Demystifying MSG-3 (Part 2)

In the first part of this series, I had dwelled upon the development and origin of MSG-3 and what were the factors that led up to it being recognized as the industry standard for development of a scheduled maintenance program. In today’s write up I would like to undertake a deeper dive into MSG-3 and what are the fundamental principles driving it.

To recap MSG-3 is a tool utilized by the Maintenance Review Board to develop the initial set of scheduled maintenance.

The objectives of an MSG-3 based scheduled maintenance program is as follows

-????????? To ensure inherent reliability of safety and reliability

-????????? To restore safety and reliability when deterioration has occurred

-????????? To accomplish the above two goals in the most economic manner.

An MSG-3 analysis consists of three primary separate analysis.

Systems Analysis, Structural Analysis and Zonal Analysis. Zonal Analysis also includes Enhanced Zonal Analysis (EZAP) and EWIS (Electrical Wiring Interconnection System).

Systems Analysis

The Systems analysis consists of the following steps

1.?????? Identification of MSI (Maintenance Significant Items) – This uses a Top-down approach to identify all functional areas starting at the ATA level and working down to sub-ATA and individual System level. Each item thus identified is called as candidate MSIs.

2.?????? MSI Definition: From Candidate MSIs we need to select the MSI.

Each MSI that will be selected needs to answer “YES” to at least one of the following questions

-????????? Could failure be undetectable by crew as part of their normal duties?

-????????? Could failure affect safety?

-????????? Could Failure have an Operational impact?

-????????? Could Failure have an Economic Impact?

Once an MSI has been selected its detailed functional features and descriptions are documented. This would include System description, interfaces, component descriptions, high level block diagrams, protective features and crew alerting systems associated with the function.

?3.?????? Functional Failure analysis: (Level 1) Beginning with the detailed review of the function of the MSI, an analysis is carried out to determine, if the function is evident or hidden and whether the resulting functional failure will be evident or hidden. The next step is to determine the effect of the functional failure. The failure effect analysis determines the effect of the functional failure by asking a series of question based on a logic diagram as shown below:

The lowest blocks define the five Failure effect categories (FEC):

FEC 5/FEC 6/FEC 7/FEC 8/FEC 9.

?4.?? Failure Cause Analysis & Task Definition (Level 2)

The next step is to determine the causes of the failure and select the task(s) suitable for the mitigation of the Failure effect. ?A series of questions are applied from the MSG-3 logic diagram to determine tasks that are both Applicable and Effective. When the risk cannot be reduced by either of the tasks or a combination thereof, redesign of the system is mandatory.

The type of Logic diagram to be used depends on the FEC Category and I provide one such example below.

Depending on the result of the Level 2 analysis one of the following or a combination of the tasks is possible

a.?????? Lubrication/Servicing (All categories)

b.?????? Operational/Visual Check (FEC 8 or 9) – Hidden Failure Related.

c.?????? Inspection/Functional Check (All Categories)

d.?????? Restoration (All Categories)

e.?????? Discard (All Categories)

f.??????? Combination (FEC 5 & 8 only)- Safety related

5.???? Task Interval determination: Once a task has been identified the final step is to determine its Interval. The interval is expresses in Flight Hours/Flight Cycles/ Calendar based i.e. Days, Months, Years or in some cases individual component life such as Engine Hours/Engine Cycles/APU Hours/APU Cycles.

The interval is determined using a multitude of factors such as but not limited to consequences of failure, ability to detect degradation, potential failure to functional failure curves, historical or predicted reliability, ?design maintenance and engineering judgement, vendor recommendations, manufacturer/operators experience etc.

Structural Analysis

The structural analysis considers fatigue, corrosion, environmental deterioration and accidental damage. It recognizes new damage tolerance, multiple failures, affect on adjacent structure, crack growth and supplemental fatigue related inspections. The MSG-3 logic ensures that CPCP is included as part of the structural analysis.

Each structural items is assessed for significance to continuing airworthiness, susceptibly to damage and ease of inspections. The are termed as SSIs (Structural Significant Items). SSI is defined as any element or assembly contributing significantly to carrying flight loads, ground loads, pressure loads or control loads where the failure could affect the structural integrity of the aircraft.

For SSIs there are three proposed Inspection Levels

GVI: General Visual Inspection, A visual examination of an interior or exterior area, installation or assembly to detect obvious damage, failure or irregularity. This level of inspection is made from within touching distance, unless otherwise specified and under normal lighting conditions.

?DET/DVI : Detailed Visual Inspection: An intensive examination of a specific item, installation or assembly to detect damage, failure or irregularity. Available lighting is normally supplemented with a direct source of good lighting at an intensity deemed appropriate. Inspection aids such as mirrors and magnifying lenses may be necessary. Surface cleaning and elaborate access procedures may be required.

?SDI : Special Detailed Inspection, An examination of a specific item, installation, or assembly making use of specialized inspection techniques such as Non Destructive Testing (NDT) and/or equipment (e.g. boroscope, videoscope, tap test) to detect damage, failure or irregularity. Intricate cleaning and substantial access or disassembly procedures may be required.

?As part of structural analysis each SSI is reviewed for Accidental Damage and Environmental Damage and accordingly provided a rating. Rating and associated intervals are determined by means of a rating table.

Accidental Damage assessment takes into consideration, type of damage, probability of damage and probability of detection of damage. Environmental damage assessment takes into consideration the exposure of an SSI to adverse environmental conditions and the visibility of damage. The environmental damage review will also include the CPCP review.

As part of structural analysis if the selected inspection level is GVI , the respective SSI may be considered as a candidate for a transfer to Zonal Program. Furthermore all structure items not classified as SSI are automatically included as part of the Zonal Program.

Zonal Analysis

The Zonal Analysis Program consists of the Standard Zonal MSG-3 and the Enhanced Zonal Analysis Procedure (EZAP).

The intent of the zonal program is to cover security and general condition of areas not covered by the Systems or Structures Program. Zonal Inspections are mostly of ?GVI type. Zonal Inspections would typically look for corrosion, security of installation (of plumbing, wiring, ducting, cables etc.), indication of any fluid leakage. The guidance material in the MPD/MRBR as well as associated AMM will details the requirements of zonal inspections. EZAP in combination with Standard Zonal MSG-3 looks at inspections of Electrical Wiring Interconnection Systems.

Each zone and sub zone Is analyzed for susceptibility to damage, inspectability (how dense and accessible is the zone in question) and impact of environment. ?

Based on the above Inspection and Damage rating are assigned to each zone and appropriate inspection interval is determined based on Rating Tables.

The function of the EZAP (Enhanced Zonal Analysis Procedure) is to ensure the functionality of the Electrical Wiring Interconnection System and reduce potential for corresponding arcing. The likelihood for buildup of combustible material will result in a Cleaning Task as part of the Zonal Analysis.

On a final note the Zonal Tasks also includes considerations from L/HIRF Lightning and High Intensity Radiated Field analysis. This analysis looks at the long term robustness of the HIRF protection systems and their degradation. LHSIs are assessed for environmental and accidental damage and accordingly GVIs are identified which are incorporated as part of the Zonal Inspection Tasks. ?

The above was a scratch on the surface of the immensely detailed procedure that is MSG-3. The idea was to give aviation professionals who are not directly involved in Maintenance Program as overview of the MSG-3 Process.

In my final article in this series i will discuss how the industry uses MSG-3 to create the MRBR and finally the MPD.

to be contd.......................