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.

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.