Aircraft Maintenance and Repair Industry Terminology

A-Check

A short-interval scheduled maintenance visit performed at or near the gate or overnight, focused on routine inspections, servicing, and minor defect rectification. Intervals vary by type and operator (often hundreds of flight hours or calendar days).

- Overnight A-check performed at 600 FH interval - Service fluids, inspect wheels/brakes, and clear minor defects - Conduct general visual inspections at the gate before first flight

AD (Airworthiness Directive)

A legally enforceable rule issued by an aviation authority (e.g., FAA, EASA) mandating inspections, modifications, limitations, or actions to correct an unsafe condition.

- FAA AD mandating fan blade inspections on specific engine models - EASA AD requiring avionics software updates within 30 days - Logging AD compliance with references and next due in aircraft records

AMM (Aircraft Maintenance Manual)

The manufacturer’s primary maintenance manual that provides approved procedures, torque values, test steps, and limits for aircraft-level maintenance.

- Perform elevator rigging per AMM 27-31-00 - Apply torque value from AMM for flap actuator installation - Execute post-replacement test per AMM after pump change

AMP (Aircraft Maintenance Program)

The operator’s approved set of scheduled maintenance tasks and intervals derived from MRBR/MSG-3, MPD, ADs, and operational data; forms the basis for planning checks.

- AMP task 32-12-00-200-001 scheduled every 1,200 FH - Interval escalation justified via reliability program and authority approval - Align AMP with latest MPD revision during annual review

AOG (Aircraft on Ground)

A status indicating an aircraft is grounded due to a maintenance issue or missing part, requiring urgent support to return to service.

- Aircraft AOG at JFK due to hydraulic leak - AOG desk expedites shipment of a replacement LRU - Borrow part via pool to clear AOG within 6 hours

ATA Chapter (ATA iSpec 2200/Spec 100)

An industry standard that structures technical documentation by system (e.g., ATA 21 Air Conditioning, 27 Flight Controls), ensuring consistent navigation across manuals.

- Refer to ATA 32 for landing gear tasks - Use ATA 21 to navigate air conditioning procedures in AMM - Record defect under ATA 36 (pneumatics) in the logbook

Back-to-Birth Traceability

A complete documentation trail for a part from manufacture through every install/removal and maintenance event—critical for life-limited and safety-critical parts.

- Review full life history of a fan disk LLP from manufacture to current cycles - Verify 8130-3/Form 1 tags, shop reports, and removal/installation records - Reject rotable lacking BTB documents for critical application

Base Maintenance

Deeper, scheduled maintenance performed in a hangar environment (e.g., C- and D-checks), including structural inspections, major repairs, and modifications.

- Perform 12-day C-check in a hangar with deep structural inspections - Embody cabin modification and corrosion prevention tasks - Conduct fuel tank structural repairs under controlled conditions

Borescope Inspection

An internal visual inspection of engines or structures using a flexible scope to assess condition without disassembly; often driven by trends or ADs.

- On-wing inspection of HPT blades after EGT trend deterioration - Post-FOD borescope of engine core to assess damage - AD-driven borescope inspection at defined intervals

C-Check

A significant base maintenance visit performed at multi-month/year intervals to inspect structures/systems, embody SBs, and conduct deep cleaning/repairs.

- Perform C2 check at 18-month interval - Execute CPCP tasks and extensive corrosion inspections - Incorporate multiple service bulletins during the visit

Cannibalization (Part Robbing)

Removing a serviceable part from one aircraft to install on another to restore service; controlled to minimize cascading delays and configuration risks.

- Remove VHF transceiver from parked aircraft to return another to service - Use controlled cannibalization with approvals and tracking - Raise requisition to replenish the robbed LRU promptly

CBM (Condition-Based Maintenance)

A maintenance philosophy that uses real-time condition data to schedule tasks based on actual wear/health rather than fixed intervals.

- Replace brakes based on wear sensor readings instead of fixed hours - Trigger bearing inspection from vibration trend exceedances - Oil analysis results drive filter change intervals

CDL (Configuration Deviation List)

A list of external parts that may be missing or inoperative under defined conditions, with performance penalties and limitations specified.

- Dispatch with a missing gear door fairing with speed penalty applied - Operate without a belly panel per CDL 53 limitations - Coordinate MEL and CDL requirements before release

CMM (Component Maintenance Manual)

The OEM or approved manual detailing shop-level repair, overhaul, and test procedures for a component, including limits and tooling.

- Overhaul hydraulic pump per CMM 29-11-01 - Perform test bench run using limits defined in the CMM - Identify replacement subcomponents from CMM illustrated parts list

CRS (Certificate of Release to Service)

An authorized statement (or tag) that maintenance is complete and the aircraft or component is returned to an airworthy condition in accordance with regulations.

- Sign aircraft CRS in the tech log after A-check completion - Issue component CRS via EASA Form 1 following shop overhaul - Authorized certifying staff releases work package to service

D-Check/Heavy Maintenance Visit (HMV)

The most extensive scheduled visit (often years apart) involving deep structural inspections, major repairs, and broad system refurbishment; now often termed HMV.

