4.1 Part-145 Approval Scope, Ratings & the Maintenance Organisation Exposition (MOE)

Regulatory Architecture and Statutory Scope of EASA Part-145

Under European civil aviation legislation, aircraft maintenance must be conducted within an institutional framework characterized by verified technical capability, standardized safety management, and statutory accountability. EASA Part-145, enacted as Annex II to Commission Regulation (EU) No 1321/2014, establishes the implementing rules and certification specifications for approved maintenance organisations (AMOs) tasked with maintaining aircraft and aeronautical components.

Historically rooted in the Joint Aviation Authorities (JAA) JAR-145 code, the standard was incorporated into Community law under Regulation (EC) No 2042/2003 and subsequently recodified under Regulation (EU) No 1321/2014. Operating beneath the constitutional umbrella of the Basic Regulation (Regulation (EU) 2018/1139), Part-145 interfaces directly with Part-M (Continuing Airworthiness Management), Part-CAMO (Continuing Airworthiness Management Organisations), Part-66 (Certifying Staff Licensing), Part-147 (Maintenance Training Organisations), and Part-21 (Certification of Aircraft and Related Products, Parts and Appliances).

Mandatory Applicability and Thresholds

Compliance with EASA Part-145 is legally mandatory for organisations performing maintenance on:

1. Commercial Air Transport (CAT) Aircraft: All aircraft operated under an Air Operator Certificate (AOC) pursuant to Regulation (EU) No 965/2012 must be maintained and released by an approved Part-145 organisation (or an approved Part-CAO organisation where expressly authorized for specific light commercial operations).
2. Complex Motor-Powered Aircraft (CMPA): Defined in Article 3(31) of Regulation (EU) 2018/1139 (not Article 140, which contains transitional provisions). CMPA means:
   • Aeroplanes with a maximum certificated take-off mass exceeding 5 700 kg, or certified for a maximum passenger seating configuration of more than 19, or certified for operation with a minimum crew of at least two pilots, or equipped with one or more turbojet engines or more than one turboprop engine;
   • Helicopters certified for a maximum take-off mass exceeding 3 175 kg, or for a maximum passenger seating configuration of more than nine, or for operation with a minimum crew of at least two pilots (the criterion is not “multi-engine”, and it does not depend on the operation being commercial);
   • Tilt-rotor aircraft.
3. Components for Fitment: Any engine, auxiliary power unit (APU), propeller, or aeronautical component intended for installation on CAT aircraft or CMPA must be maintained and certified on an EASA Form 1 by an approved Part-145 (or Part-CAO) component maintenance organisation.

---

The Rating System and Classes (Appendix II to Part-145)

An organisation cannot hold an open, unconstrained approval to maintain any aeronautical product. The Competent Authority—either the National Aviation Authority (NAA) of an EASA Member State or EASA itself for organisations located in third countries—issues an EASA Form 3-145 Approval Certificate. This certificate delimits the organisation's statutory privileges using a standardized taxonomy of Classes and Ratings set forth in Appendix II to Part-145.

Detailed Breakdown of Class C Component Ratings (C1 through C22)

The Class C rating structure covers discrete aeronautical components maintained off-wing in specialized workshops:

• C1 Air Conditioning & Pressurisation: Inflow valves, pack cooling machines, heat exchangers, outflow valves.
• C2 Auto Flight: Autopilot servos, flight guidance computers, autothrottle actuators.
• C3 Communications & Navigation: VHF/HF transceivers, VOR/ILS receivers, TCAS computers, weather radar antennas.
• C4 Doors & Hatches: Passenger entry doors, cargo door mechanisms, emergency exit hatches, door slide bustles.
• C5 Electrical Power & Lights: Generators, integrated drive generators (IDGs), starter-generators, inverters, landing lights.
• C6 Equipment & Furnishings: Passenger seating, flight crew seats, galley inserts, lavatory modules, escape slides.
• C7 Engine / APU Components: Fuel nozzles, bleed air valves, ignition exciters, mechanical oil pumps, starter motors.
• C8 Flight Controls: Primary and secondary flight control actuators, elevator feel units, flap drive gearboxes, PCUs.
• C9 Fuel: Fuel boost pumps, engine-driven fuel pumps, fuel quantity transmitters, refuel/defuel shutoff valves.
• C10 Helicopter — Rotors: Main rotor heads, swashplates, pitch links, tail rotor blade assemblies, dampeners.
• C11 Helicopter — Transmission: Main gearboxes (MGB), intermediate gearboxes, tail rotor drive shafts.
• C12 Hydraulic Power: Hydraulic pumps, reservoir modules, accumulator bottles, hydraulic distribution manifolds.
• C13 Indicating & Recording Systems: Primary flight displays, engine indication units, flight data recorders (FDR), CVRs.
• C14 Landing Gear: Shock struts, bogie beams, retraction actuators, main landing gear wheels, carbon brake assemblies.
• C15 Oxygen: Crew oxygen masks, passenger oxygen regulators, high-pressure oxygen cylinders, chemical oxygen generators.
• C16 Propellers: Variable-pitch propellers, constant-speed governors, feathering pumps, de-icing slip rings.
• C17 Pneumatic: High-pressure bleed ducting, air cycle valves, pneumatic starter ducts, cross-bleed valves.
• C18 Protection (Ice/Rain/Fire): Engine nacelle anti-ice valves, windshield wiper assemblies, fire extinguisher bottles.
• C19 Windows: Cockpit windshields, heated forward vision panels, passenger cabin passenger windows.
• C20 Structural: Nacelle cowlings, fairings, radomes, flight control surfaces, pylon structural panels.
• C21 Water Ballast: Water ballast tanks, dump valves, fill valves (principally used in high-performance sailplanes).
• C22 Propulsion Augmentation: Afterburners, rocket assist systems, thrust vectoring nozzles.

The Operational Envelope of a Class A Aircraft Rating

A central concept tested extensively in EASA Part-66 Module 10 examinations concerns the exact operational envelope of a Class A aircraft rating:

• Fitted Components: An organisation holding a Class A rating is authorized to perform maintenance on the complete aircraft, its installed engines/APU, and any fitted component, provided the work is executed strictly in accordance with the Aircraft Maintenance Manual (AMM).
• Temporary Detachment to Facilitate Access: The organisation may temporarily remove an engine, APU, or component to gain physical access to adjacent structures or systems, provided the removed part remains under the direct physical, environmental, and procedural custody of the organisation.
• Strict Legal Prohibition: A Class A approval does not permit the organisation to perform standalone bench overhaul, workshop repair, or component testing in a back-shop, nor can it issue an EASA Form 1 for an uninstalled component. Overhauling an uninstalled component or releasing it on an EASA Form 1 legally requires a Class B (Engine/APU) or Class C (Component) rating.

Class D1: Specialised Services (Non-Destructive Testing)

A Class D1 rating differs from all other ratings because it is not tied to a specific aeronautical product, engine, or system. Instead, it is granted exclusively for Non-Destructive Testing (NDT). A Class D1 facility performs specialized NDT inspections on behalf of other maintenance organisations. Under Appendix II to Part-145, all technical personnel conducting and evaluating NDT under a D1 approval must be qualified and certified in accordance with EN 4179 (or equivalent American standard NAS 410) under the statutory oversight of a recognized National Aerospace NDT Board (NANDTB).

---

Line Maintenance vs Base Maintenance

Under AMC 145.A.10, an organisation holding a Class A aircraft rating may be approved for Line Maintenance, Base Maintenance, or both. The regulatory distinction dictates the required physical facilities, certifying personnel qualifications, and operational control:

Line Maintenance

Line maintenance comprises any maintenance carried out before flight to ensure the aircraft is fit for the intended flight. Its scope encompasses:

• Troubleshooting and Defect Rectification: Diagnosing operational snags, running built-in test equipment (BITE), and replacing Line Replaceable Units (LRUs) using flight-line tooling.
• Component Replacement: Replacing external assemblies, wheels, brake packs, navigation bulbs, igniters, and straightforward mechanical/avionic LRUs.
• Scheduled Minor Checks: Servicing, pre-flight, transit, daily, and weekly checks, as well as scheduled minor checks up to and including the calendar or flight-hour thresholds specified in the approved MOE (typically minor checks up to and including A-checks).
• Minor Modifications and Repairs: Simple repairs and modifications that do not require extensive structural disassembly, specialized hangar dockings, or prolonged aircraft downtime.

Base Maintenance

Base maintenance encompasses heavy, scheduled structural maintenance that demands extensive hangar docking, heavy ground support equipment, specialized environmental controls, and extended aircraft downtime. It includes:

• Deep scheduled inspections (e.g., C-checks, D-checks, 6-year structural checks, 12-year structural tear-downs);
• Major structural repairs, fatigue crack replacements, and major Supplemental Type Certificate (STC) embodiments;
• Corrosion Prevention and Control Programmes (CPCP) requiring extensive cabin interior de-trimming, insulation blanket removal, and aircraft jacking;
• Rectification of complex structural defects requiring access to internal fuel tanks, high-pressure hydraulic distribution manifolds, or flight control primary cable runs.

---

The Maintenance Organisation Exposition (MOE - 145.A.70)

The Maintenance Organisation Exposition (MOE) is the definitive statutory document of an EASA Part-145 organisation. Under 145.A.70, the MOE serves as the legally binding contract between the maintenance organisation and the Competent Authority. Every operational procedure, managerial line of accountability, technical process, and quality standard practiced by the organisation must be formally codified within the MOE. Deviating from the approved MOE constitutes a regulatory violation under 145.B.50, exposing the organisation to Level 1 or Level 2 audit findings, suspension, or formal revocation of its approval.

Standard MOE Architecture (AMC 145.A.70(a))

The MOE must follow a standardized six-part structural template mandated by EASA:

---

Approval and Amendment Procedures: Direct vs Indirect Approval

The MOE is an evolving document that must reflect changing organisational realities, fleet modernizations, and regulatory revisions. However, amendments cannot be enacted without strict oversight. EASA establishes two distinct legal approval pathways under 145.A.70(c) and 145.A.85:

1. Direct Approval by the Competent Authority

Significant changes that affect the fundamental foundation of the organisation's approval require prior direct application, evaluation, and formal written approval by the Competent Authority before implementation. These include:

• Any change of the Accountable Manager;
• Any change of the Nominated Persons (Postholders) listed in MOE Part 1.3 (e.g., Base Maintenance Manager, Line Maintenance Manager, Compliance Monitoring Manager, Safety Manager);
• Alterations to the Rating Classes or additions of new aircraft type ratings (e.g., adding an Airbus A350 rating to an existing A320 approval);
• Relocation, significant extension, or addition of primary base maintenance hangar facilities or engine test cells;
• Fundamental restructuring of the safety management system or compliance monitoring framework.

2. Indirect Approval Procedure (MOE 1.11)

To avoid administrative bottlenecks for minor procedural updates, the Competent Authority may grant the organisation the privilege of Indirect Approval. This privilege is subject to stringent prerequisites:

• Prerequisites: The organisation must establish an explicit indirect approval procedure in MOE Part 1.11, which itself must be directly reviewed and approved by the Competent Authority.
• Oversight: The indirect approval process is administered by the Compliance Monitoring Department. The Compliance Monitoring Manager evaluates each proposed change to verify that no regulatory non-compliance is introduced.
• Permissible Scope: Minor procedural revisions, typographical and editorial corrections, updates to sample company forms, routine updates to internal working instructions, and additions of individual component part numbers to the Capability List (provided the components fall strictly within already-approved Class C rating scopes).
• Notification: The Competent Authority must receive a copy of all indirectly approved amendments within a defined timeframe specified in the MOE (typically within 10 to 14 calendar days).

Dynamic Component Capability List Governance

For organisations holding Class B or Class C ratings, listing thousands of individual part numbers directly on the EASA Form 3-145 approval certificate is impractical. Instead, the certificate states the broad rating category (e.g., C14 Landing Gear), and the organisation manages an internal Capability List in MOE Part 5. Before adding a new component part number to the capability list under indirect approval, the organisation must conduct and document a comprehensive Four-Pillar Capability Assessment confirming that:

1. The applicable OEM Component Maintenance Manual (CMM) is current and on file;
2. All specialized tooling, test benches, and ground support equipment specified in the CMM are available and calibrated;
3. Qualified certifying staff with documented type/component training and active authorisations are assigned;
4. Suitable physical workshop facilities, cleanrooms, and environmental controls are active.

---

Practical Maintenance Scenario & Module 10 Exam Tips

Maintenance Practical Scenario: An EASA Part-145 line maintenance organisation holding a Class A1 rating for the Boeing 737 Next Generation discovers a cracked elevator balance tab during a pre-departure transit check. Under the Class A1 approval, the line certifying engineer may detach the damaged balance tab, install a serviceable balance tab drawn from stores, perform the required flight control balance and rigging checks per the AMM, carry out an independent inspection per 145.A.48(b), and sign the aircraft CRS in the technical logbook. However, the maintenance team cannot take the removed cracked tab into their workshop, fabricate a sheet-metal doubler patch, re-rivet it, paint it, and sign an EASA Form 1 for it. That structural repair on an uninstalled component requires a Class C20 (Structural) rating.

Module 10 Exam Tips:

• Class A covers Aircraft, Class B covers Engines/APUs, Class C covers Components, and Class D covers Specialised Services (D1 NDT).
• Class A1 is for aeroplanes above 5,700 kg; Class A2 is for aeroplanes 5,700 kg and below; Class A3 is for Helicopters; Class A4 covers other aircraft (sailplanes, balloons).
• Technicians performing D1 NDT must be qualified to EN 4179 or NAS 410 under a National Aerospace NDT Board.
• The Accountable Manager's signed Corporate Commitment statement is located exclusively in MOE Part 1.1.
• Indirect approval cannot be used to change the Accountable Manager, nominated postholders, base facilities, or scope of aircraft type ratings—these require direct Competent Authority approval.