Aircraft maintenance explained: line maintenance, A-check, C-check and D-check

A Lufthansa Airbus that doesn’t show up on any flight tracker for three weeks. A leisure airline aircraft standing in front of a hangar without engines. A jet with a gray rudder that returns to its route after a short ferry flight. Behind observations like these there is almost always the same cause: aircraft maintenance. This guide explains how the maintenance of an airliner is structured – from line maintenance between two flights through the A-check and C-check to the heavy overhaul in the D-check – and what of it you actually get to see at the fence.

Aircraft maintenance: the short answer

The maintenance of an airliner is a tiered system. At the bottom is line maintenance: walkarounds and short checks that accompany daily flight operations and are mostly done on the apron or at night. Above that are the hangar checks, so-called base maintenance: the A-check comes often and is done quickly, the C-check less often and much more thoroughly, and the D-check is the heavy overhaul in which the aircraft is almost completely taken apart.

The higher the tier, the longer the aircraft sits, the more is opened and the less often the check comes. Exactly how often is not set in a general law but in the maintenance program of the respective aircraft type, which manufacturer, airline and aviation authority define together.

How a maintenance program comes about

Before a new type enters service, it is defined which components have to be inspected, lubricated, tested or replaced, and when. The basis for this on modern aircraft is the MSG-3 method, developed by manufacturers, airlines and authorities. It derives from safety and failure analyses which tasks are necessary and assigns an interval to each individual task – in flight hours, in flight cycles (one takeoff plus one landing) or in calendar time.

The result ends up in the Maintenance Review Board Report and in the manufacturer’s maintenance planning document. From this, each airline builds its own maintenance program, approved by the authorities. The letters A, C and D are really just packages: tasks with similar intervals are bundled so that the aircraft doesn’t have to go into the hangar separately for every little thing.

Why this is interesting for you as a spotter: a short-haul jet accumulates many cycles with few flight hours, a long-haul jet many hours with few cycles. That is why they age differently. An Airbus A320neo on European routes stresses landing gear and fuselage with many pressurization cycles a day, a long-haul aircraft rather engines and systems through long running times.

Line maintenance: maintenance between flights

Line maintenance is the part of maintenance that you can actually watch at the airport. It begins with every flight.

• Walkaround before the flight: Before departure, a crew member or a technician walks once around the aircraft. Tires, brakes, landing gear, engine air inlets, probes, antennas, flaps and possible damage from bird strikes or ground equipment are checked.
• Transit check: Between two rotations, a technician looks specifically at points the program prescribes and works through reported defects from the technical logbook.
• Daily and weekly check: At fixed intervals, more extensive inspections are added: fluid levels, oil, hydraulics, emergency equipment in the cabin, lights and functional tests. Depending on the program, these intervals are regulated differently.
• Defect rectification: A tire is worn down, a reading light is broken, a sensor reports a fault. Much of this is replaced between two flights, often within a few hours.

If a defect cannot be fixed right away, the Minimum Equipment List (MEL) regulates whether and under which conditions the aircraft may still fly. For each type, it states which systems may be temporarily inoperative and which may not.

Many of these jobs run in parallel with ground handling, as you know it from the article on the turnaround at the airport. Airlines shift larger line work to the night, when there is no flying anyway – at many German airports also because of the operating restrictions explained in the guide to the night flight ban.

The hangar checks at a glance

In the industry, the big checks are named by letters. The following table shows the usual orders of magnitude. Important: these are guideline values from technical literature and reference works. The specific interval of an aircraft depends on the type, usage and the airline’s approved program.

Check	Typical interval	Typical duration	Where	What happens
A-check	every few hundred flight hours or every few months	usually one night	hangar at the home base	systems, landing gear, engines from the outside, cabin, lubrication, filters
B-check	formerly every few months	one to three days	hangar	integrated into the A-checks on modern types
C-check	roughly every two to three years	one to several weeks	maintenance base or MRO facility	structure, corrosion, cabin, extensive system tests
D-check	roughly every six to twelve years	several weeks to months	specialized shop	heavy overhaul down to the structure, often with paint stripping

A-check

The A-check is the smallest check in the hangar. For the A320 family, the literature cites intervals of 750 flight hours, 750 cycles or four months – whichever comes first. For other types, the value differs. According to common figures, an A-check comprises a few dozen labor hours and is planned to fit into one night. So ideally the aircraft is missing from no rotation.

Among other things, landing gear and brakes, fluid levels, filters, the engines from the outside, the cabin with seats, emergency equipment and lavatories as well as control surfaces and their actuators are checked. On top of that come all tasks whose interval happens to be due.

B-check

You find the B-check today mainly in older descriptions. On modern types, its tasks are spread across several A-checks. When an airline speaks of a “B-check” today, it is more internal planning language than a standalone package.

C-check

The C-check is the big routine check. For the A320 family, the literature gives intervals of up to 36 months or a few thousand cycles; for the Boeing 737-800 and other types, their own values apply. In general, around 20 to 24 months are often cited for the C-check. For this, the aircraft sits in a large hangar for one to several weeks, and the effort is a few thousand labor hours.

