Demystifying MSG-3 Logic (Part-1)

When a new aircraft is designed it is the responsibility of the OEM to prepare the Instructions for continued Airworthiness (ICA). In EASA this is enshrined in Certifications Specifications CS-25.1529. From the generic requirement the aviation industry uses an Industry document called Maintenance Steering Group-3 (MSG-3) as the logical process for for analysis and development of the baseline scheduled maintenance requirements.

In this first of my Three part series i will first discuss the evolution of the MSG process and how we arrived at MSG-3. I will attempt also to provide a brief overview of the MSG-3 process. In the second part ( releasing by next week) i will discuss in more detail of how MSG-3 actually works. Finally the third part part will cover how the MRB utilizes the MSG-3 to prepare the MRBR. A noteworthy point is that the MSG-3 is the tool used by the Maintenance Review Board (MRB) team consisting of OEM, Operators, Vendors and Regulators to create the Maintenance Review Board Report (MRBR) to create the baseline scheduled maintenance program.

Initial maintenance programs were simple and devoid of any analysis. Gradually as aircraft complexity grew programs as well as their associated regulation was developed. Usually a combination of hard time and on-condition maintenance was used.

In hard time maintenance 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 restored 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:

1. Scheduled overhaul has little effect on the overall reliability of a complex equipment unless the equipment has a dominant failure mode, and

2. There are many items for which there is no effective application for scheduled hard-time maintenance.

The findings of the task force led to the development of a second primary maintenance process defined as On-Condition (OC). On-Condition maintenance 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.

Maintenance Steering Group (MSG) process

In 1968 the Maintenance Steering Group (MSG) was created with a mandate to formulate a decision logic process used for development of the 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 manufacturers, suppliers, and FAA representatives.

MSG-1

That same year representatives of the steering group developed “MSG-1 - Maintenance Evaluation and Program Development", which for the first time used a decision-logic diagram to develop the scheduled maintenance program for the new Boeing 747 aircraft. Both hard-time and on-condition processes were used for development of the aircraft’s routine maintenance tasks.

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MSG-2

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 characteristic 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 variation 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 characteristic 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 the 1970s. Maintenance tasks were derived from one of three processes: 1.) Hard-Time, 2.) On-Condition, and 3.) Condition-Monitoring or some combination of the three processes.

?Problems with MSG-2

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 & B767). Additionally, the task force identified a number of shortcomings in MSG-2 decision logic, key among them:

• MSG-2 did not differentiate between maintenance being done for safety reasons versus economic reasons.
• An MSG-2 program became very unwieldy and difficult to manage because it required so many components to be individually tracked.
• MSG-2 did not effectively deal with the increased complexity of aircraft systems.
• MSG-2 did not address regulations related to damage tolerance and fatigue evaluation of structures; these are currently accounted for in Corrosion Prevention and Control Programs (CPCP) and requirements mandated through an Aging Aircraft maintenance program.

MSG-3

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:

a.) separating safety-related items from economic

b.) defining adequate treatment of hidden functional failures.

?Under MSG-3 logic, activities are assessed at the system level rather than the component level.

?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 component 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 minimizes 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.

?Prior to MSG3, the Corrosion Prevention Control Program (CPCP) was mandated by Airworthiness Directives. Under MSG3, the CPCP has been integrated into the baseline MRB program and included as part of the structures maintenance program. This integration into the MRB program significantly eliminated duplicative tasks.

?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.

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For 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 a hierarchy of difficulty and cost, from lowest to highest. 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.

With this i hope i have managed to shed some clarity of the evolution of MSG-3 and why it is has become the default industry standard process.

In the next article i will discuss how MSG-3 actually works

..................(to be contd....)

PS: I cannot take credit for the diagrams, they originate from an unknown source, so due credit to the creator of those diagrams.