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Volume 3, Issue 1

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Creating Initial Scheduled Maintenance Plans
Using the MSG-3 Aircraft Systems and Powerplant Analysis Process to Develop an Initial Scheduled Maintenance Plan

The MSG-3 guidelines have been developed to provide the aircraft industry with a logical framework for creating initial scheduled maintenance plans that will be acceptable to regulatory authorities, operators and manufacturers. This article provides a general overview of MSG-3 and discusses the systems and powerplant portion of the analysis in more detail. Finally, we introduce ReliaSoft’s MPC 3 software, designed to assist MSG-3 working groups to accurately and efficiently complete such analyses.

Background
Prior to the development of MSG methods, the airline industry used “hard time” preventive maintenance programs, which set fixed intervals for the overhaul of specific portions of the aircraft. When serious questions began to be raised as to the efficiency and cost-effectiveness of these procedures, the Federal Aviation Administration (FAA) and the airline industry formed a Maintenance Steering Group to develop a new approach for establishing initial scheduled maintenance plans for aircraft. 

In 1968, the MSG-1 guidelines were adopted for the Boeing 747 aircraft. New versions of these guidelines (MSG-2 and EMSG-2) were developed in the 1970s to cover different types of aircraft. Based on experience with MSG-1 and MSG-2 and with the multi-national cooperation of regulatory authorities, aircraft/engine manufacturers, airlines and the U.S. Navy, a revised approach was developed. The MSG-3 guidelines were adopted in 1980 and revised in 1987, 1993 and 2001. Currently, the Air Transport Association’s ATA Operator/Manufacturer Scheduled Maintenance Development (MSG-3) Revision 2001.1 document is accepted by the FAA (and similar regulatory authorities in other countries) as a guideline for scheduled maintenance program development. 

MSG-3 Guidelines
The MSG-3 guidelines take a “top down” approach that looks at the potential effects of a functional failure and on the ability to detect the failure, as well as the costs of failure and of maintenance actions. Based on this analysis, inspections and other maintenance tasks are recommended to be performed at specified intervals. According to these guidelines, the objectives of an efficient scheduled maintenance program are:

  • To ensure realization of the inherent safety and reliability levels of the aircraft. 
  • To restore safety and reliability to their inherent levels when deterioration has occurred. 
  • To obtain the information necessary for design improvement of those items whose inherent reliability proves inadequate. 
  • To accomplish these goals at a minimum total cost, including maintenance costs and the cost of resulting failures. 

There are several entities involved in the development of the aircraft maintenance program, including the Maintenance Review Board (MRB), Industry Steering Committee (ISC) and Industry Working Groups (IWGs). Each of these groups consists of representatives from the participating operators, the prime manufacturer and the regulatory authority. The IWGs perform the detailed analyses while the ISC and MRB provide various levels of oversight. 

MSG-3 provides guidelines for systems and powerplant analysis, aircraft structural analysis, zonal analysis and lightning/high intensity radiated field (L/HIRF) analysis. The scheduled maintenance tasks that are identified in each type of analysis are combined to create the initial scheduled maintenance policy for the aircraft, which can then be adjusted by the operator as applicable based on operating experience. The systems and powerplant analysis, which requires a thorough analysis of maintenance significant items in the aircraft systems and powerplant, is the largest portion of the MSG-3 effort. An overview of the procedures for this analysis is presented next. Although the complementary processes for structural, zonal and L/HIRF analyses are not described here, the requirements are presented in the ATA documentation. 

MSG-3 Systems and Powerplant Analysis: Defining Maintenance Significant Items (MSIs) 
The first step in MSG-3 systems and powerplant analysis is to define the systems, subsystems, subsubsystems and parts that make up the aircraft. For each component, the analysts review the available technical data, such as reliability and maintainability characteristics and description and operation documentation. The analysts can then determine the “maintenance significant items” (MSIs) by answering the following questions:

  • Could failure affect safety on ground or in-flight?

  • Could failure be undetectable or not likely to be detected during normal operation? 

  • Could failure have significant operational impact? 

  • Could failure have significant economic impact?

If the answer to any of these questions is yes, then according to the MSG-3 guidelines, the item warrants specific analysis to determine the maintenance tasks, if any, that will be applicable and effective to detect and/or prevent failure. The analysis is performed at the lowest level of the system configuration above the individual part level (e.g., subsubsystem, subsystem, etc.). 

