Scheduled Maintenance Tasks

March 1, 2000

Scheduled Maintenance Tasks

Working through the development process with the Maintenance Steering Group

By Jack Hessburg

March 2000

Jack Hessburg has over 40 years experience in aircraft maintenance. He is recently retired from The Boeing Co. where he served as Chief Mechanic. Hessburg holds an A&P certificate and a degree in Mechanical Engineering.

How are maintenance tasks developed? The airworthiness regulations identify a requirement titled Instructions for Continuing Airworthiness. For example, FAR 25.1529 and FAR 25, Appendix H cover transport category airplanes. The contents of these FARs impose, among other things, the following obligations upon the applicant for a Type Certificate:

"Scheduling information for each part of the airplane and its engines, auxiliary power units, propellers, accessories, instruments, and equipment that provides the recommended periods at which they should be cleaned, inspected, adjusted, tested, and lubricated, and the degree of inspection, the applicable wear tolerances, and work recommended at these periods. Also, "The recommended overhaul periods and necessary references to the Airworthiness Limitations, which set forth each mandatory replacement time, structural inspection intervals, and related structural inspection procedures. In addition, they must include an inspection program that includes the frequency and extent of the inspections necessary to provide for the continued airworthiness of the airplane." And, "ÉAccumulated flight hours, calendar time, number of operating cycles or the number of landings are the generally accepted measurements used when specifying maintenance intervals."

Maintenance Steering Group dynamics
A group known as the Maintenance Steering Group (MSG) develops the maintenance requirements identified by these FARs. This group is composed of representatives of airplane, component and engine manufacturers; air carriers, both foreign and domestic; and FAA and foreign regulatory agency observers.

The MSG is divided into three bodies; the steering committee, working groups, and FAA and foreign regulatory observers.

The steering committee supports and directs working groups. It defines the number, type, and composition of working groups; documents group proposals into a unified maintenance inspection plan; and coordinates MSG activities with the certifying FAA region.

The working groups identify and analyze maintenance significant items, beginning with the original list prepared by the manufacturer. The number of groups is determined by the steering committee. Usually there are separate groups for each system, i.e. hydraulics, flight controls, structures, etc. Membership in each group is composed of specialists from the interested disciplines. The exact size and composition of the working groups is quite flexible and depends upon the needs of the moment as the analysis proceeds. Sub-groups may be formed to resolve complex issues. The manufacturer's personnel play a large role, counseling the groups regarding details of the design.

FAA observers, usually members of the FAA Maintenance Review Board (MRB), offer guidance and participate in the work of the steering committee and working groups. This has several advantages such as the (hopeful) resolution of regulatory issues regarding the recommendations and time saved discussing details of the analysis to name a few, when the MSG formally submits its recommendation to the FAA. Today, observers from various foreign regulatory agencies such as the JAA also participate in the process. MSG methods

The methods used by the MSG are described in a document issued by the Air Transport Association (ATA) titled Maintenance Program Development Document MSG-3 and include defining Maintenance Significant Items (MSI)

Very early in the design, the manufacturer begins by drafting a preliminary list of Line Replaceable Units (LRUs), system installations and items of aircraft structure. The list contains items considered "significant" in their maintenance requirements to justify establishing required maintenance inspections/checks. The beginning of MSI isn't very scientific -- it consists of gathering a collection of old heads with graybeards and lots of experience in design and maintenance who look at the preliminary design. Based upon their collective wisdom and experience, they construct this initial list — it's not rocket science. Also, the list is not static. It changes as a result of the working group's analyses and inputs, design changes, etc. throughout the design and certification.

MSG-3 uses a methodology called "decision tree" logic and is organized to uncover hidden failures and separate the safety-related items from economic failure in the design. Methods for defining servicing and lubrication tasks are included. The list is analyzed by the working groups for importance of the following:

Safety-related items: Any system or component malfunction that results in the loss of airworthiness is by definition safety related. Is the malfunction readily apparent to mechanics or pilots? Is it hidden? The answer to these and other questions help to define tasks.

Potential economic impacts: The analysis examines such issues as high initial design, manufacturing and ownership cost, high maintenance cost, premature removal rates, significant access problems, potential for mechanical dispatch delays, etc.

System or component redundancies more than that required for airworthiness maintenance recommendations: The product of the working groups' activities is a list of inspection and check tasks along with recommended check intervals. Inspection intervals may be expressed as flight time, calendar time, takeoff and landing cycles, or pressurization cycles as may be appropriate to the item analyzed.

The maintenance-inspection recommendations are assembled into a document called The Maintenance Requirements and Review Proposal. The manufacturer submits the proposal to the FAA in fulfillment of the requirements contained in Part 25, Appendix H quoted previously.

The FAA convenes an internal Maintenance Review Board (MRB) for examination and approval of the proposal. Final issues are identified and resolved and the product of this review is known as the Maintenance Review Board Report (MRB). This is a living document. Once the airplane is placed in service and experience is gained with the design, items may be added or deleted from the report. Intervals for task accomplishment may be escalated.

Developing the scheduled maintenance tasks for a new airplane is long and very costly. Consider that for the 777 airplane, the project of defining tasks was begun in early 1990. The original MRB document was adopted by the FAA in 1995, just two weeks before the airplane was certified. The project involved several hundred people from all over the world. It included mechanics, design engineers, maintenance engineers, regulators and countless other skills. I can't begin to imagine the total labor hours expended or the total dollars used.

The MRB is the framework around which each air carrier develops its own individual maintenance program for the airplane. The essential elements of the MRB are included in the airline's Operations Specifications Part D, which defines the maintenance program. Ultimately, the MRB items are translated into a series of task cards that you and I use when we are doing "Scheduled Maintenance Tasks."