# Presentation: 555768

## Maintainability:Theory and Practice

**NASA ISHEM Forum**
**Napa, California**
**7 November 2005**

## Introduction

**Definition**
**Theory**
**Metrics**
**Practice**
**New Directions****	****	****	****	**

## Definition

**“****The relative ease and economy of time and resources with which an item can be retained in or restored to a specified condition when maintenance is performed by personnel having specified skill levels, using prescribed procedures and resources, at each prescribed level of maintenance and repair.  In this context, it is a function of design.”**

**Photo: Courtesy of DoD DefenseLink**

## Definition

**1) A characteristic of design and installation, expressed as the probability that an item will be retained or restored to a specified condition within a given period of time, when maintenance is performed in accordance with prescribed procedures and resources.**
**2) The ease with which maintenance of a functional unit can be performed in accordance with prescribed requirements.**
**(Source:  Federal Standard 1037C and MIL-STD-188)**

**Photo: Courtesy of DoD DefenseLink**

## Definition

**Reliability****:**
**The ability of a system or component to perform its required functions under stated conditions for a specified period of time.**

**Photo: Courtesy of DoD DefenseLink**

## Definitions

**Mean Time To Repair**
**The arithmetic mean of the time required for maintenance actions divided by the number of actions**

**Photo: Courtesy of DoD DefenseLink**

## Reliability and Maintainability

**The Ideal**

**The All To Often Reality**

## Theory

**Use and Development of Theory in Maintainability centers around the use of statistics and sampling theory to *****predict***** aspects of ‘ease and economy of resources’:**
**Mean Time to Repair **
**Availability**
**Mainly used as basis for estimation during system design and development**

## Metrics

| Measure | Description |
| --- | --- |
| Mean Time to Repair (MTTR) | The arithmetic average of the maintenance cycle times for the individual maintenance actions of a system (excludes preventive maintenance) |
| Mean Preventive Maintenance Time | The arithmetic average of the maintenance cycle times for the individual preventive maintenance actions of a system (inspection, calibration, planned replacement, etc) |
| Median Active Corrective Maintenance Time | The value of corrective maintenance time that divides all downtime values for corrective maintenance so that 50% are equal or greater than the median |
| Mean Active Maintenance Time | The mean or average elapsed time needed to perform maintenance (both preventive and corrective) excluding logistic and administrative delays |
| Maximum Active Corrective Maintenance Time | That value of downtime below which one can expect a specified percent of all corrective maintenance to be completed.  Must be stated at a given percentile, usually 90 or 95th, primarily related to a lognormal distribution. |
| Mean Time to Restore System | For highly redundant systems, this is the time need to switch to a redundant backup unit |
| Mean Down Time | The mean or average time that a system is not operational due to repair or preventive maintenance.  This includes logistics and administrative delays. |
| Maintenance Labor Hours per Hour/Cycle/Action/Month | A measure of labor hours expended, based on operating or calendar time, maintenance actions or operating cycles |

**Source:  The RAC Maintainabilty Toolkit, pg 5**

## Metrics

**Maintenance Information Systems generally record actions by organizations**
**Maintainability Metrics are lagging measures from operational databases**
**Often report Symptoms vs Causes**
**Accuracy is Generally Low **
**Completeness (more than 1 Discrepancy?)**
**Metrics are used to drive system changes, but improvement comes hard**
**Diffuse responsibilities and actions**
**Platforms develop age related degradation**

## New Directions

**Maintainability and Reliability are growing in importance to initial design**
**But developing basic design principles  for maintainability is still somewhat elusive**
**Adapting “Lean” and “Theory of Constraints” methods has improved execution of legacy systems**
**Owner/Operators are driving responsibility for improvement back to OEMs through innovative support methods:**
**Performance Based Logistics**
**Condition Based Maintenance**

## New Directions

**Technology will be incorporated to improve:**
**Fault Identification and Fault Tolerance**
**Technical Information and Training**
**‘****Sense and Respond’ Support Actions**