Aircraft Maintenance - What is MSG?

Hello guys, we're going to talk about what is MSG today.

What is MSG

MSG, Maintenance Steering Group, is a document with maintenance design concepts issued by ATA, Air Transport Association of America.
As I told you last time, Airline will compose their Maintenance Requirement by Maintenance Planning Document, which is provided by Manufacturer.
Refer to: Aircraft Maintenance - Why do we maintenance?
MSG-3 is used to determine initial scheduled maintenance requirements.
MSG will be an original consideration for the MPD. We may talk about all the resource for MR in the future.

History of MSG

As the figure below, you can see the MSG developed by time and aircraft type.
It's interesting that once european try to make a similar document called EMSG for european aircrafts, but they also follow the newest MSG-3 in the end.

Before MSG

Before MSG appeared, people only consider that the failure come from abrasion, so they only had the plan to replace parts at specific timing (Hard Time). For the unscheduled maintenance, it counted on mechanic's experience only. It's costly and inefficient for maintenance.

MSG-1/MSG-2

In MSG-1 and MSG-2, people developed a logic to each component.
They divided the maintenance process into three category:

• Hard Time (HT)
• On Condition (OC)
• Condition Monitoring (CM)

As I mentioned above, Hard Time means replacing the parts in accordance with the schedule.

On Condition means there are scheduled checks for the parts, and the mechanic will replace the parts in accordance with the result of the checks!

Condition Monitoring is the same thing I mentioned in Reliability Program.
Engineer monitors the aircraft condition through maintenance records, pilot reports etc.
Someone described it as finding a dead tree in a forest.
I draw this concept as a picture below.

You can also understand the logic of MSG-1/MSG-2 more by the flow chart below.

MSG-3

As time passing by, it became confused to interpret the three maintenance category, so people developed MSG-3, which took a "from top to down" logic.

Of course, the key ideas are:

• SAFETY
• ECONOMIC

In MSG-3, the maintenance tasks were divided as the picture below:

  

Compare the tasks of MSG-3 to MSG-2:

Three Analysis in MSG-3

There are mainly three analysis in MSG-3.

System Analysis

In system Analysis, the first thing needs to do is to find out the MSI, Maintenance Significant Items. The experts will find it out through ATA chapter systems, then sub systems, sub-sub systems, into the end parts. That's why we called it "from top to bottom" logic. This logic will make the process be higher manageable. 

Then, the experts will try to analyze each MSI:

1. The normal functions of each MSI
2. What failures may happen 
3. What the failure may result
4. What's the cause of the failure

The experts will do the level 1 analysis through function failures in to 5 categories.

They will do the level 2 analysis from the failure cases in order to select which maintenance task need to be done to prevent or solve it.

Structure Analysis

Before talking about how the experts analyze the structure items, it's also very important to know the structure design development.

1. Safe Life: it's the same concept of Hard Time. The structure is stronger then it's supposed loading, and it was designed to be discarded before specific period.
2. Fail Safe: there are a pair of parts with the same capability for the supposed loading. As the result, if one of them failed accidentally, it's still safe for passengers.
3. Damage Tolerance: It's the main idea for the modern aircraft structure. The part was designed stronger then it supposed to be. However, it's allowed to have certain failure on the part until the limit. If the failure is a crack, we may call it critical crack at the limit. The picture shows the simple idea of damage tolerance, but in reality, mechanic will fix it before the crack growing into a critical crack.
4. The newest concept is CPCP, Corrosion Prevention and Control Program. The experts considered more on prevention at this moment. This is a big topic that we may talk about it in the future article.

Similar to system analysis, the first thing is to find out SSI, Structural Significant Items.

SSI is an detail, element or assembly which may affect safety if it's failed.

PSE, Principle Structural Element is any element which may lead catastrophic consequence if it's failed.

For the structure items which are not classified to SSI are defined within zonal boundaries.

After finding SSI, the experts will consider the three damage source to find out all the possible structural failures, and the consequences of the failures.

Then they will find out the methods to prevent, control the failures, and define the inspection thresholds and repeat intervals.

Zonal Analysis

Zonal analysis feels like picking up the remaining issues which were not considered in the previous analysis.

You can refer to the figure below for how they analyze. I didn't put everything here. It's just a simplified process.

The mains task in Zonal Analysis are GVI, general visual inspection, and DET, detailed inspection.

Conclusion

After all the introduction above, let's make a comparison for MSG-2 and MSG-3.

This article took me over a week to prepare all the pictures.

I hope you guys would like it.

Please leave message to me if there is something wrong, or something you want to know more, or discuss with  me.

I also found a very professional article about the application of MSG-3.

Please refer to that article if you guys want to know more about MSG-3.

http://www.aviationtoday.com/am/repairstations/Understanding-MSG-3_33062.html

Thank you, and see you next time. Ciao!