Aircraft Component Maintenance – Main Issues

The maintenance of aircraft components is a story of its own, and in this introductory post, I will try to go through some of the basics and the general idea of what an aircraft component actually is and who can do maintenance work on it.

Aircraft Component Maintenance

Let’s start with the second question – who can do maintenance on aircraft components? A Part 145 approved maintenance organization, with an A-class approval (for aircraft) can carry out maintenance on aircraft. However, an aircraft is – quite obviously – made up of hundreds or thousands of components. So how can deep can an aircraft maintenance organization go? It’s quite obvious that we can remove an engine from an aircraft. But can we remove a fuel pump from the engine? And if so, are we allowed to replace the driving shaft of that fuel pump, would this still be aircraft maintenance?

Maintaining components as part of the aircraft

The answer, as usual, lies within the aircraft technical documentation and relevant legislation, Part 145 in this case, for our EASA example. The legislation lists what maintenance manuals should be used with reference to aircraft maintenance, and those include mainly the AMM (aircraft maintenance manual) and IPC (illustrated parts catalogue). However, also the CMM (component maintenance manual) is listed as possible aircraft maintenance data. So what can we do?

In fact, this is actually quite simple. The AMM will tell you what you are allowed to do on an aircraft and hence, what technical actions are considered aircraft maintenance rather than component maintenance. It is possible to perform maintenance tasks in accordance with a CMM provided the AMM refers to it and tells you to perform those tasks. The regulation itself allows for component maintenance in accordance with a CMM under A category approval provided the component does not need to be removed from the airframe in order to undergo such maintenance unless it is being removed only for the purpose of obtaining better access. This may sound complex, but it really isn’t – the general idea is that you can do component maintenance under the aircraft category in two cases:

  1. When the AMM tells you to do so by providing appropriate instructions or by pointing to a relevant CMM
  2. When the maintenance of the component does not require the unit itself to be removed from the airframe except for the purpose of improving technical access. This means that you cannot perform any component maintenance which requires shelf tests or mechanical finishing, for instance.

Furthermore, the IPC which I mentioned earlier, gives you a list of all aircraft parts and components. This means that all the part numbers which are listed in the IPC can normally be replaced on the aircraft. In other words, if the IPC lists a component as one part number and does not list the specific pieces which make up a given component, it should generally be considered as a standalone unit, which needs to be maintained under a component maintenance approval.

So now we know what component maintenance is not. So how do we go about fixing the components which are being removed from the aircraft?

Maintaining components on their own

The ability to fix components requires a different kind of Part 145 approval – an approval within the C category. There are several of those categories, numbered C1, C2, C3 .. and so on and they are more or less structured by ATA chapters (so you get categories like hydraulics, or landing gear). This approval allows you to do something, that is not allowed when you maintain components in the A category as described above – it allows you to issue a component release certificate in the form of an EASA Form One or equivalent (like the FAA Form 8130).

The components release certificate is a document stating that the component you have just performed maintenance on is ready for release to service within the limits prescribed in relevant documentation or set forth by your limitations (workshop limitations). The EASA Form One is mandatory for operators (your customers) to be able to fit the component to their aircraft.

Typical component maintenance events

There are three main component maintenance events which are generally performed by specialized shops:

  • Aircraft Component Overhaul
  • Aircraft Component Repair
  • Aircraft Component Modification

All of those have their own specifics and require a slightly different approach from both the operator / airworthiness provider and the workshop. Let’s look at them in slightly more detail.

The overhaul of aircraft components

Even though modern aircraft strive away from hard time maintenance, and condition monitoring is becoming the main approach for maintenance steering groups, many components still require regular overhauls once in a while.

An overhaul is sometimes called a major repair, although personally I don’t like this term as it is highly confusing. In most cases, when an overhaul is required, the operator will provide the workshop with a component which is generally serviceable except for the fact that it has reached it’s hard time limit. This means that there will be no defect syndromes to take care off and no defect description as such.

