Maintenance, repair, and Overhaul (MRO)

Airlines MRO

Maintenance, repair, and Overhaul (MRO) in the aviation industry is a complex process that has strict and precise safety standards defined by airworthiness authority to guarantee the safety of passengers and aircrew. For example, airlines must perform a pre-flight maintenance check to ensure that the aircraft is fit for the intended flight.  

I’m not a technician but I did my best to summarize this subject for you as aviation professionals due to the importance of MRO which represents approximately 12-15% of airlines operating costs. 


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Operator’s Continuing Airworthiness Responsibilities

ICAO Annex 6-part I, chapter 8,8.1* required operators* (Airlines) by procedures acceptable to the state of registry, to ensure that each aircraft they operate is maintained in an airworthy condition. For example, EASA part M is concerning specifically the continuing airworthiness of aircraft and aeronautical products. This regulation must be reflected in the European States Civil Aviation Authorities and must be adhered to by all aircraft operators registered in the European Union.


Professional note: 

* Operator. The person, organization, or enterprise engaged in or offering to engage in an aircraft operation.

* Refer to ICAO annex 6-part I, Chapter 8 — Aeroplane Continuing Airworthiness.


Maintenance, repair, and Overhaul (MRO) Definition 

  • Maintenance*: The performance of tasks on an aircraft, engine, propeller, or associated part required to ensure the continuing airworthiness of an aircraft, engine, propeller, or associated part including any one or combination of overhaul, inspection, replacement, defect rectification, and the embodiment of a modification or repair. 


Professional note: 

*This definition was applicable from 5 November 2020, see ICAO Annex 6 — Operation of Aircraft.


  • Repair*: The restoration of an aircraft, engine, propeller, or associated part to an airworthy condition by the appropriate airworthiness requirements, after it has been damaged or subjected to wear.


Professional note: 

*This definition was applicable from 5 November 2020, see ICAO Annex 6 — Operation of Aircraft.


  • Modification: A change to the type design of an aircraft, engine, or propeller.


Airline MRO Market 

For marketing experts, the market segments are very important to help them customize their products and services for each segment taking into consideration all business factors. The most recent market research has been dividing airlines' MRO into many types as the following:


1- MRO Service type, this includes:

  • Engine overhaul (which is expected to account for the largest share of global aviation maintenance in 2023).
  • Airframe maintenance
  • Line maintenance
  • Modification
  • Components

2- MRO aircraft type, this includes:

  • Fixed-wing aircraft

- Wide-body aircraft

- Narrow-body aircraft

- Regional jet


  • Rotary wing aircraft

Also, there is MRO geographical type. 


Now, I will give you a summary of my reading about scheduled and unscheduled aircraft maintenance.


Airline MRO | Scheduled and Unscheduled Aircraft Maintenance

Aircraft maintenance can be divided to:

1-   Scheduled Aircraft Maintenance

2-   Unscheduled Aircraft Maintenance

I will start with, Scheduled Aircraft Maintenance.


Scheduled Aircraft Maintenance

It is a preventive action to ensure the product functions properly at preset intervals. i.e Airbus 320 Scheduled Maintenance.


Scheduled Aircraft Maintenance includes routine and detailed inspections called transit, daily, 48h, A, B, C, and D checks. Also, can be divided into line and base maintenance categories. 


I will further explain it,continue reading.


Category 1Line Maintenance

Transit, daily, 48h Maintenance check

When (an average): After each stop or when the aircraft is on the ground for more than 4 hours.


Description: Normally includes a visual inspection of the aircraft to look for obvious damage and deterioration.


Examples: Check fluid levels, and emergency equipment, and inspect wheels and brakes.


A check

When (an average): i.e 500 Flight Hours or 2 months.


Description: Accomplished at a designated maintenance station and includes the opening of access panels to check and service certain items. Some limited special tooling, servicing, and test equipment are required.


Examples: General external visual inspection of aircraft structure for evidence of damage, deformation, corrosion, or missing parts; crew oxygen system pressure check; operationally check emergency lights, etc. 


B check

When (an average): i.e 1100 Flight Hours or every 4-6 months.


Description: This is a slightly more detailed check of components and systems. Special equipment and tests may be required. It does not involve detailed disassembly or removal of components.


Examples: Modern aircraft do not generally need it.


Category 2Base Maintenance


C check

When (an average): i.e Every 4000 Flight Hours or 20-24 months.


Description: This is an extensive check of individual systems and components for serviceability and function. It requires a thorough visual inspection of specified areas, components, and systems as well as operational or functional checks. It is a high-level check that involves extensive tooling, test equipment, and special skill levels. 'C' checks remove the airplane from the revenue schedule for 3 to 5 days. The 'C' check includes the lower checks, i.e., 'A,' 'B,' and Daily checks.


Examples: Visually check flight compartment escape ropes for condition and security; check the operation of the DC bus tie control unit; visually check the condition of entry door seals; operationally check flap asymmetry system, etc.


* Some airlines take advantage of this check to introduce certain innovations and renew the cabin (multimedia elements, lighting systems, reordering classes, etc.).


D check | Heavy Maintenance Visit (HMV)

When (an average): i.e Every 25000 Flight Hours or 6 years.


Description: This can also be referred to as the structural check. It includes detailed visual and other non-destructive test inspections of the aircraft structure. It is an intense inspection of the structure for evidence of corrosion, structural deformation, cracking, and other signs of deterioration or distress and involves extensive disassembly to gain access for inspection. Special equipment and techniques are used. Structural checks are man-hour and calendar-time intensive. The 'D' check includes the lower checks, i.e., 'A,' 'B,' 'C,' and daily checks. This check removes the airplane from service for 20 or more days.


Examples: Inspect stabilizer attach bolts; inspect floor beams; detailed inspection of the wing box structure.


Reference: Maintenance, Repair, and Overhaul (MRO) Fundamentals and Strategies: An Aeronautical Industry Overview, By Prof. Darli Rodrigues Vieira and Prof. Paula Lavorato Loures, International Journal of Computer Applications (0975 – 8887), Volume 135 – No.12, February 2016


Unscheduled Aircraft Maintenance

It is not-planned aircraft maintenance, but is is required when an item has failed or broken down. 



All types of aircraft maintenance protocols are based on maintaining the aircraft’s airworthiness certificate to ensure maximum passenger and aircrew safety and aircraft product reliability. 


Airlines have no other options than fulfilling these maintenance protocols, regardless of the aircraft's apparent conditions. If they don’t, they will lose the relevant airworthiness certificates, and the aircraft will be grounded.


For general readers, the air transport is the safest mode of transport due to the strict regulations and the daily operation control of aircraft and future development. I hope you find this article informative, for more articles continue reading our website. 

Maged Saeed AL-Hadabi

Air Cargo / IATA Dangerous Goods | CGO, DGR, SMS Chief Instructor | Internal Auditor | DG Inspector linkedin

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