In the aviation world, understanding the language of maintenance, repair, and overhaul (MRO) is essential — especially when navigating both commercial and military environments. Each sector comes with its own set of acronyms, regulations, and procedures.
MRO is crucial in both the commercial and military aviation with the International Air Transport Association (IATA) estimating that the worldwide MRO market is expected to reach $85 billion by 2033.
“With fleet expansion slowing, airlines are extending the operational lifespan of mid-life and end-of-life aircraft, intensifying the need for comprehensive maintenance, repair and overhaul (MRO) services,” wrote Aleksandr Celiadin, founder of GetJet Airlines, in Forbes. “It’s time for the aviation industry to reprioritize MRO services as part of a strategic approach to address these current challenges.”
On the military side, sustainment is fundamental to U.S. success in future engagements, with some calling MRO the long tent pole in military aircraft readiness. As former commandant of the Marine Corps Gen. Robert H. Barrow once said: “Amateurs talk about tactics, but professionals study logistics.”
Whether you're supporting commercial fleets or military squadrons, speaking the language of MRO is critical to reducing downtime, ensuring airworthiness, and maintaining operational readiness. Here’s a comprehensive guide to some of the most essential terms used across both sectors:
Core Terms (Shared by Both Commercial & Military)
- A&P – Airframe and Powerplant. FAA-certified mechanics who perform and approve maintenance on aircraft structures and engines.
- AOG – Aircraft on Ground. Urgent condition where an aircraft is grounded due to maintenance needs.
- MOC – Method of Compliance. Documentation and steps followed to satisfy an airworthiness directive or service bulletin.
- MRO – Maintenance, Repair, and Overhaul. Encompasses routine servicing, major overhauls, component repairs, and inspections that keep aircraft airworthy.
- OEM – Original Equipment Manufacturer. Refers to the maker of original parts or systems installed during aircraft manufacture.
- PMA – Parts Manufacturer Approval. FAA-approved parts that serve as alternatives to OEM components.
- Rotable – Parts that can be repeatedly repaired or overhauled and returned to inventory.
- SSI / MSI – Structural or Maintenance Significant Items. Parts critical to the safe operation and airworthiness of an aircraft.
- USM – Used Serviceable Material. Parts that have remaining life and meet safety standards for reuse.
Military-Specific MRO Terms
- ALC – Air Logistics Complex. A major military facility (e.g., Tinker AFB) where depot-level MRO is conducted for entire aircraft fleets.
- AFTO Form 781 – Air Force Technical Order. A series of forms used to log flight hours, inspections, discrepancies, and corrective actions for U.S. Air Force aircraft.
- Cannibalization (Military) – A controlled practice in which a part is removed from one aircraft to restore another to a mission-capable status. Strictly tracked under military logistics protocols.
- Depot-Level Maintenance (D-Level) – In-depth repair and overhaul typically done at major facilities like military logistics centers or contractor depots. Involves extensive disassembly and inspection of systems.
- FOD – Foreign Object Debris/Damage. Especially critical in military operations, FOD refers to debris that can damage aircraft on runways or in flightlines. The military uses strict FOD prevention programs.
- Form 2400 Series – Standard U.S. Army maintenance forms used to document inspection, serviceability, and faults on rotary and fixed-wing military aircraft.
- GTIMS – Ground Training Integration Management System. A software system used by the U.S. Air Force to schedule, track, and record training and maintenance.
- MXG – Maintenance Group. A U.S. Air Force organizational unit responsible for executing aircraft maintenance across multiple squadrons.
- NMC / FMC – Not Mission Capable / Fully Mission Capable. Indicates whether a military aircraft is ready to perform its assigned mission. A critical readiness metric.
- SRU / LRU – Shop Replaceable Unit / Line Replaceable Unit. LRUs can be swapped quickly on the flight line; SRUs require bench-level repairs in a shop environment. Common in both military and commercial MRO.
- TMDE – Test, Measurement, and Diagnostic Equipment. Used in military aviation to test aircraft systems during scheduled maintenance or troubleshooting.
Key Regulatory and Industry Terms
- EASA – European Union Aviation Safety Agency. Oversees airworthiness in the EU and works with FAA on mutual recognition of certifications.
- FAA – Federal Aviation Administration. U.S. governing body that regulates civilian air safety and certifies aircraft and maintenance providers.
- FAR Part 145 – Defines the regulatory standards for repair stations, including personnel requirements, tools, and quality control systems.
- IA – Inspection Authorization. FAA designation for senior A&P mechanics authorized to sign off on annual inspections and major repairs.
- IATA – International Air Transport Association. Develops industry policies and safety standards for airlines and related services.
MRO Facility Types and Providers
- Contract Maintenance Organizations – Third-party contractors hired by military branches or airlines for supplemental maintenance labor or depot work.
- FBO – Fixed Base Operator. Privately operated service providers at airports offering fueling, maintenance, hangaring, and other services.
- Independent Repair Shops – Non-airline-affiliated MRO facilities that serve general aviation, military, or commercial operators.
Logistics and Supply Chain Terms
- DD Form 1577 / 1348 – Military forms used to tag unserviceable parts (1577) or issue requisitions and receipts for logistics items (1348).
- DLA – Defense Logistics Agency. Provides logistics, acquisition, and technical services for the U.S. military, including parts procurement and distribution.
- JIT – Just-in-Time. A strategy to deliver parts and materials precisely when needed, reducing inventory costs and improving aircraft uptime.
- NSN – National Stock Number. A unique identifier for military parts used across NATO and U.S. Department of Defense supply chains.
Emerging Concepts in MRO
- Digital Twin – A virtual model of an aircraft or system used to simulate performance and schedule maintenance. Increasingly used by OEMs and defense contractors.
- Condition-Based Maintenance (CBM) – Maintenance performed based on the real-time condition of a part rather than fixed intervals — especially valuable in mission-critical military aircraft.
- Predictive Maintenance – Uses sensor data and analytics to anticipate maintenance needs before failure occurs, reducing AOG events. Gaining traction in both commercial and military aviation.
Source One Spares: We Speak the MRO Language
From "AOG" to "AFTO 781s," understanding the language of MRO is essential for efficiency, safety, and mission readiness — especially when supporting both commercial and defense aviation platforms.
At Source One Spares, we speak that language. Our team supports operators across industries with hard-to-find parts, just-in-time delivery, and deep expertise in both military and commercial supply chains. Whether you’re keeping a passenger jet flying or maintaining a fleet of combat-ready aircraft, Source One Spares is your trusted MRO partner.
Contact Source One Spares today for just-in-time parts and components that can keep your commercial and military aircraft around the world aloft.