Systems Maintainability Concepts and Metrics

1. Systems Engineering Program Department of Engineering Management, Information and Systems EMIS 7305/5305 Systems Reliability, Supportability and Availability Analysis Systems Maintainability Concepts and Metrics Dr. Jerrell T. Stracener, SAE Fellow Leadership in Engineering

2. Maintainability References • Systems Engineering and Analysis Benjamin S. Blanchard and Walter J. Fabrycky Prentice Hall, 3rd edition 1998, ISBN 0-13-135047-1 • Maintainability Principles and Practices Benjamin S. Blanchard and E. Edward Lowery McGraw-Hill • MIL-STD-470 Maintainability Programs for Systems and Equipment • MIL-STD-471 Maintainability Verification / Demonstration / Evaluation • MIL-HDBK-472 Maintainability Prediction

3. Objective of maintainability • Design and develop systems/equipment which can be maintained • in the least time, at the least cost, and with a minimum expenditure of support resources, without adversely affecting the item performance or safety characteristics • Maintainability greatly influences reliability and availability of a system or subsystem. • Maintainability must be addressed early in the design stage to • prevent or reduce failure or down times of the system.

4. Maintenance and Design • The system’s design determines its requirements for maintenance • Reliability (How often maintenance) • Configuration (How much time for access) • Built in Test (Fault Isolation Time) • Subassembly life span (Inspection/forced replacement) • Adjustment/alignment requirements (Inspection) • Capacity/fill rate (Servicing) • Corrosion susceptibility (Inspection/repair)

5. Maintainability Definitions Maintainability is an inherent design characteristic of a system or product and it pertains to the ease, accuracy, safety, and economy in the performance of maintenance actions. Maintainability is the probability that a failed system will be restored to specified performance within a stated period of time when maintained under specified conditions. 5

6. Maintainability Definitions continued Maintainability is a characteristic of an item, expressed by the probability that preventive maintenance (serviceability) or repair (repairability) of the item will be performed within a stated time interval by given procedures and resources (number and skill level of the personnel, spare parts, test facilities, etc.). Maintainability is the ability of an item to be retained in, or restored to, a specified condition when maintenance is performed by people having specified skill levels, using prescribed procedures and resources. 6

7. Maintainability analysis: The sequential development and review of data – concurrent with, or preceding design development – to aid in describing the optimum design characteristics of the equipment or system. The elements considered in the review are (1) quantitative requirements, (2) support resources, (3) cost, (4) operational objectives, and (5) safety. The results of the review are translated into criteria which are applied to the design process. Maintainability engineering: An organization that is associated with the functions of maintenance engineering, maintainability-design liaison, systems analysis, design services, support documentation, systems planning, safety engineering, and systems integration and test. Maintainability Definitions continued 7

8. Maintainability Metrics Times MTTR : Mean Time to Repair T5o% : Median Time to Repair TMAX : Maximum Time to Repair (usually 95th percentile LDT : Logistics Delay Time SDT : Supply Delay Time MDT : Mean Down Time DTM : Down Time for Maintenance DTS : Down Time For Supply Events MTBM : Mean Time Between Maintenance MTTPM : Mean Time to Preventive Maintenance MTBPM : Mean Time Between Preventive Maintenance Manpower CS : Crew Size MMH/FH : Man-hours per flight hour Diagnostics FD : Fault Detection FI : Fault Isolation FA : False Alarms 8

9. System Time Relationships TotalTime InactiveTime ActiveTime UpTime DownTime Corrective MaintenanceTime Maintenance Time Not Operating Time Preventive Maintenance Time ModificationTime AlertTime SupplyDelayTime DelayTime InactiveTime Administrative DelayTime Time to Restore Functions During Mission InactiveTime

10. Maintenance Categories Maintenance Preventive Maintenance(retain item functionality) Corrective Maintenance(reestablish item functionality) • Failure detection • Failure isolation • Repair • Functional test • Test of all relevant functions, • Inspect to detect hidden failures • Service to replace consumables • Activities to compensate for drift andto reduce wear out failures • Overhaul to increase useful life • Time Change • Prognostics health management: monitor and repair before failure 10

