The definition of an aviation MRO

An MRO is a Maintenance and Repair Organization. Therefore, naturally, an aviation or aircraft MRO is a company which specializes in performing maintenance actions on aircraft and their components, such as jet engines and landing gear.

Definition of an aviation MRO

In this brief article I will go through what defines an aviation MRO, what requirements have to be fulfilled by MROs and what the difference is between maintaining aircraft and aircraft parts.

An aviation MRO must be certified

Like almost anything in aviation, an MRO must have proper certificates which allow it to perform certain tasks on aircraft in accordance with their approval schedule.

In EASA countries, the MRO must be approved with accordance to EASA Part 145 – a regulation which defines how a maintenance and repair organization must be run in order to provide the highest, acceptable level of flight safety. There are several aspects to obtaining a Part 145 approval, and I will go through those in a different post, but here are some of the basics.

The Part 145 approval is quite specific. It accurately describes what aircraft, engines or components the MRO is allowed to service and also in which scope the approval is valid. In other words, some MROs will have the ability to perform line and base maintenance on a large variety of aircraft, others will be approved only to do line maintenance on a 737NG type, while even others will only be approved for the overhaul of hydraulic actuators of a given part number.

The Part 145 certificate, which is the result of an NAA or EASA certification process, contains a scope of approval which specifies what a company is allowed to do. The scope of approval is divided into 4 main categories (A – Aircraft, B – Engines, C – Components other than complete engines or APUs, D – Specialized Services).

The categories above are divided into subcategories. For example, for category “A – Aircraft” we have subcategories A1 – Aeroplanes above 5700kg, A2 – Aeroplanes below 5700kg, A3 – helicopters and A4 – Aircraft other than A1, A2 and A3.

The remaining categories are also subdivided, particularly category C (Components) is quite specific with, for example, C8 being flight controls and C16 – propellers.

Category D deserves a special mentioning because the term “Specialized Services” seems very vague. This category is mainly used for aviation MROs which specialize in NDT (non-destructive testing) techniques and sometimes do not have an approval for any of the other Part 145 categories.

Aircraft MROs

Aircraft MROs are probably the first thing that comes to mind when considering aviation maintenance and repair organizations. You guessed it – they deal with aircraft. Depending on their scope of approval, they can do line or base maintenance, and at that, they may also be limited to a specific set of tasks which they are allowed to perform.

Base maintenance organizations will generally be quite large companies, owning a hangar (which is required for base maintenance) and employing a vast group of people to perform base maintenance tasks. Such organizations will have the ability to carry out heavy checks, commonly known as C-checks and structural checks (sometimes referred to as D-checks). Such maintenance checks can take from a few weeks to even a few months, are complex and their costs are calculated in millions of dollars.

Line maintenance organizations are mainly required to provide necessary, minor maintenance tasks in between flights. Those would include regular daily and weekly checks as well as some defect rectification. Line maintenance MROs will be able to replace wheels, brakes and most LRUs (line replaceable units) on the aircraft. They will also release the aircraft to service before its further flight.

Line maintenance companies, unlike their base maintenance counterparts, need to be fairly small and mobile. In order to satisfy the airlines, they need to be available at many airports, often 24 hours a day and with an ability to react instantly to unforeseen circumstances, such as a sudden aircraft defect. They are often based out of a small office and all their tools and supplies can fit on a single van.

Engine and landing gear MROs

These two types of aviation MROs deserve a specific mentioning, as they are very important in the life of an airline. In most cases, such MROs specialize only in engines (or only in landing gear) and at a specific family type at that. This is due to the fact, that the overhaul and repair process of such major aircraft components is really complex and sophisticated. Therefore – it is also very expensive.

Engine and landing gear MROs will have their own big facilities and they often have a production line much similar to those found in factories. They rarely have a standstill. On the contrary, they will rather be overwhelmed with customer requests to complete the overhaul of their equipment in a very short period of time. An engine is a multimillion dollar asset and no reasonable airline will want it to sit around in an MRO rather than earning its lease rates.

Engine and landing gear MROs also need very skilled and highly trained workers to perform the required work, and also often have their own NDT personnel.

Component MROs

The last type of aviation MROs is probably the easiest one to get one’s head around. Component MROs have to be certified like all the other types, but in many cases the type of maintenance they provide does not require as sophisticated an approach as that from engine or landing gear MROs. There are exceptions to that, of course, particularly in the field of hydraulics, avionics and emergency equipment (like emergency door slides, for instance) but in general – if you dream of having your own maintenance and repair organization, perhaps it would be best to start off with some easily maintainable components.

