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TRANSCRIPT
Imagination at work
Reliability: Myths & Realities
Melanie Cox May 5th 2015
Melanie Cox Principal Reliability Engineer/ Design for Reliability Leader
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Business & General Aviation portfolio
3
Turboprops Light jets Mid-size jets Large cabin
M601 H80 family
CFE738 CF34
Passport HF120
P r o d u c t s
S e r v i c e a n d S u p p o r t
A proactive, personal
relationship that’s
simple and easy to do
business with.
Highly reliable
engines, with service
offerings that give you
peace of mind.
Customer
Experience
Lifecycle Value
A comprehensive
support network that
keeps you flying.
Rapid Response
What is Reliability?
The probability that equipment will not suffer a failure
over a given length of time and with a defined set of
usage conditions.
Reliability
Environment
Time
(life)
Probability
Logistical
Mission (Dispatch)
• Probability of being failure free in a given set of conditions
• Probability of performing a function over a specified mission
Two sub-sets of this definition:
What is Reliability?
Customer Requirements
Maintenance Resources
Regulatory Requirements
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Engine Reliability vs Time
1973 1978 1983 1988 1993 1998 2003 2008 2013
Ave. Interval between engine shut-downs
% of North Atlantic flights on 2 engine aircraft 0% in 1983
30% in 1989
56% in 1994
77% today 67% in 1997
What is included in Logistical Reliability? • All failures (not just mission critical)
• All time units (not just mission time)
• Any failure that requires maintenance action
Reliability Theory vs Field Reliability
Reliability theory based on “part count” concept
Less Reliable
More Reliable
Reliability Theory vs Field Reliability
4 parts where the
environment is understood
and the design / analysis
process for designing that
part is mature
1 part that
was not
designed for
the correct
temperature
In-service unreliability typically driven by a small number of
items that are not able to fully withstand the field usage
conditions for the required time.
Less
Reliable
More Reliable
Reliability Prediction vs. Field Data
0.00%
20.00%
40.00%
60.00%
80.00%
0.0%
5.0%
10.0%
15.0%
20.0%
25.0%
30.0%
35.0% In theory, reliability is a
function of the number of
parts in the system. Each
component contributes to
failure rate in proportion to
its complexity / number of
parts
In service, unreliability is
dominated by a single
component. Predicted
contribution < 1%, actual
contribution >70%.
Reliability Theory vs Field Reliability
• Field reliability performance is driven by our ability to:
–Understand the operating environment and duty cycle before
the product enters service and
– Design and manufacture the correct product for those
conditions
Which system is more reliable?
Compone
nt A
Two components –
both must function for
the system to function
Compone
nt B
Compone
nt A
Compone
nt B
Compone
nt A
Compone
nt B
Four components, one
“component A” and one
“component B” must
function for the system
to function
Less permutations for system success!
How is Reliability Measured?
MTBF (Mean Time/Cycles Between Failures): how often a repairable item fails defined by the average
interval between failures.
MTBF: A motor is repaired and returned to service six times
during its life and provides 45,000 hours of service. What is the
motor’s MTBF?
hours7,5006
45,000
failuresof#
timeoperatingTotalMTBF
Example:
How is Reliability Measured?
Failure rate (): 1/ MTBF - the expected number of
failures per a measure of the length of operation (time or
cycles).
hours
failuresRateFailure
7*24*1000
2 hourfailures /
000,168
2
= 1.19E-5 failures/hr
1000 power meters were used constantly for a week. 2
meters fail. What is the Failure Rate?
Failure Rate (for constant failure rate) Example:
Example:
When MTBUR MTBF
MTBUR = Mean Time Between Unscheduled Removals
No Fault Found
Removals
Convenience Removals
Indication vs Real Fault
MTBUR < MTBF
How is Reliability Measured?
Reliability: probability of
being failure free over a given
time. For a constant failure
rate: R = e-t
Probability of failure:
how likely an item is to
fail over a given time
F(t) =1-R.
Reliability Example: If failure rate for the power meters is 1.19e-5
failures / hour, what is the reliability of a single meter over one week?
R = e-t
R = e-(1.19e-5 x 24 x 7) e-1.99e-3 = 0.998 or 99.8%
Probability of Failure Example: What is the P(fail) for the above meter
over the same duration?
P(fail) = 1-R = 2e-3 or 0.2%
NB: For very small failure rates, P(fail) approximates to t:
1.19e-5 x 24 x 7 = 1.999e-3
Examples
Product Life Phases
MTBF and Failure Rate metrics assume that failure rate is constant
Time (Product Life)
In
sta
nta
ne
ou
s
Fa
ilu
re R
ate
At the beginning of product
life, failure rate is decreasing
as weaker components fail
(known as “infant mortality”)
and are re-designed so that
overall product reliability
improves.
Towards the
end of product
life, failure rate
increases as
components
start to
experience
“wear out”
After the initial break-in period, failure
rate reaches a constant value for what is
known as the “useful life” of the product
True or False?
Time (Product Life)
In
sta
nta
neo
us
Fa
ilu
re R
ate
Life vs. Reliability
Q: If a component has an MTBF of 20, 000 hours,
what do we know about the component’s life?
Nothing! Component life is the expected usage time
that the component should be designed for.
Life & Reliability are connected, but not the same
If the MTBF for an component is 100,000 hours...
Reliability as a Function of Mission Time t (Exponential Distribution)
36.8% probability of survival past a time of one MTBF
Mission Length
(hours)
Reliability*
1 000 99.0 %
10 000 90.5 %
50 000 60.7 %
100 000 36.8 %
Reliability vs. Life Consider a Mechanical Pencil:
Reliability
• Failure: lead breaks
• How do I “maintain” the
pencil after a failure?
• What factors would influence
the reliability of the pencil?
Life:
• What affects the life of the
pencil? How can I extend
this?
Reliability vs. Life Contrast to a Traditional Wooden pencil:
• What’s different about
maintaining this pencil?
• Can I extend the life?
What effect does this have?
where:
• MTBF = Mean Time Between Failure
• MTTR = Mean Time To Repair & Return to Service
The probability that the product is ready to serve
Mean Availability (uptime / total time) = MTBF
MTBF + MTTR
Availability
The ability of an item to be preserved or restored when
prescribed procedures and resources are used to perform
maintenance
Maintainability
Corrective Maintenance
Preventative Maintenance
Reliability Centered
Maintenance
Intelligent maintenance
Full flight data to map
degradation vs. specific
parameters
Repair a fault
Take action before a fault occurs
Statistically describe failures &
plan to prevent faults occurring
Maintainability: On Condition vs Hard Time
Effectiveness of hard times
depends on population
variation
P(fail) = 0.021 @ 4300
P(fail) = 0.262 @ 4300
10% expected to function
@ 6450
10% expected
to function @
8500
Cost Trade Analysis
Cost of failure
& repair
Cost of
maintenance:
Total cost
Co
st
Time
Optimum time for
scheduled
maintenance
Cost of Operation per Hour vs. Time
Optimum maintenance time is at the beginning of wear-out
failures
Questions?
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