These findings have led to a new maintenance philosophy. Instead of limiting the life of the equipment, the new approach was directed toward detecting the potential failures through scheduled inspection. This new concept became commonly termed as On-condition Maintenance.
During the late 1950s and early 1960s, attempts were made to organize what had been learned from the various reliability programs and to develop a logical and generally applicable approach to the design of preventive maintenance programs. That new approach generated a technique that followed a given decision diagram. Further refinements of this technique were embodied in a handbook on maintenance evaluation and program development. This document became known as the MSG-1.
Evolution of the Maintenance Steering Group
The MSG-1 development set new ground rules for the development of scheduled maintenance programs, a process commonly known as the Maintenance Steering Group (MSG), and its later derivatives, MSG-2, and MSG-3. Figure 3, on page 56 provides a historical layout of the MSG development.
Each new derivative of the MSG improved on its predecessor in terms of ease of analysis and strength of the logic diagnosis. The MSG philosophy also led to a closer association with the MRB (Maintenance Review Board) process. At present, the MSG-3 method and the MRB process are inseparable. The MRB process is clearly defined in AC 121-22A5.
Maintenance Review Board
The Maintenance Review Board was initially associated with a group of regulatory inspectors, each with specialized skills, who were charged with the responsibility of approving the initial maintenance program for new commercial aircraft. Presently, MRB approval is still in the hands of the regulators, but the process is a joint venture between the manufacturer, vendors, operators and regulators. The process entails an Industry Steering Committee (ISC) assembled with representation from manufacturer-vendors, operators and regulatory authorities. An operator chairs the ISC. The ISC delegates the MSG-3 analysis work to the Industry Working Groups (IWGs) that have similar participation to the ISC and are chaired by an operator. Normally, there are five IWGs (Systems, Structures, Avionics-Electrical, Propulsion and Zonal). The working groups analyze the aircraft using the MSG-3 method and the MSG-3 analysis reports are then submitted to the ISC for approval. The end result of this effort is an initial scheduled maintenance program. The initial scheduled maintenance program is submitted by the ISC to the MRB as the draft MRB Report. Upon MRB approval, the MRB Report forms the initial minimum scheduled maintenance requirements. The MRB Report can be used on its own or as part of the Maintenance Planning Document (MPD). It is important to mention that both the ISC and the Working Groups membership consist of a selected elite of the most qualified personnel in terms of knowledge and experience. Participation in the ISC and the IWGs process is recognized as a great privilege. Furthermore, this unique partnership between the Manufacturer-Supplier Operator and Regulator is extremely beneficial to all the parties involved. It improves communication and understanding of each other's needs. It is a reflection of a democratic system where objectives and goals can be achieved in a free and friendly way.
The evolution of MSG-1, 2 & 3 and the advances in reliability science led to the development and recognition of a number of maintenance philosophies. The maintenance philosophies can be looked upon as evolutionary processes in the field of aircraft maintenance, as they relate to the historical development of MSG, reference Figure 2. Hard-Time being the oldest, utilized only to a limited degree in MSG-1 and 2, and Task Oriented being the most recent utilizing MSG-3 and RCM. These maintenance philosophies are listed below with their definitions:
• Hard-Time: Scheduled removal of all units of an item before some specified maximum permissible age limit
• On-Condition: Scheduled inspections, tests, or measurements to determine whether an item is in, and will remain in, a satisfactory condition until the next scheduled inspection, test, or measurement.
• Task Oriented RCM/
MSG-3: A scheduled maintenance program consisting of a set of tasks each of which is generated through RCM/MSG-3 analysis.
The major difference between MSG-3 and its predecessors is in its holistic approach and being task oriented. The holistic approach looks at consequences of failure to a Maintenance Significant Item (MSI). An MSI is normally considered a complete system or subsystem. The analysis is task oriented as it only allows selecting tasks that are applicable and effective for a given failure consequence. MSG-1 and 2, being maintenance processes oriented, explain ÒhowÓ maintenance is applied to individual components. There is no applicability and effectiveness evaluation applied. Each individual component is evaluated separately, resulting in large number of tasks that bring no specific benefit.
In order to understand better the differences between the maintenance philosophies, let's have a closer look at some of them.
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