Calculating MTTR - Mean Time To Repair - MIL-HDBK-472, Procedure V, Method A.


Michael Busha

I put this in the Statistics forum first but maybe it's better put here. Does anyone know a quick and dirty way to calculate MTTR? We have two kinds of solenoid valves, one is ISO rated, the other is not. I want to prove that the MTTR of the ISO rated valve is not significantly different from that of the non ISO rated valve. We have the capability of actually disassembling and reassembling both valves to get an accurate measure. Is there a formula I can plug the values into? Thanks for any help. The results of this analysis could save my company $40,000.
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Dan De Yarman

Have you contacted the manufacturer(s) to see what they recommend? If you are a QS-9000 TE Supplement registered company you should be requiring your suppliers to follow Section I of the TE Supplement requirements manual. That includes MTTR, MTBF, and whatever else you specify on your purchase order.

Keep in mind that MTTR and MTBF assume that the peson(s) reairing the item (the valve in your case) has both the knowledge and the tools to perfom the repair unless your Customer tells you otherwise. In terms of a "nice and easy" formula, I don't know of one. Sorry I couldn't be of anymore help.

Anyone else know of a way?



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mean time to repair (MTTR): The total corrective maintenance time divided by the total number of corrective maintenance actions during a given period of time.

corrective maintenance: 1. Maintenance actions carried out to restore a defective item to a specified condition. 2. Tests, measurements, and adjustments made to remove or correct a fault.

Also see MIL-HDBK-472, Procedure V, Method A.

Maintainability and Availability

Maintainability Standards & Handbooks

MIL-STD-470B Maintainability Program Requirements for Systems and Equipment
This document includes application requirements, tailorable maintainability program tasks, and an appendix with an application matrix and guidance and rationale for task selection. The topics covered are program surveillance and control, design and analysis, modeling, allocations, predictions, failure mode and effects analysis, and maintainability design criteria. Each task item includes a purpose, task description, and details to be specified. Software maintainability is not covered by this document.

MIL-STD-471A Maintainability Verification/Demonstration/Evaluation
This document provides procedures and test methods for verification, demonstration, and evaluation of qualitative and quantitative maintainability requirements. It also provides for qualitative assessment of various integrated logistic support factors related to and impacting the achievement of maintainability parameters and item downtime, e.g. technical manuals, personnel, tools and test equipment, maintenance concepts and provisioning.

MIL-HDBK-472 Maintainability Prediction
This document is to familiarize project managers and design engineers with maintainability prediction procedures. It provides the analytic foundation and application details of five prediction methods. Each procedure details applicability, point of application, basic parameters of measure, information required correlation, and cautionary notes. The highlights of each maintainability prediction procedure are presented in a clear and intelligible manner and include useful supplementary information applicable to specific procedures. Maintainability Prediction Procedures I and III are applicable solely to electronic systems and equipments. Procedures II and IV can be used for all systems and equipments. In applying Procedure II to non-electronic equipments the appropriate task times must be estimated. Procedure V can be used to predict maintainability parameters of avionics, ground and shipboard electronics at the organizational, intermediate and depot levels of maintenance.

DOD-HDBK-791 Maintainability Design Techniques
This handbook supplies information on incorporating maintainability into Army materiel design. It defines maintainability and discusses its importance, quantitative measurement, and incorporation into the design process. Other subjects discussed in detail cover simplification, standardization and interchangeability, accessibility, modularization, identification and labeling, testability and diagnostic techniques, preventive maintenance, human factors, and environmental factors as they relate to maintainability.

MIL-STD-1591 On Aircraft, Fault Diagnosis, Subsystems, Analysis/Synthesis of
This document establishes uniform criteria for conducting trade studies to determine the optimal design for an on-aircraft fault diagnosis/isolation system. This document is applicable where a selection can be made between such alternatives as central computer controlled on-board centrally polled built-in test equipment (BITE), decentralized BITE, detached Aerospace Ground Equipment (AGE), etc., or combinations of the preceding. The fault diagnosis/isolation systems of interest are those used to diagnose/isolate faults at the flight line (organizational) level of maintenance. This document also provides a cost model and a maintainability labor power model.

MIL-STD-1843 Reliability-Centered Maintenance for Aircraft, Engines and Equipment
This document, which is based on the Airline/Manufacturer Maintenance Program Planning Document MSG-3, outlines the procedures for developing preventive maintenance requirements through the use of Reliability-Centered Maintenance Analysis (RCMA) for Air Force aircraft and engine systems, aircraft and engine structures and equipment, including peculiar and common Support Equipment (SE) Communications and Electronics (C-E) equipment, vehicles, weapons and other similar equipment items.

