What Is an MRO in Aviation? Get Your Aircraft Flight-Ready

Aircraft maintenance is one of the most important parts of the entire aviation process. Aircraft maintenance is a huge expense in terms of both cost and man hours for all military and government aviation programs. Tactical aircraft are particularly maintenance intensive, with some estimates as high as seventeen man-hours of maintenance per flight hour for F-16s!

While the military uses somewhat different terminology for their different types and phases of aircraft maintenance, the generic terminology used fairly universally is Maintenance Repair and Overhaul, or MRO. 

What Does Aviation MRO Stand for?

In aviation, MRO stands for Maintenance, Repair, and Overhaul. 

This generic phrase encompasses a very large and broad group of processes. 

Aviation MRO refers to the maintenance, repair, and overhaul services for aircraft, engines, and other aviation-related components. It involves restoring an aircraft or its components to a condition in which they can safely operate. 

This can include routine inspections, major overhauls, and repairing or replacing parts. MRO services are crucial for ensuring the safety and airworthiness of aircraft.

What is the Function of MRO in Aviation?

MRO services and facilities are among the most important parts of the aviation process. There is a reason why the Air Force has more enlisted aircraft maintainers than all other specialties combined. 

The function of aviation MROs is to ensure the complete airworthiness and safety of flight for all assigned aircraft through a fairly broad range of services offered and aircraft maintenance tiers. The difference categories are in the name: 

• Maintenance
• Repair
• Overhaul

Maintenance involves daily tasks, inspections, and servicing to keep the aircraft flying. It is also what we do to extend the aircraft's longevity, known broadly as preventative maintenance (PMs). Aircraft systems such as the hydraulic system are routinely serviced and checked to make sure they are in good operating condition; the fluids are clean, and so on. 

Repairs are generally considered a step above maintenance in the MRO process. They involve removing malfunctioning parts or components, replacing them with a similar part, or restoring them to their original condition (think about structural repairs like sheet metal parts). 

The overhaul process kind of rolls this all into one huge package. For the military users, this would be the Depot Level Maintenance process. In the overhaul phase, earmarked aircraft components are completely examined and then replaced or otherwise restored to OEM condition. Overhauls often take months to accomplish (on large airframes, that is), and are extremely thorough including a full strip down to bare metal and repainting. 

MRO operations are absolutely crucial to military aircraft and have added decades to their service life. Take the B-52 and KC-135 airframes, two contemporary aircraft built to complement each other. The average age of each one is pushing six decades now, and they still get out there and fly every day of the week. Regular maintenance has detected countless flaws and faults, and the specialized services and technical assistance of military MROs have kept these aircraft safely flying well past their expected service life. 

Adhering to extremely strict regulatory standards along with skilled technicians, MROs in the aviation sector contribute to the safest mode of transportation the world has ever known. Effective MRO practices are critical for passenger safety and the safety of the public at large. 

The Types of Maintenance, Repair, and Overhaul services

MRO services are mainly divided into three types: engines and parts. The MRO industry includes numerous providers of aircraft services with varied service levels. Some companies offer just one type of service, while others offer a comprehensive package containing all three major categories. MRO providers generally fall under the categories of OEMs and independent service providers. OEMs are manufacturers of aircraft products.

MRO facilities encompass a wide range of aircraft maintenance services that help guarantee the flight safety and airworthiness of aircraft. These services are broadly categorized into these distinct categories:

• Line maintenance
• Base maintenance
• Shop maintenance

Line Maintenance

Line maintenance entails the everyday checks and inspections that keep aircraft in the air. Some of these include (but are not limited to):

1. Pre-flight Checks: Inspect the aircraft before takeoff to ensure everything functions correctly.
2. Post-flight Checks: Checking the aircraft after landing for any issues that may have occurred during the flight.
3. Daily and Weekly Checks: Regularly scheduled inspections and maintenance tasks that are performed at specific intervals to ensure ongoing airworthiness.
4. Routine Servicing: Refueling, checking fluid levels (oil, hydraulic fluid), and other routine services necessary for the aircraft to continue operating safely.
5. Minor Repairs: Addressing minor issues that do not require extensive work or significant downtime, such as replacing light bulbs, minor avionics fixes, and patching small leaks.
6. Rectifying Pilot Reports: Addressing any issues reported by the aircrew after a flight.

A & B level checks are also typical of the line maintenance cycle. Their military equivalent is hourly post-flights (HPOs). 

A Checks

A checks are relatively light and frequent maintenance tasks that occur approximately every 400-600 flight hours or every 1-2 months, depending on the aircraft type and operational requirements. A checks are performed to ensure the ongoing airworthiness and safety of the aircraft. A few of the typical tasks involved in A checks include:

• General inspections of the airframe and systems.
• Checking fluid levels (oil, hydraulic fluid, etc.).
• Lubrication of moving parts.
• Inspection and replacement of filters.
• Functional checks of avionics and flight control systems.
• Minor repairs and adjustments as needed.

