Initial Scheduled Maintenance Plans
Using the MSG-3 Aircraft
Systems and Powerplant Analysis Process to Develop an Initial Scheduled
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.
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.
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
- 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
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
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.).
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.
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
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?
there a task or combination of tasks that are applicable and effective?
guidelines also provide definitions of the available task types as well as
criteria to determine the applicability and effectiveness for each type of
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
- 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.
summary of the maintenance tasks that have been assigned for the MSI.
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
2: MPC 3 system and function hierarchies
3: MPC 3 failure effect categorization and task selection/assignment interfaces
Air Transport Association. ATA Operator/Manufacturer
Scheduled Maintenance Development (MSG-3) Revision 2001.1. ATA Publications,
Benoff, Dave. “MSG-3: The Maintenance Enhancer,”
Commercial Aviation, October 2000.
“Applying MSG-3 to Out of Production
Aircraft,” Aircraft Technology Engineering & Maintenance, Issue 50,