alternatives to mtbf

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

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MTBF is a common metric among practitioners and users of reliability prediction, safety assurance, and maintenance planning. However, there are a number of significant flaws and limitations with this approach. This presentation goes through those limitations and uses that information to suggest alternatives that may provide much greater insight into product performance.

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Page 1: Alternatives to MTBF

© 2004 – 2010

Reliability Communication:

MTBF. Is There a Better Way?

2014 Avionics Maintenance Conference

Craig Hillman, CEO

Page 2: Alternatives to MTBF

© 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

Page 3: Alternatives to MTBF

© 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

Page 4: Alternatives to MTBF

© 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?

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Page 5: Alternatives to MTBF

© 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.)

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Page 6: Alternatives to MTBF

© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Only One Number

Why MTBF? (cont.)

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Page 7: Alternatives to MTBF

© 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.)

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MTBF MTBF

MAINTENANCE SAFETY

Page 8: Alternatives to MTBF

© 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?

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Page 9: Alternatives to MTBF

© 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

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Page 10: Alternatives to MTBF

© 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?

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Page 11: Alternatives to MTBF

© 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

Page 12: Alternatives to MTBF

© 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

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Common among industrial controls, auto manufacturers

Page 13: Alternatives to MTBF

© 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

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Common among moving parts that wearout (fans, motors, etc.)

Page 14: Alternatives to MTBF

© 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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Page 15: Alternatives to MTBF

© 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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Page 16: Alternatives to MTBF

© 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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Page 17: Alternatives to MTBF

© 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.)

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Page 18: Alternatives to MTBF

© 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

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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

Page 19: Alternatives to MTBF

© 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.)

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Heavlin, 2005

Page 20: Alternatives to MTBF

© 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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Page 21: Alternatives to MTBF

© 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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Page 22: Alternatives to MTBF

© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Plotting MCF

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Hogge, 2012

Page 23: Alternatives to MTBF

© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Plotting ROCOF

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Hogge, 2012

Page 24: Alternatives to MTBF

© 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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Page 25: Alternatives to MTBF

© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

ROCOF Insight

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Page 26: Alternatives to MTBF

© 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?

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MTBF

Page 27: Alternatives to 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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Page 28: Alternatives to MTBF

© 2004 - 2007© 2004 - 20109000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

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Questions ?