C-check and D-check: heavy aviation maintenance explained

A modern commercial aircraft does not age like a car or an industrial machine. Its longevity — often 25 to 35 years of service — relies on a principle that civil aviation has perfected since the 1950s: replacing the concept of unmanaged wear with that of scheduled maintenance. At defined intervals, calculated in flight hours, cycles or calendar months, the aircraft returns to the shop for operations that range from a simple visual check of a few minutes to a near-complete disassembly lasting several weeks. This gradation has a universal name in the industry: the A, B, C and D-check visits, complemented by transit and daily checks and Heavy Inspections (IL).

For AéroNéo, which is structuring three hangars on the Tiaret aeronautical base to host the entire spectrum — from A-check to D-check — understanding the mechanics of these visits is not a theoretical exercise. It is the very grid that sizes the hangars, the teams, the depth of stocks, the duration of CAMO contracts and, ultimately, the economic viability of the project.

1. The hierarchy of aircraft checks: transit, daily, A, B, C, D

Before addressing heavy maintenance, each level must be placed in the inspection pyramid. It ranges from daily (at the gate, in minutes) to exceptional (in the hangar, several weeks).

• Pre-flight check / transit check: performed before each flight by the crew and line maintenance. A few minutes. External visual inspection, fluid levels, tyres, panels, general condition. No hangar entry.
• Daily check: at 24 to 48-hour intervals. A few hours. Expands the transit with system checks, technical logbook, MEL (Minimum Equipment List).
• Weekly check: every 7 to 10 days. More detailed inspections, cabin, hold and landing gear checks. Still outside the heavy hangar.
• A-check: entry into scheduled hangar maintenance. Several days.
• B-check: intermediate, largely absorbed today into the A or C depending on the manufacturer.
• C-check: standard heavy visit, 1 to 2 weeks in hangar.
• D-check or IL (Heavy Inspection): the deepest visit, several weeks, near-complete disassembly.

This gradation is not an organisational whim. It reflects a physical reality: some checks must be done daily (tyres, fluids), others monthly (filters, calibrations), others every six years (structural corrosion, wiring). Rather than imposing the same inspection depth everywhere, the industry has stratified tasks by their optimal frequency.

Each visit receives a work card package issued by the operator’s planning office or by the CAMO. These cards translate the OEM MPD requirements into executable tasks, signed by a B1 or B2 mechanic and released by certifying staff.

2. A-check: ~125 flight hours / 600 cycles, 1-3 days, 60-80 man-hours

The A-check is the first visit that requires hangar immobilisation of the aircraft. It is traditionally triggered every 400 to 600 flight hours on older generations, and now around 750 flight hours or 200 cycles on recent generations (A320neo, A330neo, 737 MAX, 787, A350). For an airline flying 10 to 12 hours per day per aircraft, that means an A-check every 8 to 10 weeks.

In practice, the A-check lasts 1 to 3 days and mobilises 60 to 80 man-hours (sometimes up to 100 depending on standard and operator). It is usually performed overnight, on the apron or in a light hangar, without major disassembly. Typical tasks:

• In-depth visual inspections, external and internal.
• Drainage and replacement of filters (hydraulic, engine oil, APU).
• Functional tests on critical systems (flight controls, gear, hydraulic circuit).
• Wear checks (tyres, brakes, shock absorbers).
• Light cabin service, but no seat or carpet removal.
• Lubrication of specific points.
• Documentation update and closure of minor discrepancies.

The A-check is the operational backbone of a fleet. It accounts for the majority of shop visits and is often the subject of line maintenance contracts with a local MRO. Performance here is measured in turnaround time (TAT): an A-check that exceeds 36 hours is costly in lost utilisation.

3. B-check: between A and C (rare today, often integrated into A or C)

The B-check belongs as much to the history of maintenance as to its present. Initially defined by manufacturers as an intermediate visit between A and C — typically every 6 to 8 months — it has progressively been absorbed into other levels. On most modern MPDs (Airbus, Boeing), the B-check no longer appears as a distinct visit: its tasks are distributed either across enhanced A-checks (sometimes called A2, A4, A6) or into the C-check.

When it does still exist, the B-check lasts 1 to 3 days and mobilises 150 to 300 man-hours. It includes deeper inspections than the A — particularly on hydraulic, electrical and pressurisation systems — without reaching the depth of a C-check. Some cargo operators and some regional manufacturers continue to use this category for economic optimisation: spreading the load between A and C rather than concentrating too many tasks at once.

For an MRO shop, the practical takeaway is simple: do not sell a B-check as a distinct service if the applicable MPD does not include one, at the risk of creating a commercial category with no regulatory content.

4. C-check: ~20-24 months or ~6000 FH, 7-14 days, 6000-8000 man-hours

The C-check is the first real hangar campaign. It is the visit that defines most world-class MROs. Its interval is set either in calendar months (20 to 24 months depending on type), in flight hours (around 6,000 FH), or in cycles, with the first threshold reached triggering the visit.

