01Where the tasks come from
Almost every explanation of maintenance checks starts in the wrong place — with the letters. A, B, C and D are packaging labels. They describe how a set of tasks has been grouped into a visit; they say nothing about where those tasks came from or why they exist. If you want to understand maintenance checks properly, start one level up.
Every scheduled task on a modern transport aircraft can be traced to one of four origins:
- MSG-3 analysis — the great majority of routine tasks, derived by structured engineering analysis during type certification.
- Airworthiness Limitations (ALS/ALI) — mandatory life limits and inspections arising from the certification basis. These cannot be escalated or removed by the operator.
- Certification Maintenance Requirements (CMRs) — tasks required to substantiate the safety analysis behind certification, likewise mandatory.
- Directives and bulletins — airworthiness directives, plus service bulletins and service letters the operator elects or is required to embody.
Only the first of those is negotiable in the ordinary sense. The middle two are fixed by the design approval and can be changed only with the airworthiness authority’s permission. The fourth arrives continuously over the life of the aircraft and is covered in our airworthiness directives guide.
02MSG-3 in practice
MSG-3 is the methodology used to develop the initial scheduled maintenance requirements for a new type. It is maintained by Airlines for America (A4A), formerly the Air Transport Association, and it is not optional in any meaningful sense: EASA recognises it as an appropriate methodology, and the FAA requires the latest version of the ATA MSG analysis process for the development of an MRBR for all new aircraft or engines.
The analysis is performed by an Industry Steering Committee — airframe manufacturer, engine manufacturer, vendors and operators — which appoints Maintenance Working Groups covering four areas: systems and powerplant, structures, zonal inspections, and lightning/HIRF.
Why MSG-3 replaced MSG-2
The change was philosophical, not cosmetic. MSG-2 was process-oriented and bottom-up: each item was assigned to hard-time, on-condition or condition-monitoring. MSG-3 is task-oriented and top-down: for each Maintenance Significant Item it asks what task would be applicable (technically capable of addressing the failure) and effective (worth doing).
The underlying principles are worth stating plainly, because they contradict the intuition that more maintenance is safer:
Excessive maintenance does not improve reliability. Needless tasks introduce human error. Few complex items exhibit a wear-out age. Monitoring is generally more effective than hard-time overhaul. Reliability is improved by modification, not by inspection.
The decision logic
Level 1 asks whether the functional failure is evident to the operating crew during normal duties, or hidden. It then classifies the consequence. That produces five Failure Effect Categories:
| FEC | Category | Consequence | Is a task required? |
|---|---|---|---|
| 5 | Evident Safety | Direct adverse effect on operating safety | Required. If no single task is adequate, a combination is mandated — or redesign |
| 6 | Evident Operational | Operational impact, not safety | Desirable if it reduces risk to an acceptable level |
| 7 | Evident Economic | Economic consequence only | Desirable if the task costs less than the failure |
| 8 | Hidden Safety | Hidden failure which, with a further failure, has an adverse safety effect | Required. Combination task permitted |
| 9 | Hidden Non-Safety | Hidden failure, economic consequence | Desirable if economically justified |
The asymmetry between categories 5 and 8 on one hand and 6, 7 and 9 on the other is the heart of the method. For the two safety categories a task is required; for the rest it is selected on cost-effectiveness grounds. This is why an aircraft maintenance programme is not a list of everything that could be inspected — it is a list of what is worth inspecting, with a hard floor under anything safety-related.
The task types
MSG-3 produces eight task types, conventionally presented in the Level 2 logic as five paired groupings. Sources that say “five tasks” are describing the grouping, not the count.
| Group | Types | What it does |
|---|---|---|
| Lubrication / Servicing | LUB, SVC | Replenish consumables to maintain design capability and slow deterioration |
| Operational / Visual Check | OPC, VCK | Failure-finding tasks — confirm the item still fulfils its purpose. Applied to hidden failures only (FEC 8 and 9) |
| Inspection / Functional Check | GVI, DET, SDI, FNC | Detect degradation at three levels of scrutiny — general visual, detailed, special detailed — or compare a measured value against a standard |
| Restoration | RST | Return an item to a specified standard at or before an age limit — from cleaning up to full overhaul |
| Discard | DIS | Remove from service at a life limit where restoration is not feasible |
The operational and visual checks are the ones most often misunderstood in planning. They are failure-finding tasks and exist only because the failure is hidden — the crew would not know it had happened. Deferring one is not the same class of decision as deferring a general visual inspection, because the whole point of the task is that nothing else will reveal the condition.