Now things get opened: floor panels in the cabin, panels on wings and fuselage, inspection hatches in the cargo hold. Technicians look for corrosion, cracks and wear on the structure, inspect cables and lines, test systems extensively and replace parts with fixed service lives. Because the aircraft is out for a long time anyway, airlines like to use C-checks for modifications: new seats, cabin Wi-Fi, software updates or a repaint.

D-check

The D-check, also called a heavy maintenance visit, is the heavy overhaul. Depending on the type, it comes every six to twelve years. The aircraft is taken apart so far that the primary structure is accessible: cabin, insulation and many systems are removed, and the paint is often stripped so that inspectors can examine the skin without gaps. The effort is many tens of thousands of labor hours, the costs in the millions.

Because a D-check is so expensive, a fundamental decision is often made at this point: is the overhaul worth it, or will the aircraft be retired? Many aircraft go through only two or three D-checks in their lives. If the next one is due and the aircraft is already older, it is a classic moment for it to go back to a lessor, be sold or be ferried to an aircraft boneyard.

In blocks instead of one big piece

The classic picture of a C-check every two years and a D-check every ten years only holds to a limited extent today. Since MSG-3, every task has its own interval, and many airlines therefore plan on a rolling basis: tasks are distributed across A- and C-checks so that no single downtime becomes excessively long. Some operators split the big check into several smaller blocks, others move structural tasks from the D-check into individual C-checks.

For you this means: a “D-check” is not always a clearly recognizable event with a half-year break. Instead, you often see an aircraft that disappears for a few weeks every two years and comes back a little different each time.

Engines and components: cycles of their own

Not everything is maintained on the aircraft itself. Engines run in a maintenance cycle of their own: they are removed according to running time, cycles or condition, overhauled in specialized engine shops and replaced by spare engines. That is why an aircraft sometimes stands at the gate with a foreign engine, or with an engine whose cowling has a different color than the fleet. How to tell engine types apart by inlet, nozzle and cowling is explained in the guide Identifying engines.

It is similar with components like landing gear, auxiliary power units (APU), brakes or avionics units. They have their own service lives and are swapped for overhauled parts. The aircraft thus returns to service while the removed part is overhauled in the workshop.

Who may maintain? Part-145 and Part-66

In the EU, EASA determines who may work on an airliner and release it to service again. There are two terms you should know:

• Part-145 is the approval for maintenance organizations. Only an organization with this approval may carry out and release maintenance on commercially operated airliners.
• Part-66 is the license for certifying staff. At the end of every job there is a certificate of release to service, which may only be signed by someone with the appropriate license and type rating.

Part-66 category	What it stands for
A	minor scheduled line work and simple defect rectification
B1	structure, engine as well as mechanical and electrical systems
B2	avionics and electrics
B3	structure and systems of small aircraft without a pressurized cabin up to 2,000 kg
C	release after base maintenance, i.e. the big hangar checks

Above everything stands the Continuing Airworthiness Management Organisation (CAMO). It plans when which check is due, manages the life histories of components and makes sure the program is followed. If you are interested in the profession, you will find an entry point in the article on studying aerospace engineering.

Where maintenance happens

Line maintenance takes place wherever an airline flies regularly – either by its own technicians or by contracted service providers. For A-checks, a hangar at the home base is usually enough. C- and D-checks, on the other hand, move to large maintenance companies, so-called MRO facilities (maintenance, repair and overhaul).

In Germany, Lufthansa Technik, headquartered at Hamburg Airport, is the best-known name. Another major site is at Frankfurt Airport, with maintenance hangars for the long-haul fleet and a hall for the overhaul of CFM engines. Many airlines, however, have their big checks done at maintenance companies abroad, depending on where hangar capacity and price fit. An overview of interesting shop locations and how to spot there can be found in the guide Maintenance companies and shops in Europe.

What you see of maintenance as a spotter

Maintenance takes place mostly behind hangar doors. Still, it leaves traces that you will recognize with a little practice.

Particularly exciting are aircraft that go into the shop for return after a lease ends. There they are prepared for the next operator, often with neutral white or a mixed paint scheme. How this is connected is explained in the articles on aircraft leasing and hybrid liveries. If you want to know how many checks an aircraft is likely to have behind it, a look at its year of manufacture helps – the guide Finding out an aircraft’s age shows you how.

What to watch out for

• Don’t expect fixed numbers. Intervals differ between types, airlines and even individual aircraft of the same fleet. Guideline values help with orientation but don’t replace a program.
• Maintenance areas are security areas. Hangars and parking areas are behind the fence. Photograph only from public areas; shop grounds are not a spot. One legal way to get closer is organized apron tours and airport tours.
• Mind the engine runs. After engine work, engines run at special positions at high power – loud and sometimes for hours. That is exciting for photos and sound recordings, but falls into the category of lucky hits.
• Note registrations. Shop visitors are often rarities. If you record registration and date, you can later work out which check was running.

Conclusion

Aircraft maintenance is not a single event but a continuous cycle. Walkarounds and line checks keep the aircraft flightworthy in daily operations, A-checks handle the frequent routine tasks in one night, C-checks go into the structure every few years, and the D-check puts the aircraft thoroughly to the test after many years. The exact intervals are set by the program of the respective type, not by a rule of thumb. For you as a spotter, the knowledge is worthwhile anyway: if you understand why an aircraft disappears and comes back changed, you discover rarities at shop airports that others miss.