Alternatives to MTBF

© 2004 –2010
Reliability Communication: MTBF. Is There a Better Way?
2014 Avionics Maintenance Conference
Craig Hillman, CEO

© 2004 -2007
2010
9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.comWho is DfR Solutions? The Industry Leader in Quality-Reliability- Durability of Electronics
50 Fastest Growing Companies in the Electronics Industry
-IncMagazine
2012 Global Technology Award Winner
Best Design Verification Tool
-Printed Circuit Design
Key Facts
•Founded in 2005
•30+ Employees, Multiple worldwide locations
•Software, Consulting, Research, Lab Services
Over 600 Customers
Most Major Avionic OEMs and Suppliers

© 2004 -2007
2010
9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o푀푇퐵퐹= 퐻표푢푟푠표푓푂푝푒푟푎푡푖표푛×푆푎푚푝푙푒푠 푁푢푚푏푒푟표푓퐹푎푖푙푢푟푒푠
o푀푇퐵퐹= 1 퐹푎푖푙푢푟푒푅푎푡푒휆
o푀푇퐵퐹=MTTF + Mean Time to Repair (MTTR)
What is MTBF?
3

© 2004 -2007
2010
A well-manufactured and screened product (‘box’) will have no defects
AND
A well-designed product will not experience wearoutduring its operational lifetime
Why MTBF?
4

© 2004 -2007
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BIG numbers are easier to remember than small numbers
153,000hours vs. 0.00065%/hour?
(of course, why not 5.7%/year?)
Why MTBF? (cont.)
5

© 2004 -2007
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Only One Number
Why MTBF? (cont.)
6

© 2004 -2007
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oAirworthiness Requirements are Business Critical
oSafety Assessments demonstrate compliance with Airworthiness Requirements
oReliability Prediction ‘feeds’ Safety Assessments
oFAA encourages MTBF for Reliability Prediction
Why MTBF? (cont.)
7
MTBF
MTBF
MAINTENANCE
SAFETY

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oMisunderstandings are common among non-reliability experts
oMust assume a constant failure rate
oAssumes failure must occur
oEncourages use of empirical handbooks
Why NOT MTBF?
8

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oMTBF can be used for predicting reliability at the design/concept stage
oMTBF can also be used for extrapolating reliability from existing events
Key Reminder
9

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oFailure Rate
oReliability with a Confidence Interval / B10
oFailure Free Operating Period (FFOP) / Maintenance Free Operating Period (MFOP)
oMean Cumulative Function (MCF)
oRate of Occurrence of Failure (ROCOF)
What Are the Alternatives?
10

© 2004 -2007
2010
oSimply invert MTBF
oAdvantages -More intuitive No assumptions regarding constantfailure rate
oDisadvantages -More challenging to incorporatetime to repair Can just invert MTBF (has anythingreally changed?)
Failure Rate
11
Common among part manufacturers

© 2004 -2007
2010
o99% Reliability with 95% Confidence
oAdvantages -More intuitive No assumptions regarding constantfailure rate Forces a discussion on confidence levels (moves away from empirical handbooks)
oDisadvantages -More challenging to incorporatetime to repair
Reliability with a Confidence Interval
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Common among industrial controls, auto manufacturers

© 2004 -2007
2010
oTime to 10% probability of failure
oOften thought of the beginning of wearout
oA variance of reliability with confidence level
B10
13
Common among moving parts that wearout(fans, motors, etc.)

© 2004 -2007
2010
oInitially proposed in the early 1980’s
oIncorporated into MIL-STD-781D (failure-free period life tests)
oConcept was to extrapolate concepts from mechanical parts and apply them to electronic boxes
oTwo approaches
oConstant failure rate is low enough that the probability of failure is highly unlikely (below a certain value) over a given period of time (possibly brings us right back to MTBF)
oReplacement of exponential distributions with three-parameter Weibull
Failure Free Operating Period (FFOP)
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© 2004 -2007
2010
oPotentially validconcept for some mechanisms
oMajor challenge is under- standing gamma ()
oRequires large number ofsamples
oNeed to characterize change as a function of stress
oMajor benefit is changing the default conversation from ‘will fail’ to ‘will not fail’
Three Parameter Weibull
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oProposed by the UK Ministry of Defense in mid 90’s
oDefined as period of time, typically starting from initial use, where the equipment is to perform its function without any maintenance (unscheduled)
oConcept is driven by the manufacturer taking some responsibility for maintenance (similar to performance- based logistics)
oCalculating MFOP requires an estimate of survivability of the system during the maintenance-free period (MFOPS)
Maintenance Free Operating Period (MFOP)
16

© 2004 -2007
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oDifferent organizations have taken different approaches to MFOP
oSome have applied FFOP to MFOP
oOthers have overlaid MFOP over MTBF metrics (MFOPS)
oMany have used it to justify greater fault detection and fault tolerance (beyond safety)
oBenefits
oChanges the conversation, does not assume constant failure rate, better for repairable systems, provides stronger financial motivation behind any reliability prediction
MFOP (cont.)
17

© 2004 - 200107
o Designed to replace the use of MTBF/MTBUR in
extrapolating field events
Mean Cumulative Function
18
Heavlin, 2005
o MTBF assumes independent and identically
distributed lifetimes (iid)
o Recurrence data vs. life data (repairable
systems are typically not iid). The order and
duration can be critical

© 2004 - 200107
o Plot of cumulative failures vs system age (analogous to
cumulative hazard functions for non-repairable
systems)
MCF (cont.)
19
Heavlin, 2005

© 2004 -2007
2010
oOnce MCF is calculated and plotted, a number of statistical techniques are available
oCochran-Mantel-Haenszel(CMH) to identify outliers
oArchetypal analysis to separate out groups of systems, detect trends, identify outliers
oRate of Occurrence of Failure (ROCOF)
oDerivative of MCF
MCF (cont.)
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© 2004 -2007
2010
oWhy ROCOF? Operating hours / total failures equals MTBF only works if the failure rate is constant (exponential distribution)
oMCF is the expected value of the number of failures over some time interval
oROCOF is the instantaneous rate of change in the expected number of failures
oDesigned to measure the in-service performance of repairable units
Rate of Occurrence of Failure (ROCOF)
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© 2004 -2007
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oThere needs to be a discussion to determine if average MTBF captures the true pain of failures
Where Does This Leave Us?
26
MTBF

© 2004 -2007
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oThere are a number of advantages in moving away from the use of MTXX to predict and track reliability
oUse of other methodologies will improve maintenance prediction and performance
oHowever, in a regulated industry, change is difficult without the express backing of the regulator
oLook at DoDand MIL-HDBK-217!
Conclusion
27

Alternatives to MTBF

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