Everybody in aviation knows the letters. A check, B check, C check, D check, in rising order of seriousness, from an overnight once-over to the one where they take the aeroplane apart. It is one of the first things an enthusiast learns and one of the most quoted facts in the industry.

Two things about it are wrong. The first is that the B check has quietly ceased to exist. Qantas publishes its own explainer on the subject and calls it “The A, C and D of aircraft maintenance”, which rather gives the game away. The second is bigger: none of those four letters appears anywhere in the regulations. Not in 14 CFR Part 91, not in Part 43, not in Part 65, not in Part 25. They are a packaging convention the industry invented, and the industry is quietly unpacking them.

Meanwhile the money has moved somewhere else entirely. Global spending on maintenance hit a record in 2024, and the share of it going on airframe heavy maintenance — the letter checks — fell by a third in five years.

The letters are an invention, and a useful one

Go looking for a C check in the Federal Aviation Regulations and you will not find it. 14 CFR 91.409(e) requires that large aeroplanes, turbojet and turboprop multi-engine aeroplanes and turbine-powered rotorcraft be inspected under a programme chosen from the list in 91.409(f): a continuous airworthiness inspection programme, an approved aircraft inspection programme under Part 135, a current inspection programme recommended by the manufacturer, or a programme the operator has had approved. That is the whole legal requirement. No letters.

What the letters actually describe is how an operator chooses to bundle the thousands of individual tasks that programme contains. Each task has a usage parameter attached to it — so many flight hours, so many cycles, so many calendar months — derived from an MSG-3 analysis and written into the type’s Maintenance Review Board Report. Appendix H to Part 25 is the regulation that makes the manufacturer produce the underlying Instructions for Continued Airworthiness in the first place.

Grouping those tasks into named packages is an engineering and scheduling convenience, not a legal category. You pick a repeating slot — one night, one weekend, three weeks — and you fill it with every task that has come due. Call the small one an A and the big one a D and everybody in the hangar knows roughly what is being asked of them.

Which also means the letters can be redefined whenever the packaging logic changes. And it has.

Where the B check went

The B check used to sit in the gap: every six to eight months, a day to three days in a hangar, a few hundred man-hours of work too big for an overnight and too small to justify pulling the aircraft out of the schedule for weeks. It made sense when maintenance intervals were set by a committee looking at the whole aeroplane and when an A check could not absorb anything extra.

Task-based scheduling killed it. Once every task carries its own interval rather than belonging to a tier, the natural move is to slide B-check tasks into successive A checks and never stop the aircraft for a day at all. The aeroplane keeps flying, the work still gets done, and the operator buys back the downtime. Qantas describes exactly this: the B check was absorbed into the other checks on more modern aircraft.

That one sentence is a better description of how a modern airline is maintained than the whole alphabet. Twelve hours a week, every week, in the gaps between flights — and the heavy events arriving less often than the letters imply. Qantas keeps more than 3,300 engineering staff on a fleet of over 200 aircraft to do it.

The D check has been drifting the same way, just more slowly. Qantas notes it is also called a C4 or a C8 depending on type — that is, the fourth or eighth C check in a cycle, rather than a separate species of event at all. On the 787 the interval is twelve years rather than six. Lufthansa Technik and other large MROs increasingly spread what used to be a single monumental D across several C visits.

The canonical film on the subject: a British Airways 747-400 through a full D check, from the BBC’s Engineering Giants. Nearly ten million people have watched it, and it remains the clearest hour anyone has filmed on what heavy maintenance actually involves.

What a D check costs, and why the numbers disagree

Qantas says several million dollars per check and three to six weeks out of service. Published industry figures from 2018, compiled by Aircraft Value News, put actual D-check costs at roughly USD 1.0 million for a 737-800, 4.0 million for a 777-200ER, 4.5 million for a 777-300ER and 6.0 million for a 747-400 — and in every one of those cases the actual came in above the estimate, by between 50 and 70 per cent.

You will also see much larger numbers quoted: up to 50,000 man-hours and six months to a year of downtime. Those are not wrong so much as they are the far end of the distribution — an old widebody, a heavy corrosion finding, a cabin reconfiguration bolted onto the same visit. A modern narrowbody in a planned slot at a competent MRO does not look like that. When sources disagree this widely it is usually because they are describing different aircraft in different conditions, and anyone quoting a single figure for “a D check” is flattening something real.

The economics that matter are not the invoice, though. They are the downtime. An airliner earns nothing in a hangar, and three weeks of a narrowbody’s schedule is worth a great deal more than the labour bill for the check. That is why task-based packaging won, why the B check died, and why an MRO will sell you a shorter visit at a higher day rate and find customers.

Full access to a Boeing 777 going through deep maintenance, with the cabin out and the panels off.

It is also why the D check is so often the moment an airframe is retired instead. An aircraft coming to the end of a lease, facing a seven-figure heavy check and a market that will not pay for the capacity, is worth more as parts. Published work suggests the average airliner goes through two or three D checks in its life, which means the third one is frequently a decision rather than a maintenance event.

The letter checks are a shrinking share of the bill

Here is the figure that reframes the whole subject. IATA publishes an annual Maintenance Cost data eXchange report; the public edition for FY2024 carries Oliver Wyman’s breakdown of global maintenance, repair and overhaul spending. Total MRO spend reached USD 104 billion in 2024, 11.7 per cent above 2019 and a record. It is now 11.5 per cent of everything the world’s airlines spend, up from 10 per cent five years earlier.

But the segments moved in opposite directions, and airframe maintenance lost.

Engines went from 41 per cent of the bill to 50; components from 17 to 18. Both of those track utilisation, and the world flew hard after 2021. Airframe work does not track utilisation in the same way, because it is driven by calendar intervals, fleet age and how an operator chooses to manage its aircraft — and those are precisely the things that task-based packaging, longer intervals and newer airframes have been pushing downwards.

There is a concentration in there too. Widebodies are 20.3 per cent of the active fleet and 42 per cent of all MRO spending. The aeroplanes most people fly on are not where the money goes.

A full-length look inside heavy maintenance, with over five million views. It is the best illustration of what the 17 per cent actually buys.

So is the alphabet obsolete?

Not quite, and the reason is human rather than technical. A planner needs a word for the three-week event and a different word for the overnight one. Lessors write residual-value assumptions around heavy-check cycles. MRO contracts are priced per visit. Engineers hand over shifts in a shared vocabulary. The letters survive because they are good shorthand, which is what they always were.

What has gone is the idea that they are a hierarchy an aeroplane climbs. Underneath the labels there is now a list of tasks, each with its own clock, packaged into whatever slots make commercial sense for that operator on that fleet. One airline’s C check is another’s pair of heavy A checks. The B vanished because nothing in the system needed it to exist once the tasks stopped belonging to tiers.

Which is the quiet lesson in the IATA numbers. The letter checks are not becoming less rigorous. Every task still gets done, still on its interval, still under an inspection programme the regulator approved. They are becoming less visible — dissolved into the overnight routine, spread across visits, and steadily outweighed by engines. The alphabet is still on the whiteboard. It just no longer describes how the aeroplane is actually being looked after.

Sources: Qantas Newsroom, “The A, C and D of aircraft maintenance”; IATA Maintenance Cost data eXchange FY2024 Executive Report, public version, with Oliver Wyman MRO market data; 14 CFR 91.409(e) and (f) and Appendix H to 14 CFR Part 25, read in the eCFR as in force 1 October 2026; FAA AC 121-22C on Maintenance Review Boards; Aircraft Value News D-check cost survey, 2018; Kinnison and Siddiqui, Aviation Maintenance Management.