11. Maintenance Definitions Maintenance: All actions necessary for retaining an item in, or restoring it to, a serviceable condition Includes servicing, repair, modification, overhaul, inspection, and condition verification Maintenance categories: 1. Preventive Maintenance Maintenance performed to retain an item in satisfactory operational condition by providing system inspection, detection and prevention of incipient failures, overhaul, lubrication, calibration, etc Prescribe procedures to reduce the probability of failure or degradation 2. Corrective Maintenance Initiated after fault detection To Regain state of system for performing required function 11

12. Maintenance Process Top-Level Unscheduled Maintenance Process Second Level Unscheduled Maintenance Process Definition Corrective Maintenance Activity On-Equipment Maintenance &Checkout Begins On-Equipment Maintenance &Checkout Ends Cannot Duplicate ProblemMaintenance Activity Repair ComponentIn Place Repaired ComponentTo Supply RemoveComponent AcquireSpare ReplaceComponent Component BenchCheck Serviceable ComponentTo IntermediateLevel Maintenance Component BenchCheck and Repair Component Not Repairable ThisStation (NRTS) 12 (ComponentTo Depot) DEPOTREPAIR (ComponentFrom Depot)

13. Corrective Maintenance Cycle Failure Occurs Detection Failure Confirmed Preparation for Maintenance Active Maintenance Commences Location and Isolation Faulty Item Identified Disassembly (Access) Disassembly Complete or Repair of Equipment Removal of Fault Item Installation of Spare/Repair Part Re-assembly Re-assembly Complete Alignment and Adjustment Condition Verification 13 Repair Completed

14. Corrective Maintenance Task Definitions Maintenance, corrective: That maintenance performed to restore an item to satisfactory condition after a malfunction has caused degradation of the item below the specified performance. The major tasks associated with corrective maintenance are: Preparation – gathering tools; obtaining, setting up, and calibration maintenance aids; warming up equipment; etc. Localization – determining the location of a failure to the extent possible without using accessory support equipment. 14

15. Disassembly – equipment disassembly to the extent necessary to gain access to the item that is to be replaced. Interchange – removing the defective item and installing the replacement. Reassembly – closing and reassembly of the equipment after the replacement has been made. Alignment – performing any alignment, minimum tests, and/or adjustment made necessary by the repair action. Verification checkout – performing the minimum checks or tests required to verify that the equipment has been restored to satisfactory performance. Corrective Maintenance Task Definition Continued 15

16. Corrective Maintenance Detailed Process Fault Occurs Time based on Probability of Failure Latent Time Delay due to Probability of Detection Fault Detection Logistics Delay due to Resource Unavailability Fault Isolation Task Time based on Steps Issue Spare Supply Delay due to Spare Unavailability Logistics Delay due to Resource Unavailability Down Time Open Access Time based on Steps Logistics Delay due to Resource Unavailability Repair Time based on Steps Remove & Replace Logistics Delay due to Resource Unavailability Close Access Time based on Steps Logistics Delay due to Resource Unavailability Verification Time based on Steps Maintenance Time Fault Corrected Delay Time 16

17. FD/FI/FA Fault detection (FD) % of faults detected before noticeable impairment of system performance occurs Time from fault occurrence to detection Fault Isolation (FI) % of detected faults isolated to single item % of detected faults isolated to group of items False Alarm (FA) % of detected faults that can not be confirmed in system maintenance AKA CND (can not duplicate) % isolated faults that can not be confirmed in repair of item removed from system AKA RTOK (retest okay) 17

18. Maintenance Analysis FMEA : Failure Mode Effects Analysis RCMA : Reliability Centered Maintenance Analysis CM : Corrective Maintenance PM : Preventive Maintenance LORA : Level of Repair Analysis MTA : Maintenance Task Analysis FTA : Fault Tree Analysis FTA Identify CM FMEA LORA MTA Identify PM RCMA 18