Define the Functions, Failures, Effects and Causes for Each MSI 
In order to determine the appropriate maintenance tasks for each MSI, the next step is to identify the functions that the item is intended to perform. Next, for each function, the analysts determine the possible failures that could occur to prevent the item from performing its intended function. Then, for each functional failure, the analysts determine the possible effects that could result from the failure. 

The MSG-3 guidelines provide logic designed to assign each functional failure effect to one of five categories: Evident Safety, Evident Operational, Evident Economic, Hidden Safety, Hidden Non-Operational (or Hidden Economic). This is referred to as “level one analysis” and the decision-making logic is presented in Figure 1. 

MSG-3 functional failure effects categorization

Figure 1: MSG-3 logic to categorize functional failure effects

For each functional failure effect, the next step is to identify any possible causes that would result in the occurrence of that effect. Once the functions, failures, effects and causes have been identified for each MSI and each functional failure effect has been categorized, the next step is to determine which maintenance tasks, if any, are applicable and effective to detect and/or prevent the causes of failure. 

Task Selection and Assignment 
For each potential failure cause, the MSG-3 guidelines provide logic, referred to as “level two analysis,” to determine the appropriate scheduled maintenance tasks. There are five types of tasks that can be performed: Lubrication/Servicing, Operational/Visual Check, Inspection/Functional Check, Restoration and Discard. Depending on the category of the functional failure effect that the cause belongs to, some or all of the following questions must be answered to identify the tasks that should be assigned:

  • Is a lubrication or servicing task applicable and effective? 
  • Is a check to verify operation applicable and effective? 
  • Is an inspection or functional check to detect degradation of the function applicable and effective? 
  • Is a restoration task to reduce the failure rate applicable and effective? 
  • Is a discard task to avoid failures or to reduce the failure rate applicable and effective? 
  • Is there a task or combination of tasks that are applicable and effective?

The MSG-3 guidelines also provide definitions of the available task types as well as criteria to determine the applicability and effectiveness for each type of task. 

The Systems and Powerplant Analysis Report 
When the analysis for a particular MSI has been completed, the next step is to generate a systems and powerplant analysis report. The reports for all MSIs in the aircraft can be combined with the reports for structural analysis, L/HIRF analysis and zonal analysis to prepare the Maintenance Review Board (MRB) report that is submitted to regulatory authorities. For each MSI, the systems and powerplant analysis report contains the following elements:

  • A list of the items included in the analysis and documentation of the MSI selection decisions. 
  • Detailed “description and operation” documentation, which includes functional description, design features, etc.
  • Maintainability and reliability data for each item included in the analysis. 
  • A hierarchical list that contains each function, failure, effect and cause that has been identified for the MSI. 
  • The failure effect categorization results for each functional failure effect that has been identified for the MSI.
  • The task selection results for each cause that has been identified for the MSI. 
  • A summary of the maintenance tasks that have been assigned for the MSI. 

Using MPC to Automate the Process 
ReliaSoft’s MPC 3 software has been designed to support the MSG-3 process for aircraft systems and powerplant analysis and to automatically generate a report of the analysis. The software was developed in conjunction with Honeywell Aircraft Engines and Systems and it is based on Honeywell’s MSG-3 Creator 2.0 software. The application is database driven and integrated with Microsoft Word for document output. The software facilitates the ability for multiple users and/or groups of users to work cooperatively on the analysis. The intuitive hierarchical structure of the interface makes it easy to define and manage the systems, subsystems, subsubsystems and parts included in the analysis as well as the functions, failures, effects and causes. MPC 3 includes built-in logic for determining the failure effect category and for selecting and assigning maintenance tasks. The software can then generate the systems and powerplant analysis report in a few seconds, ready-to-print from Microsoft Word. Figure 2 displays MPC 3’s system and function hierarchies, which are used to manage MSIs and their related functions, failures, effects, causes and tasks. Figure 3 displays the MPC 3 interfaces for failure effect categorization and for task selection and assignment. 

MPC 3 system and function hierarchies

Figure 2: MPC 3 system and function hierarchies

MPC 3 failure effect categorization interface

MPC 3 task selection and assignment interface

Figure 3: MPC 3 failure effect categorization and task selection/assignment interfaces

References 
Air Transport Association. ATA Operator/Manufacturer Scheduled Maintenance Development (MSG-3) Revision 2001.1. ATA Publications, 2001. 

Benoff, Dave. “MSG-3: The Maintenance Enhancer,” Business & Commercial Aviation, October 2000. 

“Applying MSG-3 to Out of Production Aircraft,” Aircraft Technology Engineering & Maintenance, Issue 50, February-March 2001.