August 27, 2012


A briefing document  presented by Steven Bentley

The MSG (Maintenance Steering Group) Logic Process is now over 40 years oId.  (MSG-3 is 30 years old)

Consider that if we extrapolate backwards from the introduction of MSG we reach the 1930’s.

During the early days of aviation, maintenance programs were simple and without any real analytical process or in fact reliability programs to speak of.

As Aviation grew throughout the late 1950’s and into the 1960’s it was obvious that something had to be done in a different way. Post accident analysis also played a role in creating the demand for a more structured and regulated process.

The first steps saw the manufacturer (and in those early days of Jet Aviation there were many manufactures) The basic concept was to determine an appropriate time and to overhaul in fact all component (whether they need it or not was not a consideration).

During the latter part of the 1960’s a task force was created to investigate more cost effective (but equally safe processes) from this, led the concept we now call “on condition maintenance”.

A document was produced during 1968 by ATA ( Air Transport Association)  -titled Maintenance Evaluation and Program Development, which also became know as  “MSG-1,”  The process was trialed on the Boeing 747. The MSG-1 process used decision logic to develop scheduled maintenance.

MSG-1 / MSG -2 introduced three control process.

Hard Time limit: Maximum interval for  performing maintenance tasks on a part or unit. Such intervals apply to overhaul, but also to the total life of the part or unit.

On-Condition:  Repetitive  inspections or  tests to determine the condition of units or systems, comprising servicing,  inspecting,  testing, calibrating and replacement.

Conditioning Monitoring: Applies to items that have neither Hard Time limits nor On Condition maintenance, and simply means that the part is left to expire having been determined its failure is not of critical consequence. (however such items were considered as candidates for the Reliability Program)

MSG-1 was superseded MSG-2 in the early 1970’s and used a logic process which considered failures starting at the component level and moving up, with a focus on the understanding that all aircraft, engines and components, reach a period when they should be discarded or overhauled and returned to a “as new” condition.

Over time a number of drawbacks were found using the MGS-2 process

MSG-2 was not designed to consider economic effects, rather maintaining aircraft safety irrespective of the costs involved.

MSG 2  Hidden failures (to the pilots)  do not receive appropriate consideration

Because MSG-2 is a bottom up approach it was found to be more labour intensive

There was some inherent contradiction in the terminology for example “On Condition” and “Condition Monitored”

MSG-2 did not pay sufficient attention to modern Corrosion Prevention measures.

United Airlines in fact made a significant contribution to the further development with sponsorship from the US department of defense.  The project considered a different approach to deliver effective maintenance. This was again a logic driven process and in fact became the basis for the MSG-3 process, introduced in 1980 and still in use today (with several revisions).

1980 Original Issue, 1988 Rev 1,  1993 Rev 2,  2001.1 Rev 3,  2002.1 Rev 4,

2003.1 Rev 5,  2005.1 Rev 6,  2007.1 Rev 7,  2009.1 Rev 8,  2011.1 Rev 9.

The major difference with MSG-3 is that it is a task-oriented approach to maintenance using a methodology which looks at the various failure modes from a system level, or “top down”.

In addition economic considerations play a role  – Maintenance tasks are performed for safety, operational, or economic reasons.

The MSG-3 process provide for both preventative maintenance as well as  considering tasks to expose potential failures

Some of the major advantages of MSG-3

MSG-3 is a Top-down process, which enables a step by step systematic analysis.

MSG-3 delivers lower maintenance costs with typical savings ranging from 15% to 25% for the same aircraft type on conversion from MSG-3 to MSG-3

MSG-3 typically delivers a substantial cost reduction in hard time  component removal and replacement

MSG-3 results in fewer maintenance tasks but not the importance of managing competencies.

Some MSG-3 tasks are carried out for economic reasons, while others are carried out to deliver an improved safety level.

Based on an effective and efficient model MSG-3 has continued to be developed to include as a core consideration  CPCP (Corrosion Prevention and Control Program), Structural Significant Item Inspection, and Enhanced Zonal Analysis, which introduced the term EWIS Electrical Wiring Interconnect Systems, and Lightning/High Intensity Radiated Fields.

Development work continues with Type Certificate Holders continuing to work with aircraft operators, regulators, and the ATA to update MSG-3 to consider improvements to the methodology.

The MSG 3  process applies a function, failure, failure effects and causes analysis on each element  using a Failure Modes Effect Analysis (FMEA). Potential damage is considered possible from accident, environment or fatigue .

Working Groups consider Structure, Systems, Subsystems, Engines, and Zonal with the work being performed by the Maintenance Review Board (MRB). The output of the MRB is known as the Maintenance Review Board Report (MRBR).

The main output features of the MSG-3  include On Condition / Hard Time and a very important and much miss understood process called the Zonal Inspection Program ZIP.

The MSG system has changed significantly over the last 20 years, MSG-3 is an effective process which can be applied to both aircraft and poweplants.

The MRBR is the basis of the Maintenance Planning Document (MPD).

The “MRB process” consists of all the activities performed to produce, review, accept and amend the Maintenance  Review Board Reports (MRBR) and Supplement to MRBR.

This work instruction developed by the Industry Steering Committee provides guidelines that may be used during the development and revision of the initial minimum scheduled maintenance requirements for derivative or newly type certificated aircraft.

The same process must be followed by the Regulatory Authority of the state of the manufacturer when it has been requested by the Applicant to provide assistance to the Applicant during the Applicant’s compliance demonstration to Certification


The MRB process is applicable:

1. For Large Aeroplanes and Category A Large Rotorcraft where the EASA is the Primary Type Certifying  Authority or for Large Aeroplanes and Category A Large Rotorcraft for which an applicant has applied for  EASA type approval. The TC applicant is required, in respect of new or derivative Large Aeroplanes above  13000 kg maximum take off weight and Category A Large Rotorcraft above 9072kg to make application for a Maintenance Review Board (MRB) to the EASA Flight Standards MRB Section unless an alternative process  has been agreed (see note below).

2. For aircraft between 5700 KG and 13000 KG maximum take off weight the TC applicant may make application to the EASA Flight Standards MRB Section for an MRB.

Sofema Aviation Services is  pleased to offer Trainings in the following related subjects EASA Compliant Maintenance Planning, EASA Compliant Reliability Processes.  Please visit or email


Maintenance Steering Group, MSG-3