Overhauls are defined either in component maintenance manuals (CMM) or in specific overhaul manuals (OM) although the latter is generated mainly for major components such as engines, propellers or landing gear. Whatever its name, the technical documentation specifies exactly what needs to be accomplished during an overhaul of the given aircraft component. Such tasks generally include:

  • Dismantling of the component to given detail
  • Thorough detailed visual inspections and special inspections (such as NDT) of selected parts
  • The discarding and replacement of parts which have life limits within the components or need to be replaced whenever the component is torn down (such as seals)
  • Cleaning, fixing of known and detectable defects
  • Reassembly
  • Thorough bench testing of the component

As a result, we get a component which is treated as new without additional limits imposed. What’s worth noting, however, is that especially for major components (like landing gear or engines) there are life limited parts within a components, and their life limit is often longer than the TBO (time between overhaul) for the component itself. As a result, we are allowed to place a “used” part into a component during overhaul. In order for this to be possible, the repair shop needs to obtain from the operator or airworthiness provider accurate and properly signed life limit details on the part – this is the workshops only source of information regarding the parts inside the overhauled component. Without this, they will not be able to release the component to service and hence will have to replace all parts with brand new ones, which is generally quite expensive and an unnecessary cost as such.

Aircraft component repair

The repair of aircraft components is slightly different from a complete overhaul, as the workshop needs only to determine what causes a given defect, rectify that defect, run some bench tests and release the component back to service.

The trouble shooting procedures are generally available in the CMM or the CRM (component repair manual). Therefore, in most common scenarios, the procedure is as follows:

  • Check the documentation for a trouble shooting (T/S) procedure applicable to the given defect symptoms
  • Carry out all tests in accordance with the procedure
  • Determine the fault
  • Eliminate the fault in accordance with an appropriate procedure from the CMM/CRM
  • Do reassembly tests and release the unit

As simple as this may sound, there is something that every operator or airworthiness provider needs to understand and remember:

Whenever a component is torn down (“opened up”) in a workshop, it is the workshop responsibility to assess its general state (not only with respect to the reported defect, but as a whole). This means, that a simple defect may end up requiring even a complete overhaul if, after the components is opened, it turns out that several parts and subassemblies need replacing due to excessive wear and tear. This always increases costs and is very difficult to plan for because, for parts which are not hard time controlled, it is almost impossible to assess their condition without bench testing them at least.

Aircraft component modification

Quite often, components need to be modified. Sometimes the modification is intended to improve the usability of a given component and is voluntary, at other times a modification is required for instance during every overhaul.

Modifications can have several forms, from introductions of new, modified component sub-assemblies to the replacement of software in modern, electronic aircraft components.

A modification is rarely done on its own – most often it is performed during an overhaul or a repair (generally speaking – during a shop visit) of the component. It is either ordered by the operator or airworthiness provider if it is voluntary, or performed by the workshop automatically if it is mandatory.

The fact that a modification has been applied to a given component must be made clear on the release document (EASA Form One) and recorded in the operator’s airworthiness record system.

I hope that this brief post was somewhat helpful. If you have any ideas, questions or anything you would like to share – please leave a comment below or email me!

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Posted in Maintenance, Maintenance of Components
27 comments on “Aircraft Component Maintenance – Main Issues
  1. Malcolm Hales says:

    You`ve forgotten Inspected/Tested!

    • Mike says:

      Hi Malcolm, thanks for pointing that out! Indeed, per the EASA Form One tag we also have the Inspected / Tested category. It was my intention to write about why components are being sent to shop and what can actually be done with them. But you are correct, and I will update it in my future articles, as I want to go back and write a bit more about component maintenance. The inspection is important, as it can be an MPD task or even an AD requirement.