A C-check lasts 7 to 14 days, sometimes 21 if the scope includes major modifications or heavy Service Bulletins. The workload is on the order of 6,000 to 8,000 man-hours, mobilising 50 to 80 mechanics, structures specialists, avionics technicians, painters and NDT operators simultaneously. Typical tasks:

• Removal and detailed inspection of structural access panels.
• Inspection of spars, frames, floors, engine attachment points.
• Non-destructive testing (NDT) on sensitive zones: eddy current, ultrasound, dye penetrant.
• Partial removal of seats and carpets for floor and wiring (EWIS) inspections.
• In-depth maintenance of the landing gear, sometimes full removal for shop overhaul.
• Detailed avionics checks, software updates (CMS, FMS, IRS).
• Application of overdue airworthiness directives (AD) and Service Bulletins.
• Final functional tests and ground runs (engine runs, pressurization, hydraulic).
• Full update of the aircraft logbook and Aircraft Status Sheet.

The C-check is also the preferred moment for transformation operations: P2F conversion, cabin retrofit, Wi-Fi installation, livery change, new IFE. With the aircraft already grounded, the slot is used to stack tasks that would otherwise require another stop.

Economically, the C-check is the heart of the independent MRO market. On an A320 or 737, its budget typically falls between USD 0.8 million and USD 1.5 million depending on depth and manufacturer, parts excluded; on a widebody (A330, 777), it can reach USD 3 to 5 million.

5. D-check: ~6-10 years, 4-6 weeks, 30,000-50,000 man-hours (near-complete overhaul)

The D-check is the deepest visit of the maintenance cycle. Its interval is 6 to 10 years depending on aircraft type and operator standard, meaning two or three times during the commercial life of an airframe. Its duration spans 4 to 6 weeks, and the workload can reach 30,000 to 50,000 man-hours. By volume, it is the equivalent of a mini-construction.

In a D-check, the aircraft is disassembled to a degree no other visit reaches:

• Complete cabin removal: seats, partitions, ceiling panels, carpets, galleys, lavatories.
• Partial removal of interior linings for access to primary structure.
• Inspection of frames, stringers, floors, engine and gear attachments.
• Extensive NDT on the entire airframe, including hard-to-access zones.
• Full repaint of the aircraft: stripping, surface treatment, primer, livery.
• Overhaul or full replacement of the landing gear.
• Cabin redesign if the operator decides to (major retrofit).
• Inspection or overhaul of very long interval components (tanks, control surfaces).
• Application of all outstanding manufacturer Service Bulletins.

On exit, the aircraft is considered “near new” at the structural level. This regeneration has a price: a D-check on a widebody can represent USD 5 to 12 million, partly for labour, partly for parts and associated modifications.

“A D-check is no longer a visit: it is an industrial refoundation of the aircraft. On exit, you certify an airframe that can begin a full new exploitation cycle.”

It is also why the D-check is sometimes the moment of economic switch: for an operator, paying USD 8 million on an aircraft whose market value is USD 12 million may be less rational than buying a younger airframe. Skipped D-checks are one of the main reasons for early aircraft retirements towards storage and recycling streams.

6. IL (Heavy Inspection): equivalent to D-check under OEM standards

The Heavy Inspection (IL — Inspection Lourde) is the francophone nomenclature, inherited from French standards and Airbus OEM documentation, equivalent to the Anglo-Saxon D-check. In practice, IL and D-check describe the same industrial reality. Depending on OEMs and aircraft generations, related terms also exist:

• HMV (Heavy Maintenance Visit): generic term covering enhanced C-check and D-check.
• Major Check: Boeing terminology for the deepest visits.
• IL1, IL2, IL3: subcategories used in some military and francophone commercial fleets.
• 4C-check, 8C-check, 12C-check: modern Airbus nomenclature that no longer uses “D” but multiples of C, each multiple matching a threshold of additional tasks.

This terminological diversity does not change the underlying logic: at a long interval (6 to 10 years), the aircraft undergoes a near-exhaustive structural inspection that conditions the continuation of its commercial life.

Check	Typical interval	Downtime	Man-hours
A-check	400–750 FH / 200–600 cycles / ~2 months	1–3 days	60–100 h
B-check	~6–8 months (often integrated into A or C)	1–3 days	150–300 h
C-check	20–24 months / ~6,000 FH	7–14 days (up to 21)	6,000–8,000 h
D-check / IL	6–10 years / multiples of C	4–6 weeks	30,000–50,000 h

7. The MPD (Maintenance Planning Document) issued by the OEM (Boeing AMM, Airbus AMM)

All visit scheduling rests on a single document: the MPD — Maintenance Planning Document, issued and maintained by the manufacturer (OEM). For each aircraft type, it sets out the complete list of scheduled maintenance tasks, their intervals, their scope and their prerequisites. Without an up-to-date MPD, no rigorous scheduling is possible.