03MRBR, MPD and AMP — the document chain
Four documents stand between the MSG-3 analysis and the task card a technician holds. Confusing them is the most common conceptual error in this area, and the relationship is not a simple linear hand-off.
| Document | Owned by | What it is |
|---|---|---|
| MRBR | Authority (Maintenance Review Board) | The minimum scheduled tasking and interval requirements for a type and its on-wing engine programme. The FAA states plainly that the MRBR is not an operator maintenance programme — it is the framework each operator builds on. |
| MPD | Airframe manufacturer | MRBR tasks plus CMRs, Airworthiness Limitations and manufacturer-recommended tasks. The working document for programme development. |
| AMP | Operator, approved by its competent authority | The operator’s customised programme: MPD content adapted to utilisation and operating environment, plus ADs, elected SBs, vendor recommendations and operator-specific tasks. Called OAMP in US usage. |
| Task cards | Operator / MRO | The executable instruction, referencing AMM procedures. |
Under EASA, the AMP requirement sits in M.A.302 of Regulation (EU) No 1321/2014: maintenance of each aircraft must be organised in accordance with an AMP, and the AMP and its amendments must be approved by the competent authority — though where a CAMO or CAO manages the aircraft, an indirect approval procedure set out in its CAME may be used. See our CAMO and Part-M guide.
04The letter checks, as traditionally described
Here is the conventional account, which you will find on every aviation website. Read it, then read the next section, because for a current-production aircraft roughly half of it is no longer accurate.
| Check | Traditional description | Typical scope |
|---|---|---|
| A check | Light check, every few hundred flight hours, performed overnight at the gate or in a hangar | General visual inspections, servicing, lubrication, filter changes, operational checks, minor rectification |
| B check | Intermediate check, every 6–8 months, 1–3 days | Deeper inspection than an A check; historically ~160–180 man-hours |
| C check | Base check, roughly every 18–24 months, aircraft out of service for days to weeks | Extensive structural and systems inspection, functional checks, component restoration, cabin work |
| D check | Heavy structural check, every 6–10 years | Near-teardown: interior stripped, panels removed, structural inspection, corrosion treatment, repaint |
05What is actually true in 2026
Two of those four rows describe categories that no longer exist in the maintenance planning documents of current-production aircraft.
The B check is effectively extinct
It is not merely “less common”. Neither the A320 family nor the 737NG maintenance structure contains a B check in the industry engineering literature; the content has been absorbed into escalated and expanded A checks. The label survives for some older types and in parts of the business-aviation world, but if you are planning a modern narrowbody, there is no B check to plan.
“D check” is now colloquial rather than technical
This is the stronger finding, and it is the one most explanations get wrong. For the A350, Airbus has removed the traditional letter checks from the MPD entirely, replacing them with task groupings at multiples of 1,200 flight hours and 36-month intervals; the base cycle runs C1 to C4 at 36-month intervals over twelve years, and only the 144-month check is likely to be a heavy visit. On the 737NG the structure is six base checks, C1 to C6, over twelve years with structural milestones at 6, 8, 10 and 12 years. On the A320 family it is six C checks over roughly 45,000 flight hours and twelve years, with 6-year and 12-year escalation packages attached to C3 and C6.
In none of those structures is there a discrete D check. Heavy structural work is delivered as a calendar-driven escalation package bolted onto a base check — commonly called a heavy maintenance visit (HMV) or simply a base check.
“D check” persists in three places where it matters commercially: lease agreements, insurance wording and maintenance reserve calculations. A redelivery condition drafted around a “D check” for an aircraft whose MPD contains no such event is a dispute waiting to happen — and the argument surfaces at redelivery, when both parties have the least room to manoeuvre. If you are negotiating a lease, define the event by its MPD reference, not by its letter.