B Checks

B checks are more comprehensive than A checks but less extensive than C checks (base maintenance). They occur approximately every 6-8 months or after a specified number of flight hours, varying by aircraft type and operational conditions. B checks include all tasks performed during A checks, plus additional inspections and maintenance activities. Tasks involved in B checks include:

• More detailed inspections of aircraft systems and components.
• Checking and testing of emergency equipment.
• Inspection of landing gear components.
• Functional tests of systems like hydraulic and electrical systems.
• Additional lubrication and servicing tasks.
• Some components may be removed for inspection and testing.

Shop-level Maintenance

Shop maintenance refers to the work performed in a dedicated workshop environment, commonly performed in independent repair stations. These repairs are conducted as specialized maintenance on individual components and systems off of the aircraft. 

A few of the most common items that are repaired in shop maintenance are:

• Brakes
• Hydraulic actuators
• Auxiliary power units
• Struts

Some of the core tenets of shop maintenance are:

1. A component overhaul is the comprehensive disassembly, cleaning, inspection, repair, and reassembly of aircraft components, including engines, landing gear, avionics, and hydraulic components. 

2. Repair: this portion addresses known defects and damage in specific components (i.e., the horizontal stabilizer actuators are known to leak).

3. Testing: Shop maintenance is a common place to test components to ensure they operate correctly. One example is “hush houses,” which are commonly used to test-run jet engines on aircraft. 

4. Calibration: Back shops are the ideal place and time to perform calibration testing, especially since many of the tools and components requiring calibration must be calibrated in a sterile environment. 

5. Modifications & upgrades: While some modifications have to be made to the airframe or the aircraft, component modifications can regularly be performed in a back shop during scheduled maintenance. 

Base-Level Maintenance

Base maintenance is a commercial aviation term, but there is considerable crossover between how independent repair stations and general aircraft maintenance conduct base maintenance and the military model. 

Base maintenance in the MRO model entails C & D level checks, roughly equivalent to phase maintenance or isochronal maintenance in the military (C checks) and Depot-level maintenance (D checks). Here are some of the highlights of base maintenance, which mirror the military inspection cycles:

C Checks

C Checks are extensive inspections and maintenance activities performed approximately every 12-24 months, depending on the aircraft type and regulatory requirements. They involve detailed inspections of the aircraft's structure, systems, and components. 

In the military aircraft maintenance model, phase maintenance is equivalent to a C check. Isochronal maintenance includes similar requirements but is conducted on a strict calendar cycle rather than the flight hours or cycle approach used in phase maintenance.

Tasks may include:

• Structural inspections, including checks for corrosion, fatigue, and wear.
• Detailed examinations of landing gear, control surfaces, and hydraulic systems.
• Overhaul or replacement of components as necessary.
• Software updates and avionics upgrades.

D Checks

This is far beyond component MRO maintenance, roughly equal to depot-level maintenance in the military heavy maintenance model. D checks and depot maintenance exist within MRO organizations to extend the operational life of aircraft through an extremely detailed inspection process at the MRO. 

These are the most comprehensive and intensive maintenance checks, typically carried out every 6-10 years, depending on the aircraft type and usage. D checks can take several weeks (or several months, depending on the airframe) to complete and involve a thorough examination and overhaul of the entire aircraft. 

A few of the common tasks are (but are not limited to):

• Complete disassembly and inspection of major components, such as engines, airframe, and landing gear.
• Detailed non-destructive testing (NDT) to detect hidden damage or wear.
• Extensive structural repairs and replacements.
• Overhaul or replacement of major systems and components.
• Full repainting and refurbishment of the aircraft. All aircraft are completely stripped down to bare metal during this level of maintenance and repainted. 

One last extremely important task is the weight and balance portion. Paint adds hundreds of pounds to an aircraft, so the weight and balance will differ before and after the stripping. 

Bottom Line

MROs are what make aviation happen. Without an effective and efficient MRO program, whether in-house or by private MRO operators, maintenance makes flying the safest form of transportation the world has ever known. It is also the backbone of the military aviation mission.

Whether it is air assault, air bridge, close air support, or air superiority, none happens without the MRO system. 

And MROs don’t function without parts. You can see that all of the different levels of general maintenance and MRO in aviation revolve around repairs or replacement parts, and for that, you have to guarantee a rock-solid supply chain. Greenwood Aerospace is the trusted partner you are looking for here, with over four decades of experience in parts procurement. Call us today at (580) 762-2580, or email us at contact@governmentprocurement.com, and we’ll talk about the best parts procurement options for your military MRO!