4. RBD Analysis

Reliability Block Diagrams (RBD) modelling is a graphical representation of the reliability groupings and interdependencies between items. RBD modelling provides several group types: Series, Parallel, Complex, Standby Redundancy, K/N, and Exclusion. RBDs are represented using a Block Diagram (similar to the FBD System Model) to display reliability groupings.

(Figure 17: RBD Analysis workflow — diagram only.)

4.1 Reliability Block Diagram (RBD) System Model

A Reliability Block Diagram is a method of representing the reliability of a system based on the grouping between items. Reliability calculations in an RBD model are dependent on the groups used and reliability parameters entered for each item. All items are set to a Series grouping by default.

To access an RBD model:

1. Right-click an item in the Project Explorer viewer
2. Select RBD Model from the menu
3. Verify that a new RBD editor showing all item blocks on the selected LOI (Level of Indenture) has opened

(Figures 18-19: Opening an RBD Model; Opened RBD Editor.)

To organize the RBD model canvas, each individual model item can be moved by:

• Click-and-dragging the item to the desired location in the RBD editor canvas, or
• Selecting the System Layout icon to revert items to their default positions based on the FBD system model

Each model item contains a reliability value below its FBD icon (e.g. R(t) = 0.999999). Dotted lines indicate the functional connections from the FBD model but do not affect the RBD editor or its groupings.

4.2 RBD Groups

RBD groups assign collections of block diagrams in their respective editors. Each group has its own reliability and settings.

4.2.1 Series Groups

A Series Group implies that all blocks within it must be functional for the whole group to function, i.e. no redundancy for all items in the editor. A Series Group is represented by a blue rectangular box.

To create a Series Group:

1. Select items in the RBD editor (use click & drag or Ctrl + left-click)
2. Right-click the canvas
3. Select Group > Series Group
   • Alternatively, select the series group icon from the icon menu, or select Modelling > New RBD Group > Series Group

To remove an item from a Series Group:

• Select (left-click) and drag a component out of the blue shaded box

To rename a Series Group:

1. Select the group to rename
2. Edit the name of the group in the Properties Viewer

To add an item to a Series Group:

• Click and drag an item into the Series group box

To delete a Series Group:

1. Right-click the Series group box
2. Select Ungroup

4.2.2 Parallel Group

A Parallel Group implies that only one path needs to be functional for the whole group to function, i.e. redundancy exists. A Parallel Group is represented by a yellow rectangular box.

To create a Parallel Group:

1. Select items in the RBD editor (use click & drag or Ctrl + left-click)
2. Right-click the canvas
3. Select Group > Parallel Group
   • Alternatively, select the Parallel Group icon from the icon menu, or select Modelling > New RBD Group > Parallel Group

The same actions for removing/adding items and deleting apply as for Series groups.

4.2.3 Complex Group — Decomposition Method

A Complex Group implies that a path holding a key element needs to be functional for the group to function. In a Complex Group, there are two distinct modes: OFF (Key Element Functioning) and ON (Key Element Failed). A Complex Group is represented by a green rectangular box with a smaller inner box for group exclusions.

To create a Complex Group:

1. Select items in the RBD editor (use click & drag or Ctrl + left-click)
2. Right-click the canvas
3. Select Group > Complex Group — Decomposition
   • Alternatively, select the Complex Group — Decomposition icon from the icon menu, or select Modelling > New RBD Group > Complex Group — Decomposition

For a Complex Group, a Key Element must be selected as it changes the reliability of the group depending on its mode (OFF or ON). For example, the failure of the Engine component as a key element shall be reflected in a revised group reliability.

To select the Key Element:

1. Select an item in the Key Element dialog
2. Select OK
3. Verify that the selected element now has a key icon in the RBD editor

To access the Properties of the Complex Group:

1. Select the Complex Group
2. Edit the details in the Properties Viewer, including: Group Name, Key Element, Key Element State

When the Key Element state is set to OFF (i.e. key element has failed), the Complex group hides this element and refactors the reliability of the group.

To exclude an item from a Complex Group:

1. Click and drag an item into the Exclusion Group box
2. Verify that the item is now excluded and the Complex Group reliability is updated

The same actions for removing/adding items and deleting apply as for Series groups.

4.2.4 Complex Group — Event Space

The Event Space Method maps out all the mutually exclusive combinations of states that the system can survive in. The Complex Event Space is realized when the K-of-N group in the rightmost column evaluates as true. This occurs when at least k of the items in that group belong to paths that are fully intact — meaning every item along each such path must survive for that path to contribute toward satisfying the k-of-N condition.

