Reliability
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Guidance on in-Service Reliability and Maintainability (R&M)
Definitive Guidance for the Acquisition Management System
Introduction
This document aims to illustrate the importance of in service R&M, to discuss reasons why in service R&M data may be collected and to provide guidance on how this may be achieved.
Definitions
Reliability can be defined qualitatively :
“The ability of an item to perform a required function under stated conditions for a specified period of time”, (Allied Reliability and Maintainability Publication — 7 (ARMP-7))
Or quantitatively:
For example Mission Reliability is defined as “The probability that an item will perform its required functions for the duration of a specified mission profile”, (ARMP-7).
Note: There is a very wide range of potential quantitative reliability metrics and care must be taken to select only those which are relevant to a particular equipment and its operating profile.
Maintainability is the probability that a given maintenance action, for an item under given conditions of use, can be carried out within a stated time interval, when the maintenance is performed under stated conditions and using stated procedures and resources, (ARMP-7).
Availability is the probability that the system or equipment used under stated conditions will be in an operable and committable state at any given time, (ARMP-7).
Note: There are a variety of sub-definitions of availability that are meaningful in different operational and management situations. Further details can be found in Def Stan 00-49/Issue 1 and ARMP-7.
The Impact of In Service R&M
The intrinsic R&M of any equipment is entirely defined by the design and the assumptions that this makes about the operating environment. Poor production quality and/or operation outside the designed environment will reduce the achieved reliability and/or maintainability below the levels intrinsic in the design.
R&M have operational, logistic and financial implications:
Operational: The reliability of equipment will determine its mission reliability (see paragraph 2.1) and will influence its availability through the number of maintenance actions that arise. The maintainability will determine the impact of each maintenance action on the availability achieved.
Logistic: The spares and manpower required to support an equipment will be determined by its R&M in the given operating environment. Poor reliability will mean that more failures, faults and other maintenance actions will arise; hence more spares will have to be provided. Poor maintainability will either reduce the availability, as the time taken to complete each maintenance action will increase, or more maintainers will have to be deployed to complete the maintenance in the required time-scale.
If, as noted in sub-paragraph b, the provision of increased levels of spares support and/or additional maintainers is required this will have a direct adverse impact on the whole life cost (WLC) of the equipment. If, as noted in sub-paragraph a, a reduced level of availability is achieved then more equipments will have to be purchase and deployed to generate a given capability. This will impact on the capital cost of the project and will also increase the cost of deploying the equipment and all of the required support.
The factors noted above are those which are driving the current trends in R&M requirements. These mainly relate to improving the mission reliability, (either through generally improved reliability or managed �Maintenance and/or Failure Free Operating Periods’) and/or improved sustainability and reduced logistic footprint and manning.
Reasons for Collecting In-Service R&M Data
Currently most one-off in-Service R&M Data Collection exercises are carried out because they are mandated by a contract. Until recently the usual method of determining whether a contractor had delivered equipment that met the R&M requirements was to conduct an In Service Reliability Demonstration (ISRD). The aim of this exercise would be to gather sufficient data from production standard equipment under in-Service conditions to demonstrate that the reliability requirements of the contract had been fulfilled. (Def Stan 00-43, Part 1 contains further information on ISRDs).
If the collection of in-Service R&M data is not contractually mandated it can be carried out for a wide variety of reasons. Excluding data gathered for other operational/ management reasons these can be grouped into three broad headings.
The simplest form of in-Service R&M data collection is the collection of simple usage and failure data to determine the current R&M levels achieved. This baseline determination can, for example, be the prelude to changing the scaling of spares, part of a Reliability Centred Maintenance (RCM) exercise or a necessary element in a sustainability assessment. The degree of granularity in the data collected (e.g. whole system, sub-system) will vary depending on the aim of the exercise.
In-Service R&M data covering usage and failures can also be used to identify opportunities for the improvement of equipment R&M. This can range from simple identification of the components that cause most failures (so that their design can be improved) to complex analysis to support the establishment of optimum Maintenance or Failure Free Operating Periods.
Finally data, other than direct usage and failure data can be collected for a variety of reasons. For example, where modelling of the physics of failure of the equipment has been carried out detailed data on the operational environment and failures can be collected either to validate the modelling or to allow life consumption monitoring.
Planning and Controlling R&M Data Collection and Analysis
If meaningful and robust measurements of in-Service R&M are to be obtained it is