Reliability Block Diagram (RBD) & Availability Block Diagram (ABD) Analyses

The RBD & ABD analysis is used to determine the reliability, mean time to

failure (MTTF), inherent availability and operational availability of model items

(including the system) as well as RBD/ABD groups. This analysis is conducted in the

RAM module and requires the user to enter pre-requisite information i.e. duration,

reliability and RBD information (See Failure Distribution Types & General

Reliability sections below).

[Image: rbdanalysis.png]

To generate a RBD analysis:

1. Select the RAM module
2. Enter all pre-requisite reliability information
3. Select "Analyses → RBD Analysis" from the main menu
4. Review results in the ABD/RBD viewer

[Image: rbdanalysis.gif]

RBD Analysis Table Columns

• Item - Displays the item or RBD group in a list or tree view.
• Reliability - The calculated reliability value for an item or RBD group.
• MTTF - The calculated mean time to failure of an item for a given operation duration.
• Inherent Availability - The calculated availability of an item based on the probability that, when used under stated conditions in an ideal support environment without consideration for preventive action, a system will operate satisfactorily at any time i.e. availability based solely on reactive maitnenance.
• Operational Availability - The period of time a system maintains its operational capability based on probabilistic values derived from system reliability. Availability calculations are based on parameters including delay time, turnaround time and spares on hand.
• MTTR - Mean Time To Repair refers to the average time taken to repair an item.
• Duration of Operation - A time duration in which an item (subsystem, component, part) will be in active operation. In MADE, this value is taken from the item advanced properties or automatically drawn from the Mission Profile if available.
• Replaceable - Indication whether the item is a Shop Replaceable Unit (SRU) or Line Replaceable Unit (LRU).
• Part Failure Rate - The failure rate of an item per million hours
• Characteristic Life - The scale parameter used in Weibull reliability.
• Slope - The shape parameter used in Weibull reliability.

Additional information regarding how the values of each reliability or availability

column is calculated is detailed in [

RBD Analysis Table](..//12-ReliabilityAvailability/12-02-ReliabilityTable.html) page.

Exponential Failure Distribution

The exponential failure distribution is a lifetime distribution used for modelling

the failure characteristics of items with a constant (or random) failure rate.

Electronic components are typically modelled with an exponential failure distribution

as they experience little to no predictable physical wear.

The reliability of an item using an exponential failure distribution is as follows:

[Image: reliability_series.png]

Where: λ = Part Failure Rate(per million hours) & t = Duration of

Operation (hours)

The sections below describe the data fields required for the Exponential failure

distribution type. These parameters are accessed from the RAM module, either from the

Properties viewer when selecting an item or from the Criticality / Reliability editor.

Mean Time to Failure (MTTF)

MTTF is the Equivalent to Mean Time Between Failure (MTBF) for non-repairable items.

In the context of an exponential distribution MTTF is the inverse of part failure rate.

Expressed mathematically:

• Exponential MTTF = [(1×10^6^) / (λ)] hours, where λ = part failure rate
• Weibull MTTF = η × Γ [(1/β) +1] hours, where η = characteristic life, Γ = location parameter, β = shape parameter/slope

Changing the MTTF value of an item in MADE will immediately affect the part failure

rate value as they share an inversely proportional relationship.

Part Failure Rate

Part failure rate refers to the total number of failures within an item population,

divided by the total number of life units expended by that population, during a

particular measurement interval under stated conditions. It is a statistical measure of

the reliability of a system element, expressed as failures per million hours. The

failure rate is assumed to be constant over a given operational scenario.

To modify the Part Failure Rate of an item using the Properties viewer:

1. Select an item in the System Model
2. Select the Properties viewer
3. Select the Exponential tab
4. Toggle the slider or numerical entry box to set the MTTF or part failure rate
5. Save changes

Changing the Part Failure Rate of an item will immediately affect the MTTF value as

they share an inversely proportional relationship.

[Image: reliability3.png]

Weibull Failure Distribution

The Weibull failure distribution is a flexible lifetime distribution used for

modelling the failure characteristics of items prone to age induced wear. A Weibull

distribution approximates three distinct portions of a typical bathtub curve depending

on the parameters entered into the Weibull equation. MADE uses a 2-parameter Weibull

curve, with the following profile:

1. Early failures with a decreasing failure rate
2. Mid-life with a constant failure rate
3. Wear out with an increasing failure rate

The reliability of an item using an Weibull failure distribution is as follows:

[Image: reliability_weibull.png]

Where: η = Characteristic Life (hours) & β = Slope

The sections below describe the data fields required for the Weibull failure

distribution type. These parameters are accessed from the RAM module, either from the

Properties viewer when selecting an item or from the Criticality / Reliability editor.

Slope

The slope of the line in the probability plot. It determines which family of Weibull

distributions provide the best fit for the reliability data provided. The table below

lists the three distinct classes of failure present:

Beta (β) value	Information Displayed
0.01 – 0.99	Burn-in period (Decreasing Failure Rate)
1.00	Normal Life (Flat Failure Rate)
1.01 – 6.0	Wear-out period (Increasing Failure Rate)

The slope of an item can be entered with the steps below:

1. Select an item in the System Model
2. Select 'Weibull' tab in the Properties Viewer
3. Toggle the slider bar or numerical entry box to set the value of the slope. (Value between 0.01 and 6.0)

Characteristic Life

For a Weibull distribution, this corresponds to a time value when 63.2% of a batch

of items have failed. This value is used in conjunction with duration to change the

slope of the Weibull curve i.e. a higher characteristic life lowers the probability of

failure for a given duration of operation. An increase in the characteristic life of an

item will result in an increase of the effective MTTF of that item. Characteristic life

for Weibull is most comparable to the MTTF parameter in an exponential distribution.

To modify Weibull parameters of an item using the Properties viewer:

1. Select an item in the System Model
2. Select the Properties viewer
3. Select the Weibull tab
4. Toggle the slider or numerical entry box to set the Slope and Characteristic Life for the Weibull failure distribution
5. Save changes

[Image: reliability2.png]

General Reliability

The general reliability page includes additional fields that contribute to

reliability. These fields include duration of operation, MTTR, delay time, turn around

time, spares on hand and failure distribution type.

[Image: reliability.png]

Note: The duration of operation field is automatically updated when a mission

profile or mission group is assigned to the system model.