The MPD is complemented by other OEM documents that together form the maintenance documentation:

• AMM (Aircraft Maintenance Manual): detailed task execution procedures, by ATA chapter.
• IPC (Illustrated Parts Catalog): spare parts nomenclature with P/N.
• SRM (Structural Repair Manual): structural repair procedures.
• CMM (Component Maintenance Manual): shop-level component maintenance procedures.
• FIM / TSM (Trouble Shooting Manual): troubleshooting procedures.
• SB (Service Bulletins): recommended modifications, or made mandatory by AD.
• WDM (Wiring Diagram Manual): electrical wiring diagrams, essential for EWIS inspections.

The MPD itself integrates the conclusions of the MRB (Maintenance Review Board), the body that brings together the OEM and the authorities (FAA, EASA) to validate initial intervals using the MSG-3 methodology (Maintenance Steering Group 3, 3rd iteration). MSG-3 reasons by functions and failure modes rather than by components: this logic has, since the 1990s, considerably reduced the maintenance burden of recent aircraft.

An operator’s CAMO (Continuing Airworthiness Management Organisation) translates the MPD into an AMP — Aircraft Maintenance Programme, specific to its fleet, missions and operating conditions. It is the AMP, not the raw MPD, that the shop executes. The AMP is approved by the competent authority (ANAC in Algeria, EASA in Europe, FAA in the United States).

8. The role of PART-145 in execution

Once scheduling is established, the execution of visits falls under a strict regulatory framework: PART-145. No A, B, C or D-check can be performed outside a PART-145 organisation approved for the type of aircraft and the scope concerned. This is what distinguishes an MRO shop from a mere technical garage.

PART-145 requires, for each visit:

• Certifying staff (B1, B2, or C for heavy visits) duly qualified on the type concerned.
• A capability list that explicitly authorises the organisation to work on the type, visit level and components.
• A MOE (Maintenance Organisation Exposition) approved by the authority, describing how the shop executes the visits.
• An independent quality system with internal audits and discrepancy tracking.
• Full traceability through signed work cards and the CRS (Certificate of Release to Service) at the end of the visit.

Without PART-145, the release of the aircraft is legally void: the airframe cannot return to flight. This is what makes PART-145 the central lock of any heavy maintenance activity.

9. ANAC Algeria: supervisory authority for maintenance visits in Algeria

In Algeria, the competent authority for the oversight of maintenance visits is the ANAC (Autorité Nationale de l’Aviation Civile). It approves the maintenance programme of Algerian operators, approves PART-145 organisations established on the national territory, and oversees the proper execution of visits on aircraft registered 7T-.

The ANAC framework relies on Law 98-06 on civil aviation, its implementing texts, and progressive alignment with ICAO standards (Annexes 6, 8, 19) as well as on EASA and FAA frameworks in their international applications. In practice, an Algerian operator running an A330 can only schedule a C-check or D-check by complying with an AMP validated by ANAC, executed either by an approved Algerian PART-145 shop or by a foreign shop recognised through bilateral arrangements.

For an MRO under structuring in Algeria, the order of steps is clear:

1. ANAC PART-145 approval covering an initial scope (A1 on certain types, for example).
2. Progressive scope extension (B1, C, D depending on ratings) as audits and demonstrated capabilities allow.
3. International recognition (EASA, FAA) in a second phase, through bilateral agreements and external audits.

Algerian alignment first, international opening next, is the only realistic path to building a credible MRO actor on national soil.

10. AéroNéo: C-check and D-check capacity projected across the three hangars

AéroNéo is in pre-launch on the Tiaret aeronautical base. The industrial plan envisages, at maturity, the operation of three hangars sized to cover the full heavy visit spectrum: C-check on narrowbody (A320, 737) and widebody (A330, 777, A350), D-check in progressive ramp-up, with tooling and personnel calibrated to the segment.

The siting logic relies on several elements:

• Favourable Saharan climate: low humidity, low precipitation, high solar exposure, naturally suitable for storage, repainting and long structural inspections.
• Geographical positioning: midway between Europe, sub-Saharan Africa and the Middle East, on transcontinental routes that make ferry flight repositioning to the shop economically viable.
• Runway availability: Tiaret has the airfield infrastructure to accommodate A330, 777 and A350.
• Workforce pool: the Algerian B1/B2 training network gradually enables the buildup of certified mechanic teams required for C and D-check operations.

The planned sequencing includes:

1. Start-up with A-check and C-check capacity on narrowbody (A320, 737).
2. Extension to widebody (A330, 777) as contract volumes ramp.
3. Progressive activation of D-check capacity, complemented by OEM partnerships for major structural modifications.
4. Integration of complementary streams (P2F, USM, recycling, long-term storage) that share infrastructure and amortise fixed costs.

AéroNéo’s ambition is not limited to offering low-cost C-checks. It aims to build an integrated heavy maintenance ecosystem capable of absorbing the full life cycle of a commercial aircraft on a single site. This vertical integration is what distinguishes reference MROs from one-off providers. It is also what justifies an initial investment in three hangars rather than one.

Understanding C-check and D-check visits, their intervals, scopes, durations and costs, is therefore the first step in decoding AéroNéo’s industrial trajectory and Algeria’s ambition to move up the global aerospace value chain. Heavy maintenance is no residual activity: it is a market worth tens of billions of dollars per year, structuring for any sovereign aerospace industry.