      Thank you for pointing this out! 🙂

    • Leonard Abbott says:

      Hi Mike – a very interesting article, I have one question in respect of a component requiring inspection / test. If a component is removed and replaced from an aircraft that is due an inspection in accordance with the aircraft maintenance program and the inspection criteria is documented in the aircraft maintenance manual (AMM); Is it acceptable to utilize the aircraft AMM inspection procedure to return the component to serviceability on an Component Release Certificate such as EASA Form One or FAA 8130?

      • Mike says:

        Hi Leonard,

        Interesting question. It’s probably best if you consult your CAA, but I highly doubt you can do that. You can certainly use the AMM procedure to do maintenance on said component, install it back on the aircraft and sign it off on the aircraft maintenance task card. But I don’t think it would be aceptable to sign it off on an EASA Form One and then, for example, sell it. But again – it all depends on your internal procedures which must be approved by your CAA, so maybe they could help.

  2. Keith Anderson says:

    Hi Mike, A very informative article, thank you. So to enable a non 145 company to service components and issue Form 1, (Battery overhaul, Test and repair of thermal switches/sensors etc), the company needs to specify what parts it wants to overhaul and apply for Part-145 sub-part C in those categories. Is that correct please? Just been tasked by a spares supplier to set up a Battery bay and basic electrical components test workshop. All handy hints gratefully accepted!

  3. Mike says:

    Hi Keith. Yes, if you wish to service any components, including batteries, your company will need to be Part-145 certified. To do this, you will have to comply with the regulation, of course. This includes creating the manual (MOE), obtaining all approved maintenance data for the components you wish to service, hiring the right personnel, etc. … It’s a bit of work, but it’s the only way (if you wish to service the European market, which I assume is the case).

  4. adam says:

    Hi Keith,

    You have written:

    The legislation lists what maintenance manuals should be used with reference to aircraft maintenance, and those include mainly the AMM (aircraft maintenance manual) and IPC (illustrated parts catalogue). However, also the CMM (component maintenance manual) is listed as possible aircraft maintenance data.

    1. Does this mean the AMM should include a list of all applicable CMM’s referenced throughout the AMM and IPC?

    2. Should the AMM also detail the revision numbers of those CMM’s referenced thought the AMM?

    • Mike says:

      Hi Adam, thank you for your questions.

      1. No, I don’t think the AMM would typically include a list of CMMs. But it may reference them throughout the text.

      2. Again, very unlikely. The “reference” I mentioned above will likely be in the form “do something in accordance with the applicable component maintenance manual”. It is up to the operator to obtain the most current revision of the proper CMM.

      • Dave says:

        From my experience as an OEM AIPC author, any component having a CMM should have the ATA/CMM number referenced within the description section. But not the revision level of the CMM.

  5. Rowland says:

    Why some CMM have a lot of listed PNs? is it mean same Make/Model?

    • Mike says:

      It is possible that one CMM is issued for several different PNs. Those will, typically, be the same part but one that was modified and improved over the years, or one that fits different aircraft types and is “almost” the same. The CMM body then most often contains separate sections for different part numbers.

      Also, in most cases, the differences in the PN will only be in the dash number. So you could have a PN like XXXX-01, XXXX-02, XXXX-03 etc which are very similar components with only some very minor differences.

  6. Sohail says:

    The person who inspect the component after coming from workshop he is known as?

  7. Mike says:

    Hi Sohail,

    Probably an incoming inspector or something similar. This is typically done in the shipping / receiving department. Every component that arrives, be it from a shop or from the manufacturer, needs to undergo an incoming inspection, to check for the state of the component, the package, paperwork, etc.

  8. Bahhar says:

    Hi Mike,

    A very interesting article, thanks for those valuable highlights.
    As I’m working on a project of improving the quality of CMM, we receive a lot of customer claims ( from Airlines or Airframers) about some mistakes in our technical publications ( espacially CMM), so every time we need to update our CMMs to take into acccout those technical mistakes and that costs a lot ! . So Is there any clues that you can help on that matter ?

    really appreciate your help,

    With regards

    • Mike says:

      Hi Bahhar,

      Thank you for your comment. Unfortunately, I dont think I can be of much help in this case. The CMM needs to be accurate, and according to the regulations the operator must inform you if they find any problems, and as the manufacturer, you must ammend the CMM accordingly. Maybe a brainstorming session, or some other communication with the airlines / your clients would help to understand how the problem could be tackled?


