Hard-Time vs On-Condition
Hard-Time, being a scheduled removal of all units of an item before some specified maximum permissible age limit, indicates that we are not considering each unit on its own merits (probability of failure) and generalize to assume that all units or items have the same age characteristics that will end with wearout. One may also conclude that since we are not examining the item prior to its removal we have very little data to evaluate the item for escalation or product improvement.
Take an example of a brake assembly. Under the Hard-Time concept, the brake will be removed at a given fixed time interval. No attention is paid to the condition of the brake, whether it is partially worn, fully worn, or damaged beyond repair. Under the On-Condition concept, the brake assembly is designed for On-Condition inspection. The technician will periodically inspect the wear indicator and the general condition of the brake. At a given limit of the wear indicator, the technician will remove the brake from service for restoration. Moreover, if the brake exhibits unexpected problems, a product improvement can be introduced.
Making the transition
The maintenance on engines is a prime example of the general move from Hard-Time to On-Condition. On the early jet engine, accessories and the engine were removed at fixed intervals for overhaul. The move to go On-Condition allowed detection of potential failures, in many cases, before they became functional failures. For example, under the Hard-Time concept at a given fixed-time interval the engine will be removed from service for a complete overhaul. The engine will be taken apart and many of its components will be replaced even though their useful life has not been exhausted yet. Many components could be found damaged beyond repair due to consequential damage of one or two items. Ideally, On-Condition engines are designed to allow internal inspection and modular replacement. Items in the engine such as a turbine or compressor will be inspected either visually or through a borescope. When degradation is found, corrective action will be taken. Older jet engines had minimal provisions for On-Condition maintenance, while modern engines have been specifically designed for On-Condition, by providing ample access and techniques (borescope, X-rays/isotope, etc.) to detect early erosion, cracks, and loose or broken hardware. Nowadays, engine manufacturers proudly advertise their products as being designed (On-Condition Maintenance) to stay on-wing for over 20,000 hours.
In the 1960's, the move from Hard-Time to On-Condition resulted in higher reliability and safety, and above all, tremendous savings to the operators. Additionally, it allowed collection of real data for surveillance and analysis; in order to make refinements and modifications to the maintenance program. Moreover, it allowed determination of the needs for product improvement. It is well known that scheduled maintenance allows us to maintain the inherent reliability of an item, while product improvement provides means for increasing the reliability and safety of an item.
A Task Oriented Program consists of specific tasks, selected for a given functional failure consequence based on actual reliability characteristics of the equipment they are designed to protect. Tasks are selected in a hierarchy of difficulty and cost, from lowest to highest. Each task must also pass the applicability and effectiveness criteria. Depending on the on the consequence of failure (safety, operational, economic, hidden safety and hidden non-safety) a single or combination of tasks will be selected. The following is the generic list of tasks to be selected:
• Lubrication/Servicing Task of an item to maintain satisfactory operation.
• Operational/Visual Check - failure finding task of an item for hidden safety and hidden non-safety consequences only.
• Inspection/Functional Check - inspection of an item to find any potential failures (detect degradation or actual failure). The inspection includes three categories: General Visual Inspection (GVI), Detail Inspection (DI) and Special Detail Inspection (SDI). The functional check compares measured values against an established standard.
• Restoration of an item at or before some specified age limit (may vary from cleaning or replacement of single parts up to a complete overhaul)
• Discard of an item at some specified life limit.
The development of a scheduled maintenance program consists of determining which of these five categories of tasks, if any, are both applicable and effective for a given Maintenance Significant Item (MSI), Structural Significant Item (SSI) or Zone.
MSG-3 is a task-oriented methodology to derive scheduled maintenance requirements. It provides a standardized format for analysis and determination of scheduled maintenance tasks for Aircraft Systems/Powerplant, Structures and Zonal.
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