Bjorn’ s Corner: New aircraft technologies. Part 47. MSG-3 Maintenance

By Bjorn Fehrm

February 2, 2024, ©. Leeham News: We are discussing the different phases of a new airliner program. After covering the Design and Production, we now look at the Operational phase of a new airliner family.

For the operational phase, the airplane must pass scrutiny for Continued Airworthiness. The biggest item in a regulator’s Instructions for Continued Airworthiness is the required Maintenance program to keep an airliner airworthy. We started last week with how maintenance went from ad-hoc to a Hard Timed maintenance program in the late 1950s and early 1960s.

Figure 1. The maintenance manual for the Boeing 747. Source: Boeing.

MSG Maintenance Programs

For an aircraft to keep its operational certification, it must be subject to a maintenance program approved by the local regulator. A local regulator, in turn, relies on a proposed maintenance program from the OEM, which today is developed in a prescribed procedure called MSG-3 (from Maintenance Steering Group 3rd version).

Last week, we described how aircraft maintenance programs started with the introduction of the big jets (Boeing 707, Douglas DC-8) in the late 1950s. The result was a Hard Timed maintenance schedule where the aircraft systems had parts disassembled at regular intervals.

However, these maintenance procedures were not optimal. Aircraft had incidents after maintenance, as the changed parts either had early failures (the early part of a failure bath-tub curve) or were incorrectly fitted or calibrated.

In 1968, the FAA, the Airliners’ Air Transport Association (ATA), OEMs, and Maintenance organizations formed the Maintenance Steering Group, MSG, to formulate a better maintenance concept than the Hard Timed one.

The result was a new decision logic on what maintenance different parts should have, where On-Condition maintenance was an important concept. Other ideas were based on “Do not touch a running system unless there are logical reasons to do so or indications are it needs attention.”

The result was the first maintenance program using MSG-1 logic for the new Boeing 747-100. The program, together with regulator-mandated maintenance limits coming from the System Safety Assessment (SSA), that is performed to achieve Design Certification (called Certification Maintenance Requirements, CRMs) and the limits for crack growth, etc., that comes out of fatigue analysis and testing (listed in Airworthiness Limitations, ALs), formed the approved maintenance program for the 747-100.

The instruction for how the maintenance should be done on the different systems, etc., was described in the aircraft’s maintenance manual, Figure 1.

The next refinement was the introduction of Condition-Monitoring, where values from a system were monitored, and only as these went outside the normal range a maintenance task was triggered. The MSG-2 program in 1970 added Condition-Monitoring to the Hard Timed and On-Condition maintenance procedures.

MSG-3 changes the analysis approach

MSG-1 and MSG-2 used a bottom-up analysis method, which resulted in a very high number of parts that should be tracked for Flight Hours, Flight Cycles, Calendar time, or any combination thereof.

As more complex aircraft were in development, the Air Transport Association (ATA) formed a task force in 1979 to change MSG-2 to a better analysis model. The result was MSG-3, which is used today in the industry for all airliners and most business jets. Now, the analysis was made top-down.

If it could be proven that a system with a failure had no flight safety consequence, no mandatory maintenance task was defined for the failure. Still, a maintenance recommendation could be defined, but then for economic reasons, to keep the aircraft well maintained and functional. As a result, MSG-3 separated flight safety-related maintenance tasks and economically motivated tasks.

The new generation of airliners then in development, like Boeing 757, 767, and Airbus A320/A330, have their maintenance program defined by the MSG-3 top-down approach, as has all airliners since.

In next week’s Corner, we will look at how an MSG-3 maintenance program is developed for the Airbus A319/320/321 series.

6 Comments on “Bjorn’ s Corner: New aircraft technologies. Part 47. MSG-3 Maintenance

  1. Great history and detail of how it has progressed.

    We tended to the if its working don’t mess with it (caveat if we detected an issue, fix it before it became a failure)

    And then we had an Aloha is an example of, pencil whipping where its not inspected or the maint is done at all (in my case it means greasing bearings and inspecting drive belts and replacing them if worn)

    Hidden things like gear box, the oil sampling did not work and you heard it failing before anything showed up on the oil sample (or you pulled the sample and it was yuck, we don’t care not will wait, replace it with a spare and send this thing in for a rebuild).

    Engine mfgs had some good data on rebuild, at best you got 80% as good as it came out of the mfg. 60% was what was used as a do it again.

    Standby generators did not fall into the maint lists, never got enough hours on them. They tended to get old and various seals and O ring would let go as those deteriorate over time.

    Lots of issues when you tried to apply metrics to equipment that did not fit or the recommendations were CYA and wildly out of reality.

    As one guy admitted, we had to put something there so we did.

  2. I know of one sitatuion where the Aircraft mechaics knew the plugs for ground power on the aircraft were badly worn. But it was a major job to get to the panel to remove it so they ignored it.

    They set the airplane on fire finally.

    We supplied some ground power to aircraft, we measured our plugs (female end) and did a tension check. The readings were logged.

    Truly a CYA as our end was not going to fail but we had no control over the Aircraft end. And as there was no quality follow up check, I could pencil whip it if I wanted to (did not want to, had the replacement parts to correct as needed and did so).

    From past practices, people are pencil whipping it since I quit as I did all of the plugs sans a guy I was training (who quit and got a better job of course)

    As I was leaving they were working towards all Electric Converted Ground Powered Units. We had two when I left, the rest were engine driven. If it had wheels, it belonged to the GSE group. They were no better but it was not on our head and frankly that is all that counted. We had no control of the rest.

    If they did convert then the Facility Maint end (mine) would have had 10 more units to maintain. If GSE was bad our group was worse other than the few who cared about their work.

  3. Bjorn,

    You’re tending to not talk about the importance of an airline Mx and engineering department reliability organization and the statistical analysis they provide regarding how the approved Mx program is working, or not working.
    The maintenance program is set forth by the OEM, as mentioned. The regulator approves the introduction of a new jet to an operator with this standard program.
    As operational data is gained in months and years of multitude of fleet problems the reliability people provide data to an M&E MRB (Mx review board). The key driver that is discussed is fleet type MTBUR and delay/cancelation rates charged to maintenance by specific ATA codes.

    With this data organizations may adjust their Mx programs up or down depending on the value for increased economics or sometimes better reliability. Sometimes on condition may change to hard time based on the MTBUR. It can be subjective tho to the regulator approval which is required for any Mx program change.
    Hard time life limited component times with landing gear and engines never change in the life span of the OEM airframe.

    Thank you.

    • Thanks, Airdoc.

      You are right, I have not treated the airliner side of the MX work. I guess I don’t have the knowledge, but it would be very interesting to do an article about it where you and another specialist could chip in using the comments feature. I will describe a typical OEM MPD on the 9th, and then on the 16th, we look at the airline work. It would be great if you could assist with your knowledge for the coming articles./ Bjorn

      • Bjorn,
        Let me how I can help. My airline 121 background is extensive and one reason why Boeing recruited me into the 787 Goldcare program. Thanks.

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