06Real intervals and man-hours
Published figures for maintenance checks vary enormously in quality. The numbers below come from the industry engineering literature for two specific narrowbody families, and they differ materially from the figures that circulate on general reference sites.
| Parameter | A320 family | 737NG |
|---|---|---|
| A check interval | 750 FH / 750 FC / 4–6 months, whichever first | ~1,000 FH / 600–700 FC / 90–120 days |
| A check man-hours | ~150 MH average | Folded into base-check figures |
| A checks per year | 3–4 at 3,000–3,500 FH annual utilisation | ~every 3 months |
| C check interval | 7,500 FH / 5,000 FC / 24 months, whichever first | 24–36 months; typical cycle 2.5 years |
| C check downtime | ~2 weeks average; as little as 3 days (C1), up to 3 weeks (C6 with 12-year items); findings can add a further week | — |
| Light C check (routine + non-routine MH) | C1: 3,760 + 660 | C1: 686 MH |
| Heaviest check (routine + non-routine MH) | C6 + 12-year: 14,500 + 5,400 | C12: 6,044 MH |
| Materials per C check | ~$37k (C1) rising to ~$90k (C6+12YE) and ~$100k (C9+6YE) | — |
| Full base cycle | 6 C checks / ~45,000 FH / 12 years | 6 C checks (C1–C6) / 12 years |
“A checks every 400–600 flight hours.” Superseded for current narrowbodies. The manufacturer-approved figures are 750 FH for the A320 family and around 1,000 FH for the 737NG. The lower numbers reflect intervals from an earlier era that have since been escalated.
“A D check takes six months to a year and 50,000 man-hours.” Far above anything in the narrowbody engineering data, where the heaviest A320 check is roughly 19,900 man-hours over about three weeks. Those high figures appear to reflect older or widebody aircraft, and they circulate widely without that qualification.
Intervals are set per type in the MPD and per operator in the approved AMP, and they are escalated over a type’s service life as reliability data accumulates. Any single published number is indicative only. The binding limit is whichever of flight hours, flight cycles or calendar time is reached first — and which one binds depends entirely on the operator’s utilisation and average sector length. A short-haul operator flying six sectors a day will hit the cycle limit; a low-utilisation corporate operator will hit the calendar limit years before either.
07How intervals move
Maintenance intervals are not fixed properties of an aircraft type. They are engineering positions that change as evidence accumulates, and the direction of travel over the last three decades has been consistently upward. The A320 A check moved from 500 to 600 flight hours, with the C check going to 20 months and 6,000 flight hours, approved by EASA, the FAA and Transport Canada; current figures of 750 FH and 7,500 FH / 24 months represent further escalation beyond that.
Escalation is evidence-driven and authority-approved. It typically requires reliability data demonstrating that the failure modes the task addresses are not appearing at the current interval, a sampling programme, and a formal submission. It is one of the highest-return engineering activities available to an operator — a 25% interval extension across a fleet removes a proportionate slice of downtime and labour permanently — and it is one of the first things to be cut when technical services are under-resourced.
Escalation cuts both ways in a lease. An operator who has escalated its AMP beyond the MPD baseline holds an aircraft whose next-due positions do not match a redelivery condition written against the MPD. This is a recognised negotiation point: where the lease specifies return “in accordance with the MPD” and the outgoing operator has been running an escalated programme, someone has to pay to bring the aircraft back to the contractual baseline. Check which standard your lease uses before you escalate.
08Block versus phased maintenance
Two philosophies exist for turning a task list into visits, and the industry is moving from the first toward the second.
The trend is real and manufacturer-endorsed — Airbus removing letter checks from the A350 MPD is a design-authority act, not an operator preference — and it is documented at named carriers. What nobody can tell you is how widespread it is. There is no published figure for the proportion of operators running equalised programmes, and practice varies by operator, type and MRO arrangement. Light checks are variously called phases, Q checks or simply A checks depending on the airline.
What drives the shift: aircraft availability, smoother and more predictable manpower loading, avoidance of large sporadic labour peaks, and the ability to fit work into overnight or short ground windows. What works against it: more frequent visits mean more hangar entries, more paperwork events and more opportunities for a task to be missed at a segment boundary.
09Why heavy checks overrun
Every planner knows heavy checks overrun. The engineering data explains why with unusual clarity, through a single ratio: non-routine man-hours as a proportion of routine man-hours. On the A320 family that ratio climbs steeply through the base cycle.
