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OCR for page 450
A.4.1.9 Specifications and Recommended Practices Supporting Maintainability
Belicore and EKE have developed specifications which support ma~ntam ability. As wad built-
in test and testability, documentation of Me communications device is important to achieving a
low mean time to repair (AIDE. Table A.4.7-! summarizes some applicable industry
specifications.
A.4.2 Reliability, Availability, and Maintainability Planning and Estimating
Common reliability, availability, and maintainability terms, def~nidons, and calculations are
present in Table A.4.2~. These definitions are adaptions from references I-10.
In particular, Reference I., Chapter ~ I, is followed because it has smaighffonvard concepts very
similar to Lose discussed in References 7 and 8. It should be noted that reliability, availability,
failures, and maintainability modeling are statistically based and can be complex. Thus,
modeling is most accurate when verified wad field data of subsolar systems and then applied in
future modeling activities. Even Be highly regarded M~-HKBK 217F is crazed because, due
to continuing rapid technical advances, marry of the parameters in the document are outdated.
An important element of system design is He establishment of system availability goals and He
allocation (usually based on modeling) to venous system elements including subsystems,
equipment, components (where reqliiIed), mediums, etc.
Availability of a system is a function of the Mean-T~me-Between-Failure ~3E:) and He
Mean-T~me-To-Repair ~) of He elements of the system. The military, telecommunication,
and electronic ~ndustnes have developed methodologies for calculating integrated system
availability that uses MTBF, MTTH, and related parameters for system elements. As failure and
repair involve uncertainty, many advanced sophisticated statistical techniques are available to
model the many variations including component/equipment failure charactenstics, equipment
and system architectures, alternative redundancy schemes, etc. We win present a generally
accepted basic methodology that should provide adequate ITS system planning estimates.
Table A.4.2~2 presents He number of hours in "n" years.
L;wa~pt\ N=~3-51 · ~2F~R - n
A~14
OCR for page 451
Table A.4.2-2
Hours in "n" years
n years
hours (n x 365 x 24)
8,760
17.520
1
2
3
4
26280
35,040
43,800
52,560
5
6
7
8
619320
70,080
78,840
87.600
9
12
10
11
96,360
105~120
A typical telecommunication industry goal for availability is 99.98% (or better), which equates
to an uptime of 8758 hours per year (8760 x .999X) and a downtime (not available) of 2 hours
per year. What constitutes uptime (or available time) can be defined according to operational
requirements. For example, uptune can be defined as:
I) AN elements of the system operational;
2) All critical elements up and an acceptable % of non-cntical elements up; and
3) A % or defined combination of non-cntical items up, etc. Up conditions for availability
purposes could include pardal functionality. An rrS-related example would be Mat We
backbone communication subsystem be critical and local loop communication links and
signal controllers be non-cntical win independent subsystem goals as opposed to Integrate
system goals.
Table A.4.2-1 discusses serial and paraBe! system elements. Reference I, chapter ~ I, presents
simplified approximating equations for estimating the composite MTBF, failure rate (F), Ml~,
and repair rate (R). These equations are presented in Table A.4.2-3. The series element
equations can be referred to as summations for approximations for more than two element;
however, the paraRel element equations must be applied repetitively to two elements at a time.
Fortunately, most TIs system redundancy typically involves two parapet elements. The parade]
approximations calculate the estimates for overlapping (in dme) two element failures which is
L;`Na~Wba~p~\ NCHRP3-51 · Phase2F'nalRepon A4~15
OCR for page 452
Table A.4.2-1
Common Reliability Terms, Parameters. and Calculations
,
Term/Parameter/Calculation Definiti _ - _
.
Reliability The probability that a element for system) will
successfully perform a required function, under
required conditions, usually for a stated period
of time.
. _
Maintainability use of failure rate data to predict system MTTR
and to logistic requirements to maintain system.
Availability (AV) The average percent of time that a component,
(uptime) equipment, subsystem, system, etc is
AV = MTBF/(MTBF+MTTR) or satisfactorily performing its required function.
= Rt(F+R)
Unavailability he percent of time that a compliment equipment
(down time, or DT) subsystem, system, etc is not satisfactorily
DT = 1 - AV performing its required function
failure rate, F The average number of failures per hour of a
component, equipment, subsystem, system,
etc.
.
Mean-Time-Between-Failures (MTBF) The average time between failures, usually
MTBF = 1/failure rate = 1/ F for a expressed in hours of a component, equipment,
a component, equipment, subsystem, subsystem, system, etc.
system, etc.
Repair rate, R The average number of repairs per hour of a
component, equipment, subsystem, system, etc.
.
Mean-Tim+To-Repair (MTTR) The average time to return to service, usually
(also down time of element) expressed in hours, for a component,
MTTR = 1/R for a equipment, subsystem, system, etc. When
a component, equipment, subsystem, replaced by a spare, the failed element may still
system, etc. need repair but usually does not affect
availability. Element VlllR is evaluated as part
of spares inventory replacement and not system
~MTTR.
Serial elements A configuration of component, equipment,
subsystem, system, etc such that ~ one element
Parallel elements
subsystem, system, etc such that all parallel
elements must fail for the system to be
considered failed. This is essentially
redundancy.
Table A.4.2~3
Series Elements ~
F (failures/hour) Fs = F1 + F2 Fp = F. x F2 x (I /ITTR1 +MTTR2)
I\/ITBF (hours/failure) MTBFs= 1/FS M~Fp = 1/Fp
R (repairs/hr) Rs = (F1 x R1 + F2X R2) /Fs R. = R. · Rat
.
MTTR (hours/repair) MTTRS = 1/ Rs MTTRp = 1/Rp
Probability of Unavailable Fs x MTTRS Fp x MTTRp
L;`NcH~wha~n NCHRP3-51 · Phase2~malReport Al16
Representative terms from entire chapter:
failure rate
