level fault events
Source of FTA image: www.edrawsoft.com
Content
Slide 639. Constructing Fault Tree: Family Car: Engine Overheats
Content
Slide 640. FTA Analysis: Qualitative Analysis
Once the fault tree is complete, identify all possible direct and indirect hazards impacting the system and evaluate for possible system improvement.
Qualitative analysis identifies all credible, single and multiple lower level failure modes (causes) that lead to the top level event.
Analyzes multiple failures/combinations of failures
Analyzes events in parallel and in sequence
Drills down to lowest required fault levels
Describes each fault and when it occurs
Identifies Minimal Cut Sets (MCS) – The shortest paths to failure indicate where system is most vulnerable
Smallest number of basic event combinations that cause the top event
Includes only those failures which are realistic
In an MCS, all failures are needed to create top event (if one event does not occur, top event does not occur)
Ranks failures
1st: Single-point failures (one failure causes top level event)
2nd: Dual-point failures (two failures in combination cause top level event)
3rd: Three-point failures, etc. (three or more failures in combination cause top level event)
Slide 641. FTA Analysis: Quantitative Analysis
Quantitative analysis determines the probability and frequency of all combinations of lower level events that lead to the top level event, for ranking purposes.
Usually represented in terms of unreliability
Mathematical model (algorithms, MARCOV)
Calculates probability/frequency of top level event, given probability of lower level failure modes leading to the critical failure (i.e., summing probability of minimal cut sets together)
Requires knowing failure rates, down to the lowest level events that lead up to the top level event
Requires component history and lengthy analysis
Result is ranking of failure modes by contribution to top level event
Slide 642. Mitigating Fault Risk through Design
Fault Tree Analyses impact design through a risk mitigation process. By identifying the most probable and critical paths to failure, design and maintenance strategies are devised to meet Reliability requirements effectively.
AND Gate Math: Redundant Thermostat in Model 2
Where Q0(t) is the probability that the overall top event occurs at time t.
Q0(t) = Pr((F(t) G(t))
= qF(t) qG(t)
= 0.6 times 0.6
Q0(t) = 0.36
Reliability = 1 minus Q0(t)
R = 1 minus 0.36
R = 0.64 or 64%
OR Gate Math: Engine Overheats Model 2
Where Q0(t) is the probability that the overall top event occurs at time t.
Q0(t) = Pr(A(t) B(t))
= Pr(A(t) + Pr(B(t) minus Pr(A(t) Pr Pr(B(t))
= qA(t) + qB(t) minus qA(t) times qB(t)
= (0.0676 + 0.005) minus (0.0675 times 0.005)
= 0.0725 minus 0.0003375
Q0(t) = 0.0721625
Reliability = 1 minus Q0(t)
= 1 minus 0.0721625
= 0.9278375 or 92.8% rounded
Note: Changes in design, including changes to Reliability or product structure, must go back through design engineers and applicable RAM-C and RCM Supportability analyses. Updates are then made to the Logistics Product Database. These updates are coordinated through IPTs and are consolidated under the Maintenance Task Analysis to include changes to cage codes, part numbers, MTBF, replacement rates, schedules, tools, and task procedures that result from FMECA/FTA recommendations.
Content
Slide 643. Mitigating Fault Risk through Design, Continued
Content
Slide 644. Concord Disaster – Paris: Tuesday, 25 July, 2000
On Tuesday, July 25, 2000, a Concord crashed shortly after take-off from Paris. All one hundred and thirteen people on board perished.
This slide and the following one present the Fault Tree Analysis conducted during the aircraft mishap investigation to determine the chain of events leading to the catastrophic event.
Select the links:
Concorde Air Crash Investigation - Part 3 (10:00) http://www.youtube.com/watch?v=zHY2PyEwGtg&feature=fvst
Concorde Air Crash Investigation - Part 4 (10:06) http://www.youtube.com/watch?v=Zd0pN0izgF4&feature=fvwrel
Content
Slide 645. Concord Disaster: FTA Continued
Report Findings – FMECA and FTA
Content
Slide 646. Topic 6: Report Findings: FMECA and FTA
Content
Slide 647. Report Findings: FMECA & FTA
Results are summarized in a formal report and disseminated to the IPTs, per contractual requirements. These reports can be preliminary, updates or final, and are often synchronized with design reviews to determine whether the design has been improved such that it will reduce or eliminate significant or catastrophic events.
Content
Slide 648. Report & Implement Findings
Recall the FMECA/FTA process chart. During the Report Findings phase, analysis results are reviewed and approved by the IPT, and applicable data elements are entered into the Logistics Product Database for use in subsequent Supportability analyses, such as Reliability & Maintainability (R&M), previous FTAs, RCM Analysis, and Maintenance Task Analysis (MTA).
SAE GEIA-STD-0007
Content
Slide 649. FMECA Report
The results of FMEA and Criticality Analyses are presented in interim and final reports. Report contents include:
Level of analyses
Results summary of Reliability and safety critical components
System definition
Data sources and analysis techniques
Resultant analysis data
Worksheets for each failure mode:
Identification number
Function
Failure modes and causes
Mission phase and operational mode
Failure effects and their probability
Failure detection method (e.g., audible warning signs, automatic sensing devices)
Compensating provisions
Actions by operator to mitigate impact of failure
Design provisions such as redundant or back-up systems
Severity classification
Ground rules, analysis assumptions, and block diagrams
Indenture level
Ranking of failure modes by severity and probability of effects
Category I and II failures, highlighted
Recommended design changes to eliminate or mitigate consequences of failure, and a review of the effectiveness of these actions
Single point failures
Failures requiring corrective design/mitigating action
Failures not mitigated by design
Interim reports guide design maturation by highlighting:
Category I and II failure modes—ranking failures according to severity of failure on equipment operation and personal safety
Unresolved single-point failures—highlighting areas needing corrective action
Visibility of system interface features and problems
Location of performance monitoring and fault sensing test equipment or test points
Comparison of alternative designs
Content
Slide 650. FTA Report
The FTA report includes:
Executive summary
Scope of analysis (what is and is not analyzed)
System description (brief)
Description/severity bounding of top level event
Analysis boundaries (e.g., physical, operational, human, interfaces)
The analysis
Method of analysis
Software
Fault tree diagram
Data sources
Common causes
Sensitivity tests, if applicable
Cut sets
Path sets, if applicable
Trade studies, if applicable
Findings
Top level event probability
System vulnerability
Primary contributors
Possible actions to mitigate risk
Troubleshooting guidance
Conclusions and Recommendations
Risk comparisons
Additional analyses required, including methods
Content
Slide 651. Report Coordination: IPT Communication Paths
FMECA/FTA results are routed through the appropriate Integrated Product Team (IPT), which is responsible for approval of actions to resolve any issues identified. The specific IPT team accountable for addressing identified problems depends on the recommendation. For example:
Design Interface impacts are reported to:
Test & Evaluation IPT
Product Support Management IPT
Systems Engineering IPT
Maintenance Planning & Management impacts are reported to:
Product Support Management IPT
Systems Engineering IPT
Exercise
Content
Slide 652. Topic 7: Exercise
Content
Slide 653. Exercise Overview
Summary
Content
Slide 654. Topic 8: Summary
Content
Slide 655. Takeaways
Content
Slide 656. Summary
Congratulations! You have completed Lesson 6 on Failure Mode Effects and Criticality Analysis (FMECA) and Fault Tree Analysis (FTA).
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January 2013
Final v1.3
January 2013
Final v1.3
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