1. Introduction to Maintainability
What is Maintainability?
The measure of the ability of an item to be retained in or restored to specific conditions when maintenance is performed by personnel having specified skill levels, using prescribed procedures and resources at each prescribed level of maintenance and repair.

2. The Need for Maintainability Prediction
The prediction of the expected number of hours that a system or device will be in an inoperative or “down state” while it is undergoing maintenance is of vital importance to the user because of the adverse effect that excessive downtime has on mission success.
Therefore, once the operational requirements of a system are fixed, it is imperative that a technique be utilized to predict its maintainability in quantitative terms as early as possible during the design phase. This prediction should be updated continuously as the design progresses.

3. Useful Reference Materials
MIL-HDBK-472, Notice 1, Maintainability Prediction
MIL-HDBK-470a, Designing and Developing Maintainable Products and Systems.

4. Definitions
Maintenance Action – An element of a maintenance event. One or more tasks necessary to retain an item in or restore it to a specified condition. These include Fault Localization, Fault Isolation, Disassembly, Interchange, Reassembly, Alignment, and Checkout.
Fault Localization – The process of determining the approximate location of a fault.
Fault Isolation – The process of determining the location of a fault to the extent necessary to effect repair.

5. Definitions
Disassembly – The opening of an item and removal of a number of parts or subassemblies to make the item that is to be replaced accessible for removal. This does not include the actual removal of the item to be replaced.
Interchange – Removing the item that is to be replaced and installing the replacement item.
Reassembly – Assembly of the items that were removed during disassembly and closing the reassembled items.
Alignment – Performing the adjustments that are necessary to return an item to specified operation.

6. Definitions
Checkout – Tests or observations of an item to determine its condition or status.
Mean-Time-To-Repair (MTTR) – A basic measure of maintainability. It is the sum of corrective maintenance times at any specific level of repair, divided by the total number of failures within an item repaired at that level, during a particular interval under stated conditions.

7. Definitions
Mean-Time-To-Repair (MTTR)

8. Definitions
Availability – A measure of the degree to which an item is in an operable and committable state at the start of a mission when the mission is called for an unknown (random) time.
Maintenance – All actions necessary for retaining an item or restoring it to a specified condition.
Preventive Maintenance – All actions performed in an attempt to retain an item in specified condition by providing systematic inspection, detection, and prevention of failures.

9. Definitions
Corrective Maintenance – All actions performed as a result of failure to restore an item to a specified condition; Corrective maintenance can include any or all of the following steps: Localization, Isolation, Disassembly, Interchange, Reassembly, Alignment and Checkout.

10. Sample – Maintenance Tasks
This information can be found in either MIL-HDBK-472 or MIL-HDBK-470A. This example shows the remove and replace times associated with Standard Screws and other types of fasteners.

11. Sample – Remove Tasks
Above is an example of remove tasks for an Elapsed Time Meter. The total time is 4.03 minutes to perform this set of tasks. This would be considered the Remove Time for this item.

12. Sample – Replace Tasks
Above is an example of replace tasks for an Elapsed Time Meter. The total time is 5.15 minutes to perform this set of tasks. This would be considered the Replace Time for this item. The Interchange Time is the sum of the Remove Time and the Replace Time for the item of interest.

13. Sample – Prediction
Above is an example of a simple Maintainability Prediction that includes three replaceable items

14. What’s Next? This concludes a brief introduction to Maintainability.
For more information, review the other tutorials in this section to see how the ASENT software can help you automate and manage your Maintainability Analysis.