3.9 Reliability, Availability and Maintainability (RAM)
Objectives
Describing:
  • Reliability
  • Availability
  • Maintainability
  • Testability
Relationships between RAM elements
  • Failure Reporting And Corrective Actions Systems (FRACAS)
  • The use of Failure Modes Effects and Criticality Analysis (FMECA) data
  • Defence policy and procedures on RAM
  • RAM = Reliability, Availability and Maintainability
Reliability
The probability that an item will perform its intended function(s) for a specified interval under stated conditions.
Mission Reliability: is the ability of an item to perform its required functions for the duration of a specified Mission Profile.
Mission Profile: is the tasks the Mission System is required to complete within a set of given conditions in order to complete the defined mission.
The profile includes the criteria for mission success and details critical failures.
Reliability Metrics
Mean Time Between Failure (MTBF) is a measure of reliability for Repairable Items, it includes the mean number of life units that an item functions within its specified limits under stated conditions.
Mean Time Between Critical Failure (MTBCF) this is a measure of Mission Reliability which equals total mission time divided by the total number of critical failures during a series of stated missions.
Availability
The probability that an item is operable and committable at the start of a mission, Availability translates Reliability and Maintainability into an index of effectiveness.
Availability is time related, where:
  • TT  = Total time, calendar time period
  • OT = Operating time per given calendar time period
  • ST = Stand by time (not operating, but assumed operable)
  • TCM   = Total Corrective Maintenance Time
  • TPM   = Total Preventative Maintenance Time
  • TALDT     = Total Administrative & Logistics Delay Time
Availability Metrics
  • Operational Availability (Ao)
  • There are others such as:
  • Inherent Availability (Ai)
  • Achieved Availability (Aa)
Maintainability
The ability of an item to be retained in, or restored to a specified condition when maintenance is performed.  All actions necessary to retain an item in or restore to a specified (serviceable) condition given personnel with the necessary skills, using documented procedures and resources, at prescribed levels of maintenance and repair
Maintainability Metrics:
  • Mean Corrective Maintenance Time (MCMT) or Mean Time To Repair (MTTR)
  • The average time to repair an item at a given level of repair = MTTR = Total repair times /No of items repaired.
  • At a given repair level under stated conditions
  • Maximum Maintenance Time (MaxMT) or Maximum Time To Repair (MaxTTR):
  • The maximum corrective maintenance down-time within which a specified percentage (normally 90 or 95 percent) of all corrective maintenance actions can be accomplished
Testability
The ability to test and isolate a fault within an item, this impacts on Availability by reducing the time taken to perform maintenance
Failure Modes Effects and Criticality Analysis (FMECA)
Common logistics information used across many disciplines as an input to Materiel System Task Analysis.
FMECA helps develop a list of Corrective and Preventative maintenance actions.
In-Service
Need to capture all failure and repair data to:
  • highlight and prioritise areas of deficiency, which may necessitate Engineering Change Proposal or warranty action
  • input to other Maintenance Engineering Analysis (MEA) activities, such as Reliability Centred Maintenance (RCM)
  • update spares and Life Cycle Costing (LCC) models
 
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