  9. Edison says:

    Informative article. I have question, Is it true that aircraft maintenance cost grows higher and higher when the aircraft getting older and older?

  10. AdamEbook says:

    Hi Mike
    Thanks for your awesome articles to help community as well as who are thinking to reduce aircraft maintenance cost and component maintenance as well.

  11. shahriarthm says:

    Super tips for aircraft maintenance manuals
    That might be informative for you

  12. Sergey says:

    Dear Mike,

    Could you clarify the following question regarding the repair of aircraft components iaw CMM by EASA Part 145 AMO with A-rating:
    – You have mentioned that “It is possible to perform maintenance tasks in accordance with a CMM provided the AMM refers to it and tells you to perform those tasks” and “When the AMM tells you to do so by providing appropriate instructions or by pointing to a relevant CMM”. I didn’t find this requirements in EU regulation (M.A. 502 (b) and Appendix IV of Part M) or in AMC/GM/UG (M.A. 502 (b) and Appendix IV of Part M). I mean that AMM should include or refers to CMM for such repair (maintenance) by the Part 145 AMO with A-rating.

    Could you advise from where it is coming or give a reference to the regulation (document) with the requerements?

    I saw this statement in some Part 145 MOEs but why it should be added (make procedure stricter because not all AMM tasks have reference for CMM in case of repair needed) if there is no requirements in the regulation?

    Thank you in advance.

  13. Jays says:

    Hi Mike,

    Thank you for the wonderful piece of information, I’m more interested in the repair process in the assembly line. Could you please tell me if there are any standard processes that have to be followed if a defect is found during the aircraft assembly.

    Thanks in advance

  14. Vladimir says:

    I have one qwestion about components and plane registrations
    Iif two different aircraft have different registration their components cannot be changed. Where is it written?
    Thank you SIR.

  15. Carl Green says:

    It’s good to learn that plane parts listed in the IPC can be replaced on an aircraft. My brother is wanting to become an aircraft mechanic and he was wondering how he could replace specific parts on an airplane. I’ll be sure to tell him that the parts listed in the IPC can be replaced.

  16. I found it interesting when you said that the workshop needs only to determine what causes a given defect so its process is different from a complete overhaul. This is a great tip for my father who wants to purchase a 2-gallon oil dispenser to better protect his helicopter from destructive moisture. I could imagine how your tips could better help helicopter and aircraft owners.

  17. Thanks for explaining the different steps and procedures for aircraft component overhaul, repair, and modification. I think that it would also be important to consider when a component should be replaced. In that case, you would need to get a new component from a highly reputable provider.

  18. Global Aviation Infrastructure LLC says:

    Well descriptive and very informative post it was. Many people will get helpful from this one. Thanks for sharing it with us.

  19. Ava Murphy says:

    It sure was interesting when you said that compared to a complete overhaul, the repair of aircraft components will only determine the causes of the defects. This reminds me of my father who is interested in restoring his aircraft next month with the help of a professional. He mentioned that it has some components that need to be replaced to fully operate it again, so it will be beneficial for him to seek the help of a reliable aircraft restoration company.

  20. Irfan says:

    Hi Mike,
    It was a wonderful explanation. I have a question regarding IPC. As AMM is the main maintenance manual and AIPC is the supplement manual for AMM, It will have all the parts listed which are required to do the maintenance shown in AMM. Sometimes CMM will have the components and its detail parts breakdown shown in IPL. Is it required to list all the parts from CMM’s IPL to IPC or its enough to list only the maintenance parts from CMM’s IPL that aligns with the AMM.

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