Three practical consequences follow. First, slot length must be quoted with a findings allowance that grows with airframe age, and a fixed allowance applied across a fleet of mixed ages will be wrong at both ends. Second, the material forecast is unreliable by construction — you cannot order parts for damage you have not yet found, which is why heavy-check material costs rise from roughly $37k at C1 to $90–100k at the heaviest visits. Third, and most important commercially, a heavy check immediately before a lease redelivery is the worst possible sequencing: the findings ratio is at its highest precisely when the schedule has the least slack and the counterparty is watching.
10Key terms
- MSG-3
- Task-oriented analysis methodology maintained by A4A, used to develop initial scheduled maintenance tasks and intervals for a type.
- MSI
- Maintenance Significant Item — an item selected for MSG-3 analysis on the basis of its failure consequences.
- FEC
- Failure Effect Category, 5 to 9 — classifies a functional failure as evident or hidden and by safety, operational or economic consequence.
- MRBR
- Maintenance Review Board Report — the authority-approved minimum scheduled tasking for a type. Explicitly not an operator programme.
- MPD
- Maintenance Planning Document — the manufacturer’s document; a superset of the MRBR including CMRs, ALS and recommendations.
- AMP
- Aircraft Maintenance Programme — the operator’s customised, authority-approved programme. OAMP in US usage.
- CMR
- Certification Maintenance Requirement — a task required to substantiate the certification safety analysis. Mandatory.
- ALS / ALI
- Airworthiness Limitations Section / Item — mandatory life limits and inspections from the certification basis.
- GVI / DET / SDI
- General Visual Inspection, Detailed Inspection, Special Detailed Inspection — three escalating levels of inspection scrutiny.
- HMV
- Heavy Maintenance Visit — the current term for the heavy structural visit formerly called a D check.
- 6YE / 12YE
- Six-year and twelve-year escalation packages — calendar-driven structural task groups attached to a base check.
- Non-routine
- Work arising from findings during a check, as opposed to the planned routine task content.
11Frequently asked questions
What is the difference between line maintenance and base maintenance?
Line maintenance is work performed while the aircraft remains in service — turnrounds, transit and daily checks, defect rectification, and light scheduled checks that fit an overnight window. Base maintenance is work requiring the aircraft to be withdrawn from service into a hangar for an extended period, typically the C check and heavier. The boundary is practical rather than regulatory, and an A check may fall either side of it depending on the operator and the facility.
Can an operator change its maintenance intervals?
Yes, with authority approval and supporting evidence. Under EASA, the AMP and its amendments are approved by the competent authority under M.A.302, or through an indirect approval procedure set out in the CAME where a CAMO or CAO manages the aircraft. Escalation normally requires reliability data showing the failure modes the task addresses are not materialising at the current interval. The exception is Airworthiness Limitations and CMRs, which travel with the design approval and cannot be escalated by the operator.
Why do C check intervals sometimes quote both hours and months?
Because different failure mechanisms run on different clocks. Fatigue and wear track flight hours and cycles; corrosion, seal degradation and material ageing track calendar time. An interval expressed as “7,500 FH / 5,000 FC / 24 months, whichever occurs first” is addressing all three. Which limit binds depends on utilisation — and for a low-utilisation operator the calendar limit will bind every time.
What does “whichever occurs first” actually oblige me to do?
Compute every applicable clock, take the earliest resulting due point, and record which one governed. It is a hard constraint, not a planning preference, and the record of which limit drove the due date is what an auditor will ask for.
Is a heavy check the same as a D check?
In common speech, yes. Technically, on current-production types there is usually no D check to be the same as. Heavy structural work is delivered as a calendar escalation package — a 6-year or 12-year package, or a C4/C6-class check — attached to a base visit. The distinction matters when the term appears in a lease, an insurance policy or a maintenance reserve calculation, where it needs to be tied to an MPD reference rather than a letter.
Who approves the aircraft maintenance programme?
The operator’s competent authority, under M.A.302(b) in the EASA system. Where continuing airworthiness is managed by an approved CAMO or CAO, amendments may be handled under an indirect approval procedure established in that organisation’s CAME or CAE. In the FAA system for larger and turbine-powered aircraft, 14 CFR 91.409(f) sets out four selectable inspection programme options, one of which is a programme established by the owner and approved by the Administrator.