Aircraft Maintenance Program Perspective - Explained
Dr. Sasidharan Murugan
Assistant Professor at CK College of Engineering & Technology
Maintenance Program History
In the early days of aviation maintenance programs were developed primarily by pilots and mechanics. They assessed an aircraft's needs for maintenance based on their individual experiences and created programs that were simple and devoid of analysis. The introduction of the airlines as a new method of transport demanded new regulations and broader involvement of regulatory authorities in maintenance requirements. During this era not only were regulations put in place but programs were started to monitor reliability and safety. The entry of the large jet aircraft (B707 and DC-8) in the fifties focused public attention on the need for safer and more reliable aircraft. The aircraft manufacturer became the source of maintenance program development. Time limitations were established for maintenance and the entire aircraft was periodically disassembled, overhauled and reassembled in an effort to maintain the highest level of safety. This was the origin of the first primary maintenance process referred to as:
Hard-time (HT)
Hard-time processes mandated that all components be taken out of service when they reached a specified age, expressed as the number of operating flight hours, flight cycles, calendar time, or other stress units since new or since last shop visit. Removed units were routed to repair centers and effectively zero-timed, whereby the operating age was resorted to a unity of zero by means of an overhaul. In 1960, representatives from both the FAA and the airlines formed a task force to investigate the capabilities of preventive maintenance.
Two major discoveries resulted from their investigation:
The findings of the task force led to the development of a second primary maintenance process defined as:
On-condition (OC)
On-condition requires that an appliance or part be periodically inspected or checked against some appropriate physical standard to determine whether it can continue in service. The purpose of the standard is to remove the unit from service before failure during normal operation occurs. Example of an OC process is measurement of brake wear indicator pins; compare brake wear condition against a specified standard or limit. Brake wear will vary considerably among operators due to operational conditions, however the wear indicator pin on-condition check will help attain near maximum usage out of each set of brakes.
Maintenance Steering Group (MSG) Processes
In 1968, the Maintenance Steering Group was created with a mandate to formulate a decision logic process used for development of initial scheduled maintenance requirements for new aircraft. The group was composed of participants from various aviation bodies, including the Air Transport Association (ATA), airlines, aircraft manufactures, suppliers, and FAA representatives. That same year representatives of the steering group developed "MSG-1 - Maintenance Evaluation and Program Development". which for the first time used decision-logic diagram to develop the scheduled maintenance program for the Boeing 747 aircraft. Both hard-time and on-condition processes were used for development of the aircraft's routine maintenance tasks.
In 1970, MSG-1 is updated to MSG-2 to make it applicable for later generation aircraft (L-1011 and DC-10), and at the same time the methodology introduces a third primary maintenance process defined as Condition-Monitoring (CM). Under Condition-Monitoring no services or inspections are scheduled to determine integrity or serviceability, however the mechanical performance is monitored and analyzed. For example, a given operating characteristics of the equipment (e.g. vibration, oil consumption, EGT margin deterioration, etc.) is trended and compared with known "normal" operating levels. An acceptable range is established with either upper and/or lower limits, or some maximum or minimum level. As long as the trend data remain inside the acceptable level, any vibration is considered to be normal. When the trend line intersects the "unacceptable" limit, removal of the unit is required to prevent a failure in the future. A characteristics of CM is that it is not considered a preventive maintenance process; the process allows failures to occur, and the failure modes of conditioned-monitored items are considered not to have a direct adverse effect on operating safety. MSG-2 decision logic was subsequently used to develop scheduled maintenance programs for the aircraft of 1970s. Maintenance tasks were derived from one of three processes:
In 1979, the Air Transport Association (ATA) task force sought to improve on MSG-2 to address a new generation of advanced technology aircraft (B757 and B767). Additionally, the task force identified a number of shortcomings in MSG-2 decision logic, key among them:
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The work of the ATA task force led to the development of a new, task-oriented, maintenance process defined as MSG-3. The process adopted a decision tree methodology with the primary purpose of:
Under MSG-3 logic, activities are assessed at the system level rather than the component level (See FIG 3). In other words, if it can be demonstrated that the functional failure of a particular system had no effect on operational safety, or that the economic repercussions were not significant, there was no need for a routine maintenance activity.
Although, there is no actual in-service operational data available when the MSG-3 process begins for a new aircraft, there is much historical data on the performance of similar components and systems used in earlier designs, as well as test data from the manufacturer and components vendors. It's the actual in-service reliability data of similar components and systems that drives the task and interval decisions.
Another principal benefit from the MSG-3 process is that it generally produces higher safety standards. This is primarily due to the greater degree of intelligent approach to maintenance in terms of selecting tasks that are effective. The approach results in far less maintenance tasks, which minimize the infant mortality effect associated with excessive maintenance. Studies in Human Factors clearly identified correlation between excessive maintenance and induced incidents, or accidents, resulting from preventive maintenance through replacement and overhaul of components.
Today, MSG-3 is the only game in town for commercial airplane manufacturers. According to Advisory Circular AC-121-22A, FAA policy states that the latest MSG analysis procedures must be used for the development of routine scheduled maintenance tasks for all new or derivative (Part 121) aircraft. It is the only methodology accepted by the airworthiness authorities. MSG-3 has also been adopted by most major business jet manufactures, with the encouragement of the National Business Aviation Association (NBAA).
Maintenance Task Development
MSG-3 is the current method used for developing the scheduled maintenance tasks and intervals which will be acceptable to the:
The remaining maintenance, that is non-scheduled or non-routine maintenance, consist of maintenance actions to correct discrepancies noted during scheduled maintenance tasks.
FIG 3 illustrates the difference between the scheduled task development processes employed using MSG-3 versus MSG-2. Foe each potential failure cause, the MSG-3 guidelines provide task oriented logic to determine the appropriate scheduled maintenance tasks. 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 hierarchy of difficulty and cost, from lower level to higher level. Depending on the consequences of the 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: