Aviation Maintenance Glossary: 45+ EASA Part-66 Terms Every Engineer Should Know
Every industry has its own language, and aviation maintenance is no exception. From regulatory acronyms to technical manual designations, the terms you encounter as a Part-66 candidate can be overwhelming at first. This glossary defines essential terms with clear explanations and real-world context — helping you decode the documents, pass your EASA module exams, and talk the talk on the hangar floor.
Skylicence Europe Team
EASA Part-66 exam preparation specialists — helping engineers earn their Part-66 licence since 2025
Whether you are preparing for the EASA module exams, reading an AMM for the first time, or trying to understand a DOA-approved repair disposition, this glossary will help you cut through the jargon. Each entry includes a concise definition and a real-world usage context so you understand not just what the term means, but how it applies in your daily work as a certifying engineer.
For exam-specific preparation resources, see our EASA Part-66 study guide, airframe module exam FAQ, and Part-66 study plan.
A
AD (Airworthiness Directive)
A legally enforceable rule issued by EASA (or a national aviation authority within the EU system) requiring mandatory action on an aircraft, engine, propeller, or component. ADs address known safety deficiencies and may require inspection, modification, replacement, or operational limitations. Compliance is mandatory regardless of other maintenance schedules.
Context: Compliance with ADs must be recorded in the aircraft maintenance records with the AD number, method of compliance, and signature of the certifying staff. Tracking AD compliance is a core CAMO and certifying staff responsibility.
AMM (Aircraft Maintenance Manual)
The manufacturer-issued manual containing step-by-step maintenance, inspection, and repair procedures for a specific aircraft model. The AMM is the primary source of data for scheduled and unscheduled maintenance and must be current and available whenever maintenance is performed.
Context: Performing maintenance without following the applicable AMM data is a violation of the continuing airworthiness requirements. The AMM must be revised to the latest level and kept accessible to maintenance personnel.
AMC (Acceptable Means of Compliance)
Non-binding guidance issued by EASA that describes acceptable means of complying with a regulation. Following an AMC (for example the AMC to Part-66 or Part-145) is generally accepted as compliance with the underlying requirement, though alternative means may be approved.
Context: When a maintenance organisation or training organisation follows the AMC, the competent authority presumes compliance. AMCs are frequently referenced in exam answers about how requirements are met.
APU (Auxiliary Power Unit)
A small gas turbine engine mounted in the aircraft (usually in the tail cone) that provides electrical and pneumatic power when the main engines are not running. Used for ground operations, engine starting, and air conditioning.
Context: Maintenance of APUs falls under the B1 powerplant scope. APUs have their own maintenance schedules and component lives managed through the aircraft maintenance programme.
ARC (Airworthiness Review Certificate)
The certificate issued after a successful airworthiness review under Part-M.A. Subpart I, confirming that the aircraft is in a condition for safe operation. It is issued by the CAMO or an authorised airworthiness review organisation and is typically valid for one year, with the possibility of a three-year programme for certain private aircraft.
Context: The airworthiness review checks records, AD compliance, and the physical condition of the aircraft. Without a valid ARC, a non-commercial aircraft cannot operate.
ATA Chapters
The standardised numbering system for aircraft maintenance documentation developed by the Air Transport Association (now Airlines for America). Each aircraft system is assigned a chapter number — e.g., ATA 32 for Landing Gear, ATA 72 for Turbine Engine, ATA 21 for Air Conditioning. The ATA iSpec 2200 specification governs this standard.
Context: Every maintenance engineer must know ATA chapter numbers to navigate maintenance manuals efficiently. ATA chapter references appear on virtually every maintenance document.
B
Basic Module Examinations
The 17 module exams of Part-66 Appendix I (M1 Mathematics to M17 Propeller) that certify basic knowledge for the aircraft maintenance licence. Each module has a fixed question quota and time limit, and the pass mark is 75 percent (66.A.25). Module certificates are issued by approved maintenance training organisations (MTOs).
Context: Module certificates form the basic knowledge evidence for a licence application. B1.1 candidates take modules including M11, M14, M15 and M17; B2 candidates take M3, M4, M5 and M13 at the deepest levels.
Borescope Inspection
A non-destructive inspection method using a flexible or rigid optical probe (borescope) to examine internal surfaces of engines, components, and structures without disassembly. Essential for turbine engine hot-section inspections.
Context: Commonly used during scheduled engine inspections to check compressor and turbine blades for cracks, burning, and FOD damage. Borescope inspections are a standard Part-145 task on turbine engines.
C
CAMO (Continuing Airworthiness Management Organisation)
An organisation approved under Part-M Subpart G to manage the continuing airworthiness of aircraft: it establishes and maintains the aircraft maintenance programme, tracks ADs and life-limited parts, arranges maintenance with approved organisations, and organises the airworthiness review.
Context: The CAMO is the "manager" of continuing airworthiness for an aircraft, while Part-145 organisations are the "doers". Knowing the split of responsibilities between CAMO and maintenance organisations is core Module 10 material.
CRS (Certificate of Release to Service)
The document by which an aircraft is released to service after maintenance, signed by appropriately authorised certifying staff under Part-M.A.801 or Part-145.A.50. It certifies that the work was performed correctly in accordance with the approved data and that the aircraft is fit for flight.
Context: No aircraft may operate after maintenance without a valid CRS. The signature on a CRS is a legal certification — the signatory assumes responsibility for the work performed.
CDL (Configuration Deviation List)
A document approved by the competent authority that lists non-essential airframe or engine components that may be missing or inoperative for a limited period. The CDL is aircraft-specific and supplements the MEL.
Context: A missing fairing or access panel may be permissible under the CDL, but only for a specified period. The engineer must document the deviation per the operator procedures.
Certifying Staff
Personnel holding an EASA Part-66 licence (or equivalent) who are authorised by a maintenance organisation to certify maintenance and sign the Certificate of Release to Service. Categories: B1 (mechanical), B2 (avionics), A (limited line maintenance tasks), B3 (non-pressurised piston helicopters).
Context: Certifying staff sign the CRS for aircraft and the EASA Form 1 for components. Trainees and technicians without a licence work under their supervision and cannot sign.
Continuing Airworthiness
The set of processes and requirements that ensure an aircraft remains in a condition for safe operation throughout its service life: maintenance, inspections, AD compliance, repairs, modifications, and the airworthiness review. Governed by Part-M within Regulation (EU) No 1321/2014.
Context: Continuing airworthiness is the legal framework around everything a maintenance engineer does. Module 10 covers it in depth, from the maintenance programme to the ARC.
D
DOA (Design Organisation Approval)
An approval issued under Regulation (EU) No 748/2012 (Part 21) that authorises an organisation to design aircraft, engines, propellers, and modifications, and to approve design data for repairs and modifications. DOA-approved data is the European equivalent of the design approvals used in other systems.
Context: When a structural repair exceeds the limits of the SRM or AMM, an approved design from the design approval holder (DAH) or a DOA organisation is required. Repairs and modifications must be accomplished with approved design data.
Dirty Dozen
The twelve common human error precursors in aviation maintenance: lack of communication, complacency, lack of knowledge, distraction, lack of teamwork, fatigue, lack of resources, pressure, lack of assertiveness, stress, lack of awareness, and norms. Developed by Gordon Dupont in the 1990s and central to Module 9 (Human Factors).
Context: Exam questions present a maintenance scenario and ask which Dirty Dozen factor is at play. Being able to identify the factor and recommend a countermeasure is core Module 9 knowledge.
E
EFIS (Electronic Flight Instrument System)
A glass cockpit system that replaces traditional analog flight instruments with electronic displays. EFIS includes Primary Flight Display (PFD), Navigation Display (ND), and Engine Indication and Crew Alerting System (EICAS).
Context: B2 avionics engineers work on EFIS installation, data buses, and troubleshooting; B1 engineers need to understand how the systems interface with aircraft power and instruments.
EGT (Exhaust Gas Temperature)
A critical engine instrument measuring the temperature of exhaust gases. For turbine engines, EGT is a primary indicator of engine health and is monitored during start (hot starts) and throughout operation. For piston engines, EGT is used for leaning the mixture.
Context: Abnormal EGT readings — such as a slow rise on start indicating a hot start — are key troubleshooting signals on both engine types, covered in Modules 15 and 16.
Experience Requirements (66.A.30)
The practical experience required before a Part-66 licence is issued: 2 years for category A and B3, 4 years for B1.2 (3 with recognised training), and 5 years for B1.1, B1.3, B1.4 and B2 (reduced to 3 years with a recognised technical diploma, or 2 years with a recognised technician licence). Experience is documented in a logbook under the supervision of certifying staff.
Context: The experience must be relevant to the licence category and is verified by the competent authority before licence issue. Keeping an accurate logbook is essential.
F
EASA Form 1
The authorised release certificate for aircraft components, engines, and propellers. It is issued when a component is released from a Part-145 approved maintenance organisation after maintenance, or from a production organisation under its POA. The form certifies that the work was performed in accordance with approved data and that the component is in a condition for safe operation.
Context: A component without a valid EASA Form 1 cannot be installed on an aircraft. Knowing what makes a Form 1 invalid — incorrect completion, missing signatures, unauthorised alterations — is a frequently tested Module 10 topic.
FOD (Foreign Object Debris/Damage)
Any foreign object (tools, hardware, debris) left in or near an aircraft that can cause damage to engines, systems, or structures. FOD is a critical safety concern in all maintenance environments.
Context: FOD prevention programmes are standard at all Part-145 organisations. Tool control, FOD walks, and zoned inspections are standard procedures, and FOD is a classic Module 9 and Module 7 topic.
G
GSE (Ground Support Equipment)
Equipment used to service and maintain aircraft on the ground, including tow tractors, power carts, nitrogen carts, hydraulic test stands, engine stands, and maintenance platforms. GSE must be maintained and calibrated as part of the maintenance organisation quality system.
Context: Proper GSE operation and maintenance is critical — poorly maintained GSE can damage aircraft just as easily as poor maintenance can. GSE safety appears in Module 7 (Maintenance Practices).
H
HPT (High Pressure Turbine)
The section of a gas turbine engine immediately downstream of the combustion chamber. The HPT extracts energy from the hot gas flow to drive the high-pressure compressor. HPT blades operate under extreme thermal and mechanical stress.
Context: HPT inspection intervals are governed by airworthiness limitations and ADs. Borescope inspection of HPT blades is a critical scheduled maintenance task on all turbine engines.
I
ICA (Instructions for Continued Airworthiness)
The data issued by the type certificate holder that specifies how an aircraft, engine, or component must be maintained: inspection procedures, replacement times, and airworthiness limitations. The ICA forms the basis of the aircraft maintenance programme managed by the CAMO.
Context: Maintenance must be performed using the current ICA and associated approved data. Airworthiness limitations within the ICA are mandatory, not advisory.
IPC (Illustrated Parts Catalog)
A manufacturer-issued document containing exploded-view diagrams and parts lists for an aircraft, engine, or component. The IPC is used to identify and order correct replacement parts with the proper part numbers.
Context: Every maintenance engineer uses the IPC regularly to verify part numbers during installations. Using the wrong part number can have serious consequences.
IFSD (In-Flight Shutdown)
An engine shutdown that occurs in flight, either automatically or by crew action, due to a malfunction. IFSD rate is a key reliability metric for engines and airlines.
Context: IFSD events trigger mandatory investigation and reporting requirements under the operator maintenance programme and the CAMO.
M
Magneto
A self-contained engine-driven electrical generator that produces the high voltage for spark plugs in reciprocating engines, independent of the aircraft electrical system. Key components include the permanent magnet rotor, coil, breaker points, condenser, and distributor. High-tension magnetos with impulse couplings are standard on general aviation engines.
Context: Magneto timing (E-gap), magneto-to-engine timing, and impulse coupling checks are classic Module 16 topics and routine maintenance tasks on piston engines.
MEL (Minimum Equipment List)
A document approved by the competent authority that lists specific equipment that may be inoperative for a defined period while maintaining safe operations. The MEL is aircraft-specific and derived from the Master Minimum Equipment List (MMEL).
Context: When an MEL item is deferred, the engineer must placard the inoperative equipment and make the appropriate maintenance record entry. MEL deferrals are temporary.
MMEL (Master Minimum Equipment List)
The baseline document developed by the aircraft manufacturer and approved by the authority that establishes minimum operating equipment for the aircraft type. Each operator develops their specific MEL from the MMEL.
Context: The MMEL is more restrictive than the operator MEL — operators may add items but cannot remove items from the MMEL.
MRO (Maintenance, Repair, and Overhaul)
An organisation certified to perform maintenance, repair, and overhaul services on aircraft, engines, and components. MROs range from small general aviation shops to large facilities such as Lufthansa Technik and Air France Industries.
Context: In Europe, MROs are certified under Part-145. Major MROs hold multiple approvals including EASA and other national approvals.
MTO (Maintenance Training Organisation)
An organisation approved to deliver the Part-66 basic training and module examinations (the framework formerly known as Part-147, evolving under Regulation (EU) 2023/989). MTOs issue module certificates after each exam is passed and may also deliver type training.
Context: Module exams are administered by approved MTOs, and module certificates are the basic knowledge evidence for the licence application.
N
NDT / NDI (Non-Destructive Testing / Inspection)
Inspection techniques that evaluate material properties and detect defects without damaging the component. Common NDT methods include visual, dye penetrant, magnetic particle, eddy current, ultrasonic, and radiographic (X-ray) inspection, covered in Module 6 (Materials and Hardware) and Module 7 (Maintenance Practices).
Context: NDT is essential for crack detection in landing gear, engine components, and airframe structures. NDT personnel qualification in Europe follows EN 4179 / NAS 410 and is separate from the Part-66 licence.
O
OEM (Original Equipment Manufacturer)
The company that designed and manufactured the aircraft, engine, or component, and holds the type certificate (the design approval holder). OEMs produce maintenance manuals, parts catalogs, and service bulletins. Examples include Airbus, Boeing, Pratt & Whitney, and Safran.
Context: OEM-approved data (the AMM, SRM, IPC, and instructions for continued airworthiness) takes precedence over other maintenance instructions. Engineers must follow the OEM data applicable to the aircraft.
P
Part-66 (Annex III to Regulation (EU) No 1321/2014)
The regulation establishing the aircraft maintenance licence: the categories (A, B1.1-B1.4, B2, B3), the 17 basic modules of Appendix I, the experience requirements (66.A.30), the basic knowledge requirements (66.A.25, including the 75% pass mark), and the privileges of certifying staff.
Context: Part-66 defines what a certifying engineer is authorised to do — and the limits of that authority. Every candidate should know the category privileges and experience requirements.
Part-145 (Annex II to Regulation (EU) No 1321/2014)
The regulation governing approved maintenance organisations: the requirements for approval, the privileges granted (aircraft, component, or specialised maintenance), the quality system, certifying staff authorisation, and the release to service of aircraft (CRS) and components (EASA Form 1).
Context: Most certifying engineers work inside a Part-145 organisation. Understanding its requirements — from the quality system to the release to service — is core Module 10 material.
Part-M (Annex I to Regulation (EU) No 1321/2014)
The regulation governing continuing airworthiness: the obligations of owners and operators, the maintenance programme, the CAMO (Subpart G), the performance of maintenance, AD compliance, and the airworthiness review (Subpart I) leading to the airworthiness review certificate.
Context: Part-M defines the continuing airworthiness framework that drives all maintenance activity. The split between CAMO responsibilities and Part-145 responsibilities is a favourite exam question.
POA (Production Organisation Approval)
An approval issued under Regulation (EU) No 748/2012 (Part 21) that authorises an organisation to produce aircraft, engines, propellers, and parts. Components released from production under a POA carry an EASA Form 1 certifying conformity with approved design data.
Context: The EASA Form 1 from a POA certifies the component as conforming to its approved design. This is distinct from the Form 1 issued after maintenance by a Part-145 organisation.
R
RII (Required Inspection Item)
A maintenance task that requires an independent inspection by a second qualified person before the aircraft can be released to service. RII items are critical to flight safety — examples include flight control rigging, engine installation, and landing gear retraction tests. Required under the Part-145 quality system.
Context: RII programmes are part of the approved maintenance organisation quality system. Performing an RII task without the required independent inspection is a serious violation.
RTS (Release to Service)
The formal process of releasing an aircraft back to operational service after maintenance, certified by the Certificate of Release to Service (CRS) signed by authorised certifying staff under Part-M.A.801 or Part-145.A.50. Components are released with the EASA Form 1.
Context: No aircraft may operate after maintenance without the proper release-to-service records. The engineer who signs assumes legal responsibility for the work.
S
SB (Service Bulletin)
A manufacturer-issued document that provides recommendations for product improvement, modification, or enhanced inspection. Unlike Airworthiness Directives, SBs are generally not mandatory unless specifically required by an AD or the approved maintenance programme.
Context: Many operators choose to comply with SBs proactively. Some SBs become mandatory when they are incorporated into an AD or the CAMO maintenance programme.
SRM (Structural Repair Manual)
The manufacturer-issued manual that provides data for structural repairs on a specific aircraft model. The SRM defines damage limits, repair schemes, material specifications, and allowable loads for structural repairs.
Context: Repairs within SRM limits can be performed using the SRM as data. Repairs exceeding SRM limits require approved design data from the design approval holder or a DOA organisation.
SMS (Safety Management System)
The systematic approach to managing safety, endorsed by ICAO and embedded in the EASA framework, covering hazard identification, risk management, safety assurance, and safety promotion — including a just culture that encourages error reporting.
Context: Maintenance organisations integrate SMS principles with the Part-145 quality system. Safety culture, error reporting, and just culture are core Module 9 topics.
T
TBO (Time Between Overhauls)
The manufacturer-recommended interval (in flight hours, cycles, or calendar time) between major overhauls of an engine, component, or accessory. TBO intervals are established based on reliability data and may be extended under engine health monitoring programmes.
Context: Exceeding TBO renders the component unairworthy unless an approved extension programme is in place. TBO compliance is tracked in the aircraft maintenance programme.
Turbine Spool
A rotating assembly in a gas turbine engine consisting of a compressor stage and a turbine stage connected by a shaft. Most turbine engines have two spools (N1 low-pressure and N2 high-pressure), and some large engines have three (N1, N2, N3). Each spool rotates at its own speed, indicated by its own tachometer reading.
Context: Understanding spool configurations — including which compressor and turbine sections drive which — is core Module 15 knowledge for turbine engine troubleshooting.
Type Training / Type Rating
The additional training and examination on a specific aircraft type required before a B1 or B2 licence holder can certify maintenance on that type. Type training covers the type-specific systems, limitations, and maintenance practices and is delivered by approved training organisations.
Context: The Part-66 licence provides the basic knowledge; type ratings add the aircraft-specific authorisation. Certifying staff can only sign for types covered by their type training.
W
W&B (Weight and Balance)
The calculation of aircraft weight and centre of gravity position to ensure the aircraft remains within certified limits. Weight and balance data must be updated after any modification, repair, or equipment change that affects aircraft weight.
Context: Engineers performing modifications must update W&B data. The W&B report is part of the aircraft permanent records and must be current.
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