I hope that this clarifies, at least in a basic way, the definition of an aviation MRO. Please let me know what you think of this article and if I can answer any other questions – I’ll be glad to do so.

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Posted in Aircraft Maintenance Management, Aircraft Maintenance Regulations, Airline Business, Airworthiness, Airworthiness Regulations, Maintenance, Maintenance of Aircraft, Maintenance of Components, Special Maintenance - NDT, etc.
15 comments on “The definition of an aviation MRO
  1. Ben says:

    Really interesting and helpful read. I was trying to find out whether base or line MROs would carry out lightning strike checks on large aircraft though. Can you help?

    • Mike says:

      Hi Ben! Glad you liked the article. Yes, of course they can. Both a line and a base maintenance organization would normally be allowed to do that (although, with large aircraft, the line station would need to be equipped with a lift or other means of going up to the empennage for example). But this is just for an inspection. Should a repair be necessary, that would be done by a base maintenance organization. A line station could only decide whether the damage is within limits (and, for example, allowed to fly for a specific time period). Lightning strike repairs would typically require structural work, either per the SRM or manufacturer’s approval and would be performed by a base maintenance MRO.

      • Ben says:

        Thanks, that makes perfect sense.
        We are looking at working with Independent MROs to allow them access to drone tech for doing lightening checks and potentially other inspection work.

  2. Hillary Muparamoto says:

    This was helpful indeed this is exactly the kind of an organization I was thinking about I really what to start my own

    • Mike says:

      Hi Hillary, I’m very glad you liked the article. And all the best on your endeavour! Although it is a budget consuming project…

  3. rabin shahi says:

    Is approved maintenance organisation and MRO same term?

  4. Muhammad Zahidul Sayeed says:

    Dear Michal Swoboda
    Good day. I found your article interesting. Sir, I found that In the scope of Air Navigation Order (AW) Part-145 Approval Schedule Rating is there ‘A’ is for Aircraft, ‘B’ is for Engines and ‘C’ is for components. So far I understood Both for A & B ratings base and line maintenance of large aircraft, must have appropriate aircraft type rated certifying staff and support staff (qualified as C,B1,B2,B3). But for Component MROs
    ( shops) does it requires to have any such aircraft type rated certifying staff and support staff (qualified as C,B1,B2,B3)?
    Your kind reply is highly appreciated.

    M Z sayeed
    Technical Management officer

    • Mike says:

      Hi Muhammad! Thanks for your comment!
      No, you don’t need aircraft type rated certifying staff for component maintenance. The certification process of your staff will depend on your procedures, which you would write in your MOE and have then approved by your aviation authority.
      So, if you have a shop that does maintenance on, say, hydraulic valves, you would have your own procedure on how you train your personnel and who you allow to do which maintenance tasks. I believe the person finally signing the Form One would need a C license, but I’m not sure now and I would have to check the regulation. Either way, you can get the C license through the “academic” route by completing a degree and some supplementary training.

  5. Piotr says:

    Thank you for your article.
    What about simple cosmetic cabin interior maintenance?
    Would that be category C?
    Would that be line maintenance?
    Can you kindly expand on that?
    Regarding certification, would that be conducted by a civil aviation department or a private certification company?
    Thank you very much!

  6. Mike says:

    Hi Piotr! Happy to have your comment here 🙂

    Not an easy question. Typically, cabin maintenance (replacement of screws in seats, tray tables, painting of overhead bins, etc) is NOT covered in the AMM and hence cannot be done as neither line nor base airframe maintenance. Thus, it would have to be done as component maintenance under the C category.

    In practice a lot of MROs just do it as line maintenance and sign it off as “standard practices” from the AMM. Whether this is legal or not is for the CAA to decide, not me. Formally, I believe that they shouldn’t be doing it, although it can become a nightmare.

  7. Thibagar says:

    What will be the % earning for respective MRO’s ?

  8. Allen Hayward says:

    Hello Mike,

    I too enjoyed your article. Question – Is training, customer specific, included within an MRO license?

    Thank you,

  9. Kofi Danso says:

    Thanks for the nice exposition on MRO.
    Please, what is the difference between an MRO and an AMO?

    Thank you.

  10. Thanks for sharing this article. This is excellent article. Aviation Maintenance, Repair and Overhaul (MRO) Aviation Maintenance, Repair & Overhaul (MRO) services from SGS – ensure the reliability, safety and productivity of your assets. Precise surveillance and upkeep of aircraft and their supporting facilities is essential to retain public and governmental trust in the safety of our skies.

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