Maintainability Modeling

Maintainability Programs

Maintainability Assessment

Maintainability Modeling

Maintainability Demo

DRACAS

Where there are specific maintainability requirements or goals, which must be obtained for a system, then there is a need to determine the system's quantitative maintainability characteristics. This could be represented in terms of a Mean-Time-To-Repair (MTTR). Other parameters to be considered are the maximum time repair and these could be determined for each of the various levels of maintenance. The basic approach in deriving the MTTR for level one (or first line) maintenance action, would be to determine the maintenance task times for each corrective maintenance task and weight it

with their occurrence. The elapse times for each corrective maintenance task could then be calculated. This would take into consideration the various steps required to implement this task and include the time to isolate the fault to the failed unit to be removed, removal time of that unit, its replace time and the time to complete system verification.

Critical to the remove and replace times is the accessibility to the failed unit required by the maintainer. This would include the ability to use the necessary hand tools and or test equipment and the actual physical removal of the unit. Therefore, the design phase consideration must be given to the layout of the components and avoid the prospect of having to remove other components to access a failed unit. A good example of this would be the restricted engine compartment of an automobile. Where many mechanics have been faced with the prospect of having to strip out half the engine to gain access to a particular item.

Another critical consideration when determining the overall MTTR calculation is the time it takes to isolate the fault. For some systems this could be relatively straight forward, while for others it could be a more complex affair. To ensure that the fault detection and isolation components and capabilities of a system are obtained, a careful testability analysis must be performed.

The following table provides an example for a given corrective maintenance task, on how it could be broken down into specific subtasks, and elapse time.

Control Display Assembly
Task Number	CM: 107	Task Title:	Corrective Maintenance Action
Task Frequency	0.39	MTBF	3,000 Hours
Group/ Sub Task	Subtasks Details	Time	Required Support
Fault Detection (01)
CM:107:01:01	Run WRA Initiated BIT	2.00
Removal (02)
CM:107:02:01	Set Control SW on the Operator Station (OS) to OFF	0.50	Located at OS Station
CM:107:02:02	Isolate Power : Open OS CCT BRK main AC BUS phase.	2.00	OS Station electrical circuit breaker panel
CM:107:02:03	Release the DZUS fasteners on CDA	0.20
CM:107:02:04	Pull CDA from OS console	0.50
CM:107:02:05	Disconnect connector P1 and P2, install protective caps	1.00
Installation (03)
CM:107:03:01	Verify Control SW on the Operator Station is OFF	1.00
CM:107:03:02	Verify that CCT BRK is open	1.00
CM:107:03:03	Remove protective cover from P1 and P2 and connect to CDA	1.00
CM:107:03:04	Insert CDA into OS console:	1.00	Care must be taken to ensure that the cable harness does not bind with the structure
CM:107:03:05	Secure the DZUS fasteners	0.60
CM:107:03:06	Close CCT BRK	1.00
CM:107:03:07	Reset Operator Station	0.25	The Control SW must remain in the ON position.
Checkout (04)
CM:107:04:01	Run WRA IBIT	2.00
CDA WRA CMT Total		14:05 Minutes

Example of a System MTTR Calculation

The MTTR for a system can be calculated by using a weight factor against each corrective maintenance time (CMT), namely the unit's failure rate. The derived MTTR takes into consideration the elapse time (or CMT) required to perform the corrective maintenance tasks for each of the LRUs, making up the system. In the given formula, the MTTR represents the mean of the number of times to repair, weighted by the probability of occurrences:

Where:

In deriving the MTTR, a given assumption is that the required logistics support are readily available. This includes the necessary manpower and spare parts. In other words the time for administration delay are not considered.

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