- Execute 6-year HMV with full interior removal and structural NDT - Perform major corrosion repairs and repaint - Embody large modification packages across multiple systems

DER (Designated Engineering Representative)

An FAA-authorized engineer who can approve data for major repairs/alterations and issue engineering approvals within delegated areas.

- Approve data for a structural repair via FAA Form 8110-3 - Authorize alternative materials/processes for a major repair - Review and approve analysis for major alteration installation

Digital Twin

A high-fidelity virtual model of an asset synchronized with operational and maintenance data to simulate behavior and enable predictive maintenance.

- Engine digital twin predicts EGT margin loss and shop visit timing - Simulate wing loads to forecast fatigue and plan inspections - Optimize maintenance intervals using twin-based degradation models

Dual Release (8130-3/EASA Form 1)

A component airworthiness tag endorsed for both FAA and EASA, enabling acceptance in multiple regulatory jurisdictions.

- Component tagged with dual release acceptable to both FAA and EASA - MRO issues dual release after overhaul to enable cross-border use - Avoids re-certification when shipping parts between US and EU operators

EASA Part-145 Approval

European approval for maintenance organizations specifying capabilities, facilities, management system, and procedures to perform regulated maintenance.

- Maintenance organization audited for compliance with Part-145 and MOE - Capability list includes A320 family and CFM56 engines - Line station operates under the parent Part-145 approval

ECTM (Engine Condition Trend Monitoring)

Systematic analysis of stabilized engine parameters over time to detect performance deterioration and trigger maintenance actions.

- Analyze stabilized cruise data for EPR/N1/N2/EGT trends monthly - Detect step change triggering borescope inspection - Use ACMS/FOQA data to support maintenance planning

EGT Margin

The difference between an engine’s observed exhaust gas temperature at a defined condition and its redline limit; indicates remaining performance capability.

- Engine shows 40°C EGT margin at takeoff reference condition - Plan performance restoration when margin approaches minimum - Monitor margin loss rate as indicator of turbine wear

ERP (Enterprise Resource Planning)

Integrated software used by MROs/operators to manage maintenance planning, work orders, materials, tooling, labor, and financials.

- Create and manage work orders and task cards in the ERP - Track rotable inventory, tooling, and costs across bases - Integrate maintenance planning with purchasing and finance

ETOPS (Extended-range Operations)

Standards governing extended operations on routes distant from diversion airports; require enhanced maintenance, reliability, and procedures (also termed EDTO).

- Operate twinjets on ETOPS-180 transoceanic routes - Perform additional APU checks and system tasks per ETOPS program - Plan diversion time and alternates meeting ETOPS rules

FAA Form 8130-3

The FAA airworthiness approval tag used to certify a component’s release to service, conformity, or export status.

- Issue 8130-3 for return to service after component repair - Provide export 8130-3 when shipping parts internationally - Supply conformity release 8130-3 for new production parts

FC/FH (Flight Cycles/Flight Hours)

Primary utilization metrics used to control maintenance intervals, fatigue life, and engine LLP tracking.

- Control engine LLP limits by flight cycles - Schedule structural inspections at 4,000 FH - Track component life and maintenance intervals by FC/FH

FCF (Functional Check Flight)

A post-maintenance flight to verify aircraft systems performance after major work, per a defined test profile and approvals.

- Conduct FCF after heavy maintenance to verify system performance - Fly defined test profile including autopilot and stall protection checks - Obtain required approvals and risk assessment before FCF

FOD (Foreign Object Debris/Damage)

Debris in the operating area that can cause damage to aircraft/components; also the resulting damage from such debris.

- Perform FOD walk to remove loose hardware from ramp - Fan blade nick caused by runway FOD ingestion - Install covers and follow housekeeping practices to prevent FOD

GSE (Ground Support Equipment)

Tools and equipment supporting maintenance and ground operations, such as tugs, GPUs, jacks, and specialized tooling.

- Use tripod and axle jacks for gear maintenance - Connect 90 kVA GPU for power during checks - Service struts with nitrogen cart and service panel tools

GVI (General Visual Inspection)

A baseline inspection level requiring adequate lighting and direct line of sight to detect obvious damage, deterioration, or discrepancies.

- Perform GVI of fuselage skin for dents and loose fasteners - Inspect for fluid leaks and obvious damage with flashlight - Conduct GVI of empennage panels during A-check

Hangar Slot

An allocated period and space in a maintenance hangar to perform scheduled work; a key capacity planning and commercial constraint.

- Reserve a 10-day slot in May for a C-check - Resolve slot clash by shifting aircraft input by two days - Capacity plan shows four widebody bays available

Hard Time (HT) Maintenance

A maintenance policy requiring removal, overhaul, or replacement at a fixed interval of hours, cycles, or calendar time.

- Overhaul hydraulic pump every 6,000 FH per HT policy - Replace oxygen bottle at 6-year calendar limit - Remove fan disk at 20,000 cycles per certified life limit

HUMS (Health and Usage Monitoring System)

Systems (common on rotorcraft and some fixed-wing) that collect vibration and usage data to assess component health and forecast failures.

