Guidelines to Understanding Reliability Prediction -MTBF Report_24 June 2005
Alternatives to MTBF
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Transcript of Alternatives to MTBF
© 2004 – 2010
Reliability Communication:
MTBF. Is There a Better Way?
2014 Avionics Maintenance Conference
Craig Hillman, CEO
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Who is DfR Solutions?
The Industry Leader in
Quality-Reliability-
Durability
of Electronics
50 Fastest Growing
Companies in the
Electronics Industry
- Inc Magazine2012 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© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o 𝑀𝑇𝐵𝐹 =𝐻𝑜𝑢𝑟𝑠 𝑜𝑓 𝑂𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛 × 𝑆𝑎𝑚𝑝𝑙𝑒𝑠
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐹𝑎𝑖𝑙𝑢𝑟𝑒𝑠
o 𝑀𝑇𝐵𝐹 =1
𝐹𝑎𝑖𝑙𝑢𝑟𝑒 𝑅𝑎𝑡𝑒 𝜆
o 𝑀𝑇𝐵𝐹 = MTTF + Mean Time to Repair (MTTR)
What is MTBF?
3
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
A well-manufactured and screened product
(‘box’) will have no defects
AND
A well-designed product will not experience
wearout during its operational lifetime
Why MTBF?
4
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
BIG numbers are easier to remember
than small numbers
153,000 hours vs. 0.00065%/hour?
(of course, why not 5.7%/year?)
Why MTBF? (cont.)
5
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Only One Number
Why MTBF? (cont.)
6
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Airworthiness Requirements are Business Critical
o Safety Assessments demonstrate compliance with
Airworthiness Requirements
o Reliability Prediction ‘feeds’ Safety Assessments
o FAA encourages MTBF for Reliability Prediction
Why MTBF? (cont.)
7
MTBF MTBF
MAINTENANCE SAFETY
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Misunderstandings are common among non-reliability
experts
o Must assume a constant
failure rate
o Assumes failure must
occur
o Encourages use of empirical handbooks
Why NOT MTBF?
8
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o MTBF can be used for predicting reliability
at the design/concept stage
o MTBF can also be used for extrapolating
reliability from existing events
Key Reminder
9
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Failure Rate
o Reliability with a Confidence Interval / B10
o Failure Free Operating Period (FFOP) / Maintenance
Free Operating Period (MFOP)
o Mean Cumulative Function (MCF)
o Rate of Occurrence of Failure (ROCOF)
What Are the Alternatives?
10
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Simply invert MTBF
o Advantages - More intuitive
No assumptions regarding constant
failure rate
o Disadvantages - More challenging to incorporate
time to repair
Can just invert MTBF (has anything
really changed?)
Failure Rate
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Common among part manufacturers
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o 99% Reliability with 95% Confidence
o Advantages - More intuitive No assumptions regarding constantfailure rate
Forces a discussion on confidence levels (moves away from empirical handbooks)
o Disadvantages - More challenging to incorporatetime to repair
Reliability with a Confidence Interval
12
Common among industrial controls, auto manufacturers
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Time to 10% probability of failure
o Often thought of the beginning of wearout
o A variance of reliability with confidence level
B10
13
Common among moving parts that wearout (fans, motors, etc.)
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Initially proposed in the early 1980’s
o Incorporated into MIL-STD-781D (failure-free period life tests)
o Concept was to extrapolate concepts from mechanical parts and apply them to electronic boxes
o Two approaches
o Constant 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)
o Replacement of exponential distributions with three-parameter Weibull
Failure Free Operating Period (FFOP)
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© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Potentially valid
concept for some
mechanisms
o Major challenge is under-
standing gamma ()
o Requires large number of
samples
o Need to characterize change as a function of stress
o Major benefit is changing the default conversation
from ‘will fail’ to ‘will not fail’
Three Parameter Weibull
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© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Proposed by the UK Ministry of Defense in mid 90’s
o Defined as period of time, typically starting from initial use, where the equipment is to perform its function without any maintenance (unscheduled)
o Concept is driven by the manufacturer taking some responsibility for maintenance (similar to performance-based logistics)
o Calculating MFOP requires an estimate of survivability of the system during the maintenance-free period (MFOPS)
Maintenance Free Operating Period (MFOP)
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© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Different organizations have taken different approaches to MFOPo Some have applied FFOP to MFOP
o Others have overlaid MFOP over MTBF metrics (MFOPS)
o Many have used it to justify greater fault detection and fault tolerance (beyond safety)
o Benefitso Changes the conversation, does not assume constant
failure rate, better for repairable systems, provides stronger financial motivation behind any reliability prediction
MFOP (cont.)
17
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
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 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Plot of cumulative failures vs system age (analogous to
cumulative hazard functions for non-repairable
systems)
MCF (cont.)
19
Heavlin, 2005
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Once MCF is calculated and plotted, a number of
statistical techniques are available
o Cochran-Mantel-Haenszel (CMH) to identify outliers
o Archetypal analysis to separate out groups of systems,
detect trends, identify outliers
o Rate of Occurrence of Failure (ROCOF)
o Derivative of MCF
MCF (cont.)
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© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o Why ROCOF? Operating hours / total failures equals
MTBF only works if the failure rate is constant
(exponential distribution)
o MCF is the expected value of the number of failures
over some time interval
o ROCOF is the instantaneous rate of change in the
expected number of failures
o Designed to measure the in-service performance of
repairable units
Rate of Occurrence of Failure (ROCOF)
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© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
Plotting MCF
22
Hogge, 2012
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Plotting ROCOF
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Hogge, 2012
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
ROCOF vs. MTBF
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ROCOF Insight
25
© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o There 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© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com
o There are a number of advantages in moving away
from the use of MTXX to predict and track reliability
o Use of other methodologies will improve maintenance
prediction and performance
o However, in a regulated industry, change is difficult
without the express backing of the regulator
o Look at DoD and MIL-HDBK-217!
Conclusion
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Questions ?