MIL-STD-2084 Maintainability of Avionic & Electronic Systems and Equipment
This document covers the common maintainability design requirements to be used in military specifications for avionic and electronic systems and equipment.

MIL-STD-2165A Testability Programs for Electronic Systems & Equipment
This document is intended to prescribe a systematic approach for establishing and conducting a testability program. It describes a uniform approach to testability program planning, establishment of diagnostic concepts and testability (including BIT) requirements, testability and test design and assessment, and requirements for conducting testability program reviews. Relevant tasks in this document are to be applied during the conceptual phase, demonstration and validation phases, full-scale development phase and production phase of the acquisition process.

DOD-STD-1701 Hardware Diagnostic Test System Requirements
This document establishes the general procedures, terms and conditions governing the preparation and completion of a hardware diagnostic test system.

MIL-STD-2173 Reliability-Centered Maintenance Requirements for Naval Aircraft, Weapons Systems and Support Equipment
This document is used to provide procedures for a Reliability-Centered Maintenance analysis for naval aircraft, weapons systems, and support equipment. This document is used during development of new systems and equipment, and by analysts and auditors within the Naval Air Systems Command for determining preventive maintenance requirements and developing age exploration requirements. The document can also be used to update the initial reliability-centered maintenance analysis and analyze newly discovered failure modes.

MIL-STD-001591A Subsystem Analysis/Synthesis of Command, Control & Communication (C3) System Component Fault Diagnosis
This document establishes uniform criteria for conducting trade studies to determine the optimal design for command, control and communication system and component fault diagnosis/isolation subsystems, These types of systems are referred to as Fault Identification & Test Subsystems (FITS). FITS include the hardware and/or software necessary for the detection and isolation of failures.

Many systems are repairable; when the system fails — whether it is an automobile, a dishwasher, production equipment, etc. — it is repaired. Maintainability is a measure of the difficulty to repair the system. More specifically, maintainability is:

The measure of the ability of a system to 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.

Military Handbook 472 (MIL-HDBK-472) defines six components of maintainability, which are discussed below.

1. Elemental Activities are simple maintenance actions of short duration and relatively small variance that do not vary appreciably from one system to another. An example of an elemental activity is the opening and shutting of a door.
2. Malfunction Active Repair Time consists of:
1. Preparation time
2. Malfunction verification time
3. Fault location time
4. Part procurement time
5. Repair time
6. Final malfunction test time Items a–f above are composed of elemental activities.
3. Malfunction Repair Time consists of:
1. Malfunction active repair time
2. Malfunction administrative time
4. System Repair Time is the product of malfunction repair time and the number of malfunctions.
5. System Downtime includes:
1. System logistic time
2. System repair time
3. System final test time
6. Total System Downtime is a combination of the distributions of
1. Initial delay
2. System downtime

MIL-HDBK-472 provides a procedure for predicting maintainability based on the structure described above. The philosophy of the procedure is based on the principles of synthesis and transferability. The synthesis principle involves a buildup of downtimes, step-by-step, progressing from the distribution of downtimes of elemental activities through various stages culminating finally with the distribution of system downtime. The transferability principle embodies the concept that data applicable to one type of system can be applied to similar systems, under like conditions of use and environment, to predict system maintainability.

Other useful maintainability references are Military Standard 470, which describes a maintainability program for systems and equipment, and Military Standard 471, which provides procedures for maintainability verification, demonstration, and evaluation.

Availability is a measure of the readiness of a system. More specifically, availability is:

A measure of the degree to which a system is in an operable and comitable state at the start of a mission when the mission is called for at a random time.

There are three categories of availability.

1. Inherent Availability is the ideal state for analyzing availability. It is a function only of the mean time to fail, MTBF, and the mean time to repair, MTTR; preventive maintenance is not considered. Inherent availability is defined as Ai = MTBF/(MTBF + MTTR)

2. Achieved Availability includes preventive maintenance as well as corrective maintenance. It is a function of the mean time between maintenance actions, MTMA, and the mean maintenance time, MMT. Achieved availability is defined as Aa = MTMA/(MTMA + MMT)

3. Operational Availability includes preventive maintenance, corrective maintenance, and delay time before maintenance begins, such as waiting for parts or personnel. It is a function of the mean time between maintenance actions and the mean down time, MDT, and is defined as Ao = MTMA/(MTMA + MDT)

It is important to note that the type of availability being described is often not distinguished. Many authors simply refer to "availability," MTTR may be the equivalent of MMT or MDT, and MTBF may be the equivalent MTMA.


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