- Rotorcraft HUMS detects gearbox bearing fault via vibration - Perform track-and-balance adjustments using HUMS data - Use usage spectra to refine component maintenance intervals

ICA (Instructions for Continued Airworthiness)

Approved maintenance instructions issued by the design/STC holder that define how to maintain a modification or product in service.

- Add STC winglet ICA tasks to the operator’s AMP - Follow ICA wiring inspection intervals for EWIS modification - Use ICA to define post-installation checks and limits

Inventory Turnover

A business KPI measuring how often inventory is used/sold over a period; in MRO, indicates spares velocity and cash efficiency.

- Achieve 3x annual turnover on rotable stock - Identify slow-moving spares for consignment or pooling - Improve cash efficiency by right-sizing safety stock

IPC (Illustrated Parts Catalog)

The parts reference with exploded diagrams, part numbers, interchangeability, and effectivity, used for accurate parts identification.

- Identify correct door seal P/N using IPC figure and item number - Check effectivity to ensure part applicability to MSN range - Verify supersedure chain before ordering replacement

Job Card (Task Card)

A detailed work instruction derived from the AMP/AMM/CMM specifying steps, references, tooling, skills, and sign-offs for a maintenance task.

- Execute pack filter change per task card 21-51 including steps and torque - Complete RII sign-off where required on the card - Reference AMM/CMM sections listed on the job card

Kitting (Pre-draw Kits)

Pre-picking and packaging all parts, materials, and standard hardware required to execute a task or work package efficiently.

- Pre-draw kit with all parts and consumables for task 11-21-00 - Bag and tag hardware by step to reduce errors - Deliver kits to point of use before aircraft input

Lead Time

The elapsed time from initiating a purchase/repair order to receiving the part or service; a key planning and supply-chain variable.

- OEM repair lead time quoted at 90 days - Expedite order using AOG priority to reduce lead time - Plan alternates due to long lead items on critical path

Line Maintenance

Routine maintenance performed on the flight line or at outstations, including daily/weekly checks, defect rectification, and minor scheduled tasks.

- Perform daily/weekly checks on the flight line - Troubleshoot and rectify MEL item during turnaround - Conduct ETOPS pre-departure service check at outstation

LLP (Life-Limited Part)

A part with a certified life limit in cycles/hours/calendar time after which it must be removed from service; requires strict traceability.

- Track fan disk life to 20,000 cycles with strict BTB records - Record cycles consumed at each removal/installation event - Scrap HPT disk at certified limit and update engine records

LRU (Line Replaceable Unit)

A modular component designed for quick line-level replacement to minimize downtime; repaired later at shop level.

- Swap flight control computer LRU on the line to clear a fault - Return removed LRU to shop for repair and test - Use quick-disconnects for rapid LRU change-out

MEL (Minimum Equipment List)

An operator-specific, authority-approved list stating which equipment may be inoperative for dispatch and the associated conditions, time limits, and procedures.

- Dispatch with one VHF radio inoperative per MEL with time limit - Apply placards and maintenance procedures required by MEL item - Clear MEL defect within the category A/B/C/D limit

MPD (Maintenance Planning Document)

The manufacturer’s recommended scheduled maintenance tasks and intervals for a type, used to build the operator’s AMP.

- Build check packages from MPD Rev 27 task intervals - Group MPD tasks by access to optimize visit duration - Propose interval escalation based on reliability and authority approval

MRO (Maintenance, Repair, and Overhaul)

The industry sector and companies that perform maintenance, repair, and overhaul services for aircraft, engines, and components.

- Independent MRO performs C-checks for multiple operators - Engine MRO shop overhauls CFM56 modules - Component MRO repairs avionics LRUs under Part-145

MSG-3 (Maintenance Steering Group-3)

A methodology for developing scheduled maintenance programs using reliability-centered maintenance principles and consequence analysis.

- Use MSG-3 logic to derive lubrication and inspection tasks - Perform consequence-of-failure analysis to set intervals - Develop MRBR recommendations for new aircraft type

NCR (Nonconformance Report)

A formal record documenting a deviation from requirements, with dispositions such as use-as-is, rework, repair, or scrap.

- Raise NCR for out-of-tolerance fastener hole during C-check - Engineering disposition: repair with oversized fastener - QA closes NCR after verification and documentation

NDT (Non-Destructive Testing)

Inspection methods (e.g., ultrasonic, eddy current, magnetic particle, dye penetrant, radiography) that detect flaws without damaging the part.

- Eddy current inspection of fastener holes for cracks - Ultrasonic inspection of wing spar during HMV - Dye penetrant check on landing gear fork

NFF (No Fault Found)

A result where a removed component tests satisfactory in the shop; often signals intermittent faults or misdiagnosis.

- Removed avionics unit tests satisfactory on the bench - Intermittent fault suspected after NFF result - Improve troubleshooting to reduce NFF returns

OEM (Original Equipment Manufacturer)

The original designer/manufacturer of the aircraft, engine, or component; issues manuals, service bulletins, and provides approved data.

- Boeing issues AMM, IPC, and service bulletins for aircraft - Engine OEM provides CMMs and repair limits for modules - Seek OEM-approved data for major structural repairs