Reducing the complexity of a product and improving a products maintainability are activities of:

5.2.3.3.1 Task 1, Maintainability Analysis

Maintainability analysis includes all the maintainability tasks addressed in the program plan. It translates overall system operational and support requirements into detailed quantitative and qualitative maintainability requirements and evaluates how many established maintainability requirements have been achieved. It is an iterative process that begins with preliminary concepts, proceeds through the development of a preferred maintainability model for the preliminary design review (PDR), and culminates in a detailed design with well-defined quantitative and qualitative maintenance support requirements to the lowest repairable item level.

5.2.3.3.2 Task 2, Document Maintainability Requirements and Criteria

Maintainability engineers provide inputs to the design engineers to identify and integrate specified maintainability requirements. The design criteria constitute specific maintainability goals, such as modularization, standardization, accessibility, interchangeability, repair versus discard guidance, quantity and placement of test points, and degree of self-test features. These criteria are stated qualitatively or quantitatively and are used as guidelines by the design engineer. A maintainability design checklist may be provided to the design engineers and used by maintainability engineers to record their evaluations of maintainability features within the design.

The allocations and predictions are performed using one or more of the maintainability parameters listed in Table 5.5. These parameters are measured commonly in hours but the time figure could be selected based on the timescale of the system. Each of these parameters has a very precise definition that may have slightly different meanings to different people. It is important to get a clear understanding with the customer about the parameter they wish to use as a basis for maintainability requirements.

Table 5.5. Maintainability Parameters

These and other parameters can be included in a maintainability model using mathematics common to queuing theory that permits manipulation of maintainability figures in “what-if” analyses helpful in resolving maintainability concerns by adjusting values and consuming maintainability margins in alternative ways. The model can also be arranged to compute an aggregate maintainability figure from the many lower-tier figures to validate that allocations made are consistent with top-level figures. Note: Mean time between maintenance (MTBM) is similar to MTBF (a reliability parameter), but is equivalent only when there are no preventive maintenance or maintenance-induced failures.

The maintainability engineer should develop and maintain a maintainability requirements model that also can produce a report with content driven by the customer’s data needs (expressed in a data item description). Commonly, you will be required to report detailed corrective maintenance data and a summary mean time to repair (MTTR) figure plus preventive or turnaround maintenance data.

There are many ways to measure maintainability and many ways to count time in maintenance events, so the corrective maintenance times must be very carefully defined and agreed upon by the customer. These times could include only remove and replace time of line replaceable units or they could include access, checkout, and logistic delay times. The maintainability engineer should keep a tabular list (in a computer database model if possible) of allocated corrective maintenance times for each element accepted into the maintainability program.

Table 5.6 provides a fragment of an example of a corrective maintenance requirements list and reporting format using system MTTR as the maintainability parameter. Allocated failure rate numbers from the reliability analysis are multiplied by allocated repair times for each item included in the maintenance analysis. These products are added and divided by the sum of allocated failure rates to yield a measure of MTTR. In this case the MTTR figure would be 0.3500 meaning that it takes on the average 0.35×60=21 min to repair the item when it fails. MTTR is computed here from the component figures based on a weighted averaging technique.

Table 5.6. Corrective Maintenance Requirements List

The design engineer may extract the maintainability figure from this table and plug it into the requirements for his/her item. If the design respects these allocations, the system MTTR will be as advertised. The maintainability engineer may include a margin in the allocated numbers such that a target figure is actually given to the designer and a portion is withheld at each architecture level. This margin is dispensed grudgingly to resolve designer difficulty in meeting the allocated values. The equation applied in Table 5.6 is:

MTTR=∑λiMTTRi∑λi=7.080520.23=0.35,where the summation occurs from i−1ton

Preventive or turnaround maintenance requirements are developed for a system or end item using a process flow diagram and companion timeline. The times for each task are determined through analysis, estimate, or appeal to history. The times are combined in accordance with the flow diagram pattern to develop the total time for comparison with a system requirement. If the analytical time exceeds the required time, the maintainability engineer may have to assign specific time requirements for time-critical item tasks that apply to specific items involved in parts of the overall use process.

