RELIABILITY AND MAINTAINABILITY
AND REMM MODEL
Lewitowicz J.
Instytut Techniczny Wojsk Lotniczych, Warszawa, Poland
Abstract: The Reliability Enhancement Methodology and Modeling (REMM) can provide
alternative metrics from those traditionally used in the engineering community. This problem have outline the metrics available from such techniques and compares them with the more commonly used engineering metrics such as removal rate: MTTF, MTBF, MMFH, MTTR, MTBT, MTBCD.
1. Introduction
The main issues appear to be related to an inadequate understanding of reliability requirement objectives. Te process of negotiating and agreeing to requirements between a supplier and customer appears to be flawed in many cases by a lack of knowledge regarding the assumptions associated with the chosen metric. The REMM (Reliability Enhancement Methodology and Modeling) [1-4] can provide alternative metrics from those traditionally used in the engineering community. The methods take the metrics available from such techniques and compares them with the more commonly used engineering metrics such as removal rate of reliability.
The aim of the REMM to consolidate this work in three main areas[1]:
To evaluate and validate the REMM methodology and statistical
model;
To extend and refine the methods and the model; To expand the scope of REMM.
The problem with this approach is that the product reliability is not really considered. The parts count prediction is based on the number of components of each type and their corresponding failure rates and as is widely known this is all based on the assumption of constant failure rates. In edition there is a temptation when given an deferent rate requirement to use the prediction to show that it will be met without doing any engineering analysis.
However, failure rate predictions are necessary when the product being designed is part of safety critical system. In this case safety requirements must be met and need to be shown to be met. In the aerospace industry it is conventional to carry out an FMEA for safety purposes and use reliability predictions in the FMEA. These are then used to feed into the base events in a Fault Tree Analysis.
3. The REMM approach to the reliability case
In order to ensure that the REMM approach to the reliability case covered as many aspects as possible the approach taken by the consortium was to brainstorm out what should form part of the case. In REM include the old and a new rate of maintenance [5, 6]:
MTTF- Mean Time To Failures;
MTBM - Mean Time Between Failures; MTTR - Mean Time To Repair;
MTBR - Mean Time Between Repair;
MFHBF - Mean Flight Hours Between Failures;
MMHFH - Mean Maintenance Man-Hours per Flight Hours;
MTBUMA - Mean Time Between Unscheduled Maintenance Action; MTBCD – Mean Time between Confirmed Defect;
MDT – Mean Down Time;
ROCOF – Rate Of Occurrence of Failures.
Reliability rates used while analyzing operational/maintenance processes are usually selected because of some specific criterion; reliability criterion of flight safety, or an economic one. Each of these can be reduced to some synthetic criterion of selecting the reliability rate [5]. This criterion resolves itself into a statement that the of performing/accomplishing a task, e.g. an aeronautical task. The term effectiveness can be understood as: increase in the probability of accomplishing the task, reduction of time or cost of, e.g. carrying out
the servicing, repairs, overhauls, etc. For each of these notions, some conventional idea of effectiveness is formulated. It is optimized then (minimized or maximized) because of its nature.
Let conventional effectiveness refer to the time between failures. Then the MTBF paramwter is used to measure this quantity. Some specific design efforts, engineering undertakings in the aircraft manufacture or maintenance practice, etc. enable us to find final value MTBFEND –at the
same time, Δ (MTBF) = initial MTBFSTART – MTBFEND should keep growing.
In some other case, e.g. for an undertaking measured with the flight safety rate the Δ cannot take a negative value. The same procedure can be repeated for other kinds of rates.
Each rate is a derivative of cost of the manufacture and operation/maintenance of an engineering system. Any advantageous change of this rate (either decrease or increase in value thereof, depending on the rate itself) involves suitable cost. Fig. 1 shows an exemplary change in costs against the operation/maintenance effectiveness parameters. This example facilitates finding the optimum (minimum) value of the rate of costs.
Fig. 1. Cost aspects of risk – based of maintenance strategy
The structure of the REMM reliability case is shown in Fig. 2.
The REMM approach in the following process has been implemented the aerospace companies [1]:
A 380 electronics motor control; Rolls – Royce engine;
Westland helicopters.
4. Conclusion
The examples in the previous section illustrate how the REMM process provides richer information about the reliability of a new product during the design phase.
Effective troubleshooting training and system design can improve a product removal rate, but acknowledgement for this is rarely factored into reliability assessment. The REMM model can then demonstrate the effect of mitigating such concerns on the probability operation/maintenance and repair removals and thus can provide a relationship between faults and removals. By using REMM approach during design, development and operation/maintenance, analysis can encompass those product removal events where no failure confirmed.
Fig. 2. Structure of the REMM of reliability case
Strategy/Plan Management review
Risk management Progressive assurance Updatable Company procedures
Evidence
Conclusion
Compliance recommendations Definite of rate reliability
and maintenance
Products description Reliability life Environment/utilization Definite of failure
Previous experience Test/Analysis Reasoned argument REQUIREMENTS
References
1. Marshall J., Lumbard D., Davies J.: A comparison of reliability methods and REMM process, RAMS. Los Angeles USA, 2004.
2. Marshall J., Baldestone M., Davies J.; REMM evaluation process. RAMS. Tampa USA, 2003.
3. James I., Marshall J., Evans M.: Reliability metrics and the REMM model. RAMS. Los Angeles USA, 2004
4. Jones J. A., Marshall J., Newman B.: The reliability case in the REMM methodology. RAMS. Los Angeles USA, 2004.
5. Lewitowicz J., Kustroń K.: Podstawy eksploatacji statków powietrznych – własności i właściwości statku powietrznego. Wydawnictwo ITWL. Warszawa, 2003 (in polish) 6. Jadźwiński J., Borgoń J.: Niezawodność i bezpieczeństwo lotów. WKiŁ. Warszawa,