To create a Complex Event Space:

1. Right-click the canvas
2. Select Group > Complex Group — Event Space
   • Alternatively, select the Complex Group — Event Space icon from the icon menu, or select Modelling > New RBD Group > Complex Group — Event Space

To specify which items are able to appear multiple times in the Event Space diagram:

1. Select Source Block Definition to expand it
2. Drag and drop the desired items into the Source Block Definition

To add columns and K/N groups to the Event Space diagram:

1. Select the "<|>" symbol to add the number of columns desired
2. Select the "[+]" symbol to add K/N groups in the desired columns

To add items to the Event Space diagram:

• Drag and drop items into the Event Space group, either from the canvas or from the Source Block

To set the K value for each of the K/N groups:

1. Select the circle on the canvas which contains the current K/N value
2. Set the K value in the Properties window

To create connections between columns in the Event Space:

1. Select the connection-tool symbol to the right of the group name
2. Hover over an item in the Event Space
3. Drag the mouse and release when it is hovering over another item
4. Repeat the previous two steps until all connections are complete
5. Select the connection-tool symbol again to exit the Connection tool

To delete an Event Space group:

1. Right-click on a blank space in the Event Space group
2. Select Ungroup

This action will delete the Event Space group and return all items inside it back to the canvas.

4.2.5 Standby Redundancy Group

A Standby Redundancy Group substitutes an item from the group for a failed component. There are two main types of Standby Redundancy:

• Hot standby: System is active and ready to substitute a failed item
• Cold standby: System is inactive and ready to activate to substitute for a failed item

To create a Standby Redundancy:

1. Select items in the RBD editor (use click & drag or Ctrl + left-click)
2. Right-click the canvas
3. Select Group > Standby Redundancy Group
   • Alternatively, select the Standby Redundancy Group icon from the icon menu, or select Modelling > New RBD Group > Standby Redundancy Group

To modify the parameters of the Standby group:

1. Select the Standby Redundancy Group
2. Select the Properties viewer
3. Edit the group name and number of Functioning items in the group

The same actions for removing/adding items and deleting apply as for Series groups.

4.2.6 K/N Redundancy Group

A K/N Redundancy Group features items in parallel where a certain number of those items need to be active to provide functionality. The number of required items for the group to function is listed by a fraction. A Redundancy Group is represented by a slate-green rectangular box.

To create a K/N Redundancy Group:

1. Select items in the RBD editor (use click & drag or Ctrl + left-click)
2. Right-click the canvas
3. Select Group > K/N Redundancy Group
   • Alternatively, select the K/N Redundancy Group icon from the icon menu, or select Modelling > New RBD Group > K/N Redundancy Group

To edit the Properties of a K/N Redundancy Group:

1. Select the K/N Redundancy group
2. Edit the group name and number of functioning items

The same actions for removing/adding items and deleting apply as for Series groups.

4.2.7 Exclusion Group

An Exclusion Group is used to list any items that are not considered in the RBD analysis.

To create an Exclusion Group:

1. Select items in the RBD editor (use click & drag or Ctrl + left-click)
2. Right-click the canvas
3. Select Group > Exclusion Group
   • Alternatively, select the Exclusion Group icon from the icon menu, or select Modelling > New RBD Group > Exclusion Group

The same actions for removing/adding items and deleting apply as for Series groups.

4.3 RBD Analysis

The RBD analysis is used to generate the reliability of the system or a higher LOI based on Reliability Groups and individual item reliability parameters. Note: The failure distribution type shown will depend on the type selected per item.

To run a Reliability/Availability analysis:

1. Select Analysis > RBD Analysis in the main menu
2. Verify that an RBD viewer appears

Table 1: RBD Viewer fields

Term	Definition
Reliability	The duration of probability of failure-free performance under stated conditions
MTTF	Mean Time to Failure (see MADE glossary)
Inherent Availability	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
Operational Availability	The reliability of the system without considering critical items
MTTR	The mean time to repair represents the average time required to repair a failed item
Replaceable	An option to determine whether an item is an LRU/SRU
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's advanced properties or automatically drawn from the Mission Profile if available
Failure Rate	Total number of failures within an item population, divided by the total number of life units expended by that population, during a measurement interval under stated conditions
Characteristic Life	For a Weibull distribution, this corresponds to a time value when 63.2% of a batch of items have failed
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

Note: The baseline metrics indicate the inherent value, and the revised values take into account the maintenance effectiveness factor.