RBD Analysis

The RBD Analysis in the ABD/RBD viewer is used to display calculated reliabilities

and availabilities based on operational, reliability and RBD data entered into the

system model. The calculations for reliability, Inherent Availability and Operational

Availability columns are described in the sections below.

[Image: rbdanalysis.png]

Reliability Column

Reliability is defined as (a)the duration of probability of failure-free performance

under stated conditions or (b)the probability that an item can perform its intended

function for a specified interval under stated conditions.

Expressed mathematically: [Image: reliability_series.png]

Where R(t) is component reliability, t is duration of operation, and λ is

failure rate.

For systems and subsystems, reliability is calculated by calculating all

individual item reliability in lower LOIs, followed by applying a serial RBD group

reliability expression. This serial RBD group reliability is expressed as:

[Image: reliability_serial_group.png]

Where ∏ is the product, i is the item counter, n is the number of

items in a RBD group, λ is the exponential failure rate, and t is time.

For RBD groups, R(t) is calculated by applying the corresponding expression

for each RBD group and failure distribution type.

Inherent Availability Column

Inherent Availability is 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 maintenance.

Expressed mathematically: AI= (MTTF)/(MTTF + MTTR)

Where AI is inherent availability, MTTF is Mean Time to Failure, and MTTR

is Mean Time to Repair.

Mean Time To Repair (MTTR) is aggregated for groups of items using a weighted

aggregation so as to assure items that fail more often contribute more to the MTTR of

the system than items which fail rarely in the given duration of operation. The

relative weighting of an item will be proportional to the MTTF of the item.

The equation for MTTR calculation of a group containing n items is as follows:

[Image: mttr_eqn.png]

This MTTR calculation is undertaken at each subsequent level of grouping within an

RBD. Each subsequent level of grouping uses the lower constituent groups MTTR and MTTF

values as though it was a single item (representing all items below that grouping).

For systems and subsystems, inherent availability is calculated by calculating all

individual item AI in lower LOIs, followed by applying a serial RBD group

reliability expression. See serial RBD group reliability expression in the reliability

section above.

For RBD groups, AI is calculated by applying the corresponding expression

for each RBD group and failure distribution type to the calculated AI values

for all items in the group.

Operational Availability Column

Operational availability is 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.

Expressed mathematically: Ao = Uptime / Operating Cycle = (MTTF) / (MTTF

• MTTR + DT + [((TT/MTTF)^k^×TT)/(k+1)!])

Where MTTF is Mean Time to Failure, MTTR is Mean Time to Repair, DT is Delay Time,

TT is Turn Around Time, and k is no. of spares.

For systems and subsystems, operational availability is calculated by calculating

all individual item Ao in lower LOIs, followed by applying a serial RBD

group reliability expression to the calculated Ao values for all items in

the group. See serial RBD group reliability expression in the reliability section

above.

For RBD groups, Ao is calculated by applying the corresponding expression

for each RBD group and failure distribution type to the calculated Ao values

for all items in the group.

Additional RBD Analysis Options

The ABD/RBD viewer contains functionality in the form of icons in the top right of

the viewer used to update or extend the RBD analysis. These icons are described in the

table below:

Icon/Function	Icon Symbol	Description
Refresh	[Image: refreshicon.png]	Used to run the RBD analysis once more to reflect changes to parameters which may have been updated by the user
Back-Fit RCM	[Image: bfrcmicon.png]	Used as a shortcut to open the Back-Fit RCM editor for a selected item in the ABD/RBD viewer.
Poor Performers	[Image: poorperformericon.png]	Used to filter out items or RBD groups that do not meet a reliability threshold set by the user.
Export	[Image: exporticon.png]	Used to export the table in its current state to a CSV file format.

Drop-down Menu

[Image: dropdownmenu.png]

The drop-down menu in the ABD/RBD viewer is used to select several options:

• Reconfiguring the table to show a tree or list view of the analysis results
• Exporting the table to a CSV file
• Column preferences to select which columns to show/hide

[Image: colprefdiag.png]