The issue of the effects of corrosion on structural integrity of aircraft has been a question of concern for some time. The Maintenance Steering Group (MSG) system has evolved from many years of corporate knowledge. The first generation of formal air carrier maintenance programs was based on the belief that each part on an aircraft required periodic overhaul. As experience was gained, it became apparent that some components did not require as much attention as others and new methods of maintenance control were developed. Condition monitoring was thus introduced in the decision logic of the initial maintenance steering group document (MSG-1) and was applied to Boeing 747 aircraft.
The MSG system has now evolved considerably. The experience gained with MSG-1 was used to update its decision logic and create a more universal document for application to other aircraft and powerplants. When applied to a particular aircraft type the MSG-2 logic would produce a list of Maintenance Significant Items (MSIs), to each of which one or more process categories would be applied, i.e. 'hard time', 'on-condition', or/and 'reliability control'.
The most recent update to the system was initiated in 1980. The resultant MSG-3 system is based on the basic philosophies of MSG-1 and MSG-2, but prescribes a different approach in the assignment of maintenance requirements. Instead of the process categories typical of MSG-1 and MSG-2, the MSG-3 logic identifies maintenance requirements.
The processes, tasks and intervals arrived at by the use of MSG can be used by operators as the basis for their initial maintenance program. In 1991, industry and regulatory authorities began working together to provide additional enhancements to MSG-3. As a result of these efforts, Revision 2 was submitted to the FAA in September 1993 and accepted a few weeks later. Major enhancements include:
The MSG-3 structures analysis begins by developing a complete breakdown of the aircraft systems, down to the component level. All structural items are then either classified as Structurally Significant Items (SSIs) or other structure. (reference)