5.2.3.3.3 Task 3, Maintainability Quantitative Analysis to Assure Requirements Are Met

Maintainability allocations are assigned in a top-down fashion from a top-level quantitative maintainability figure to provide a quantitative goal for the designer to satisfy and to permit us to evaluate cost-effective alternatives in the integration of maintainability and other requirements into the system, subsystem, and component concepts and designs. It may be necessary to develop allocations for both corrective and scheduled maintenance actions as noted above.

As the design matures such that it is possible to imagine the maintenance actions that must be performed, the maintainability engineer adds predicted failure rate and maintenance time columns to his or her data table and computes predicted MTTR. If the system level MTTR is within the required value, all is well. If it is not, then it is necessary to identify the items that are contributing the excess maintenance time and evaluate with the designer how the times can be reduced. Later, when actual failure rate and maintenance time data become available from reliability and maintainability testing, manufacturing testing, or the user, the engineer can develop a figure for achieved MTTR.

System or end item scheduled maintenance time is computed from detailed scheduled maintenance tasks by combining them in accordance with the process flow diagram and timeline diagram as noted above. In a deterministic environment, the estimated times are simply added where they are serially accomplished and critical path times added where parallel tasks are involved. In a probabilistic environment, mean and variation numbers are required and they are introduced into a queuing model to determine aggregate values. In both the deterministic and probabilistic approaches, margins should be used to provide for risk management opportunities.

As the design concept matures, maintainability engineers study the evolving design and make scheduled maintenance predictions within the context of the planned maintenance concept. Where predictions suggest a failure to satisfy allocated values, the maintainability engineer must first determine which area of the design offers the best avenue for reduction of this figure.

There may be several alternatives that may have to be studied in a trade format. The maintainability engineer then works with the selected item principal engineer for scheduled maintenance time reduction. As a last resort, maintainability figures may be reallocated and maintainability margins attacked or margin values exchanged in other parts of the architecture.

5.2.3.3.4 Task 4, Design Surveillance/Assessment

Maintainability design inputs are formally transmitted to design engineers by conversation, tabular maintainability data, and design specifications where flow down is applicable. The design effort is monitored by attending design meetings, reviewing preliminary design data, such as sketches and drawings, and through concurrent engineering discussions with the design engineer. Issues that cannot be informally solved are resolved by engineering management in internal reviews.

5.2.3.3.5 Task 5, Participate in Design Trade-off Studies

Maintainability engineering provides inputs to alternative design concepts, support concepts, subcontractor/supplier proposals, and analysis of the effect of alternative manufacturing processes on maintainability. Normal criteria for assessing the compatibility of concepts or alternatives include relative MTTR and/or Mpt, access requirements, skill levels and number of personnel, special tools and test equipment, impact on facilities, and the relative life-cycle maintenance cost.

5.2.3.3.6 Task 6, Participate in Design Reviews

Maintainability engineering actively participates in all formal and informal in-house reviews and is also included in the distribution of all proposed design changes. Each change is evaluated for impact on quantitative and qualitative maintainability requirements. Upon approval of a design change, prediction parameter values, design criteria, or maintenance procedure documentation is updated as appropriate.

5.2.3.3.7 Task 7, Subcontractor and Supplier Control

Subcontractors and suppliers providing newly designed equipment are subject to the maintainability constraints allocated to them by the prime contractor. Based upon the results of the maintainability analysis, quantitative parameters are allocated to and incorporated in subcontractor or supplier specifications. Supplier progress is monitored and maintainability concerns highlighted for resolution.

5.2.3.3.8 Task 8, Failure Reporting, Analysis, and Corrective Action

Management may review failure trends, significant failure, delinquent actions, and corrective actions at a Program Reliability Review Board. A maintainability engineer participates on the review board in the maintainability analysis process. In-plant corrective action data may provide useful maintainability data including maintainability action timeline information.

5.2.3.3.9 Task 9, Conduct Maintainability Demonstration

The achievement of system maintainability requirements is evaluated by analysis and formal and/or informal maintainability demonstrations. The demonstration tests are conducted to support verification of contract requirements. Often the contractor will be required to demonstrate the system MTTR, which is comprised of the time for fault detection, fault isolation, removal and replacement, and repair verification.

5.2.3.3.10 Maintainability References

Table 5.7 lists several maintainability engineering documen ts primarily drawn from government sources. Some of these military standards have been dropped but they still contain useful background information.

Table 5.7. Maintainability References