19. Maintenance Task Analysis (MTA) will detail the resources required to implement effective corrective and preventative maintenance tasks for a system and/ or equipment a detailed analysis performed for each of the corrective and preventative maintenance tasks (earlier identified in the LSA process) Defines support resources that will be required to conduct each of the maintenance tasks. Determines task intervals and task elapsed times. Level of Repair Analysis (LORA), establishes at which level (or line) of maintenance that a task will be performed. . . . 19

20. support resources determined and detailed for each corrective and preventative maintenance task Support Equipment; Standard and special; Tools; Standard and special; Personnel; Facilities; Storage and packaging; Transportation consideration; provides critical input to support and address the needs other ILS elements, such as training and technical publications development. Technical publications utilize data such as the detailed task description, which also provide input for the development of maintenance training courses and support material. Maintenance Task Analysis (MTA) 20

21. MTA Example 21

22. Maintainability Metrics Times MTTR : Mean Time to Repair T5o% : Median Time to Repair TMAX : Maximum Time to Repair (usually 95th percentile LDT : Logistics Delay Time SDT : Supply Delay Time MDT : Mean Down Time DTM : Down Time for Maintenance DTS : Down Time For Supply Events MTBM : Mean Time Between Maintenance MTTPM : Mean Time to Preventive Maintenance MTBPM : Mean Time Between Preventive Maintenance Manpower CS : Crew Size MMH/FH : Man-hours per flight hour Diagnostics FD : Fault Detection FI : Fault Isolation FA : False Alarms 22

23. Maintainability Metric Definitions • Mean Time Between Maintenance (MTBM): included both preventive and corrective maintenance requirements. Relative to corrective maintenance, MTBM considers primary (random) failures, secondary (dependent) failures, quality and manufacturing defects, operator- and maintenance-induced failures, and others. • Mean Time Between Replacements (MTBR): Mean time between equipment item replacements for preventive- and corrective-maintenance purposes. This factor forms the basis for spare/repair-parts determination. It should be noted that a maintenance action (represented by the MTBM factor) does not always result in an item replacement.

24. Maintainability Definitions continued • Maintenance downtime: That portion of downtime which can be attributed to preventive- and corrective-maintenance functions. Maintenance downtime may be expressed in a measure of central tendency (arithmetic mean, geometric mean, median, and mode). It may also be expressed in terms of a maximum value relative to a percentile point of distribution of downtime. Symbols of maintenance downtime are: • Mct – Mean active corrective-maintenance time (arithmetic mean). Equal to MTTR. • Mpt – Mean active preventive-maintenance time (arithmetic mean).

25. Maintainability Definitions continued • M – mean maintenance time (function of Mct and Mpt). • MTTR – Mean time to repair. Equal to Mct. • MDT – Maintenance downtime (total time during which an equipment item is not in condition to perform its intended function). MDT includes logistics time and waiting or administrative time. • Mct – Mean active corrective maintenance time. Equal to ERT

26. Maintainability Definitions continued • ERT – Equipment active repair time. Equal to Mct. • Mpt – Median active preventive-maintenance time. • MTTRg – Geometric mean time to repair. • Mmax – maximum active corrective maintenance time (95% confidence level). • Logistics or Supply Time – That portion of nonactive maintenance time during which maintenance is delayed solely because a needed item is not immediately available. • Wait or Administrative Time – That portion of nonactive maintenance time that is not included in logistics or supply time.

27. Maintainability Definitions continued • Maintenance indices: Primary expressions of quantitative measurements of equipment-maintenance characteristics. MMH/OH – Maintenance man-hours per equipment or item operating hour. MMM/OH – Maintenance man-minutes per equipment or item operating hour. Cost/OH – Maintenance cost per equipment or item operating hour. Others are MTBM, MDT, and achieved availability.