Reliability & Availability Calculation Methods

Sharif University Of TechnologyAerospace Engineering Department

Aircraft performance courseby Dr. M.B Malaek

Presented by Javad Ghasempoor SISStd. No: 89105241

MTBF Introduction

MTBF as a reliability term based on :

a) Assumptionsb) definition of failurec) attention to these details

What is a Failure?

1) The termination of the ability of the product as a whole to perform its required function

2) The termination of the ability of any individual component to perform its required function but not the termination of the ability of the product as a whole to perform.

Infinite definitions of failure depending on the type of product andmanufacturers

Engineers are never wrong; they just make bad assumptions !

MTBF definition

MTTR definition Mean Time to Repair (or Recover) :

expected time to recover a system from a failure

include the time it takes to : diagnose the problem get a repair technician onsite physically repair the system

Reliability - Availability DefinitionReliability is the ability of a system or component

to perform its required functions under stated conditions for a specified period of time

Availability, on the other hand, is the degree to which a system or component is operational and accessible when required for use

An example

- 500,000 25-year-old humans- Throughout the year, 625 people failed (died)- operational life of the population is 500,000 x 1 year =

500,000 people years- The failure rate is 625 failures / 500,000 people years =

0.125% / year- The MTBF is the inverse of failure rate or 1 / 0.00125 =

800 years

extremely high reliability(MTBF) but a low expected service life

Rate of failure Vs Time

- failures at ‘useful life period’ could include undetectable defects low design safety factors higher random stress than expected human factors natural failures

prevent the decay curve shown in the "wear outperiod”

Ample burn-in periods for components by the manufacturers

proper maintenance proactive replacement of worn parts

Methods of Predicting and Estimating MTBF

Predict : calculate a value based only on a system design

Estimate : calculate a value based on an observed sample

of similar systems

Reliability Prediction Methods

1. MIL-HDBK 217 handbookcontains failure rate models for the various part types used in electronic systems

Parts Count Prediction Parts Stress Analysis Prediction

2. Telcordia

3. HRD54. RBD (Reliability Block Diagram)5. Markov Model6. FMEA / FMECA7. Fault Tree8. HALT

Reliability Estimation Methods

Similar Item Prediction MethodField Data Measurement Method

RelCalc for Windows software

automate the MTBF reliability prediction calculation procedure of Telcordia SR-332, or MIL-HDBK-217

Benefits • Save time: reduce prediction times by 60-95% over manual

methods• Save money • Integrate reliability analysis for better quality products

an alternative to tedious, time consuming, and error prone manual methods

RelCalc graphs

Sample report

Applications

Military and commercial product applicationsProposal, feasibility studiesDesign phase trade-offsFind likely problem spotsFinal design predictionsCost reduction analysisWarranty analysis Compare Design Alternatives Track Reliability Improvement

version updates include

Latest Telcordia and 217 handbook revision including many revised device failure rates some new device types new environments (GL) and revised

environment descriptions

- Windows 7 compatible

Library Over 44,000 IC, Semiconductor, and Other

Parts Hundreds of Thousands of

Resistors/Capacitors Various Package Types, Including SMT Various Device Manufacturers

- MTBF Consulting Services

Thanks for your attentation