Aircraft maintenance checks

Aircraft maintenance checks are periodic inspections that have to be done on all commercial/civil aircraft after a certain amount of time or usage. Airlines and other commercial operators follow a continuous inspection program approved by the Federal Aviation Administration (FAA) in the United States or by other airworthiness authorities such as the European Aviation Safety Agency (EASA). The maintenance program includes both routine and detailed inspections. Airlines and airworthiness authorities casually refer to the detailed inspections as "checks", commonly one of the following: A-check, B-check, C-check or D-check. A- and B-checks are lighter while C- and D- are considered heavier checks.

The maintenance of aircraft is differentiated into line maintenance and overhaul (base maintenance). The work that has to be done in the short term is called line maintenance. This means that the aircraft keeps to its usual schedule. The daily, weekly and monthly checks are carried out overnight, the next morning the aircraft goes back into scheduled service. Merely the C-Check that is carried out about every one and a half years takes somewhat more than one day. For an overhaul, let it be an IL (intermediate layover check) or D-Check, the aircraft is taken out of service for several weeks.

Pre-flight Check / Trip Check:

The lowest-level maintenance event is the pre-flight check that precedes every flight and involves a visual inspection of the aircraft by the cockpit crew (and mechanics if present) at the gate. The aim of this check is to detect obvious external damage, wear and oil or hydraulic leaks. Depending on the aircraft type it lasts for 15 to 60 minutes.


The next maintenance event in the hierarchy is the ramp check, in which mechanics test individual functions of the aircraft, check tire pressure and brakes and replenish fluids such as hydraulic, oil and water. A visual inspection of the aircraft is also carried out, both externally and in the cabin. Such a check, carried out on a daily basis, requires between 6 and 35 man-hours.

Next in size is the weekly service check, a combination of the work performed in the ramp check with tasks such as topping up the water, air and oil and thorough cleaning of the cabin, which takes between 10 to 55 man-hours.


This is performed approximately every one to two months, after 300-600 flight hours or 200–300 cycles, depending on aircraft type. (Maintenance checks are the same for long haul and short haul aircraft because the maintenance schedule is based on airframe hours and landings (known as cycles). Aircraft flying several short trips a day may have maintenance due earlier than aircraft flying long-haul because they have more cycles attributed to them.)

As well as general inspections of the interior and the aircraft hull, it also covers service checks as well as engine and function checks. At the same time the technicians replenish consumables such as oil, water and air and eliminate defects whose rectification has been postponed on the grounds that they did not impair flight safety. If any extensive seat repairs are required, these are also carried out in this interval inspection. It needs about 20–60 man-hours and is usually performed overnight at an airport gate. The occurrence can be delayed by the airline if certain predetermined conditions are met.

Examples of A-check items include:

• General external visual inspection of aircraft structure for evidence of damage, deformation, corrosion, missing parts
• Check crew oxygen system pressure
• Operationally check emergency lights
• Lubricate nose gear retract actuator
• Check parking brake accumulator pressure
• Perform Built-in Test Equipment (BITE) test of Flap/Slat Electronics Unit


Contemporary maintenance programs do not use this interval. The B-check, which comes into play only in some types of aircraft such as the Boeing 747-200 or the B737-200, corresponds to an enhanced A-Check with a slightly more detailed check of components and systems. Special equipment and tests may be required but it does not involve, however, detailed disassembly or removal of components. It needs about 120-150 man-hours, depending on the aircraft, and is usually completed within a day at an airport hangar. This kind of check is performed approximately every 5-8 months or after 1000 hours of flight time. A similar occurrence schedule applies to the B-check as to the A-check. However, B-checks may also be incorporated into successive A-checks, i.e.: Checks A-1 through A-10 complete all the B-check items.


The following two checks are traditionally known as heavy checks. They are normally accomplished at the main maintenance base of the airline where specialized manpower, materials, tooling, and hangar facilities are available.


C-Check on Crystal Holidays' leased charter B737 
This is an intensive review carried out approx. every 18–24 months or after about 5000 hours of flight time. The schedule of occurrence has many factors and components thus varies by aircraft category and type. It is a high-level check that involves extensive tooling, test equipment, and special skill levels. This maintenance check requires a large majority of the aircraft's components to be inspected therefore the aircraft will be partially disassembled in a hangar at a maintenance base. This entails thorough inspections inside and outside, along with meticulous examination of structures (load-bearing components on the fuselage and wings) and functions. The time needed to complete such a check is generally up to five days and the effort involved can require up to 1500 and 2000 man-hours.

The C-check includes the lower checks, i.e. A-, B-, and Daily checks.

Examples of C-check items:

• Visually check flight compartment escape ropes for condition and security
• Check operation of DC bus tie control unit
• Visually check the condition of entry door seals
• Operationally check flap asymmetry system
• Pressure decay check APU fuel line shroud
• Inspect engine inlet TAI ducting for cracks
• Operationally check RAT deployment and system

IL-Check (Intermediate Layover Check):

The classical IL-Check [or HMV (heavy maintenance visit)] is carried out every four to six years, about halfway between the D-Checks and lasts between 2 to 4 weeks. Recently lost its significance as the lifespan and reliability of the systems are continually improving. Therefore it is now possible to distribute the elements of the IL-Check over several C-Checks or to delay it to the next D-Check.

To obtain easier access to the fuselage and wing structure for inspection purposes, a number of large assemblies such as the high-lift devices are dismantled. At the same time numerous items of equipment and systems are tested and repaired as necessary. Cabin components such as seats, galleys and toilets are also completely overhauled and, if necessary, the aircraft will be repainted.


This check can also be referred to as Structural check which is due every 6 to 10 years or after about 25.000 hours of flight time.

Completely empty cabin: wall and ceiling panels all removed
During a D-check the entire structure is inspected down to the smallest detail. The engines, the landing gears and the high-lift devices are dismantled, along with the cabin interior and the wall and ceiling panels. The instruments, the electrical systems, the electronics, hydraulic and pneumatic equipment is also removed. All equipment dismantled and taken apart is closely scrutinized and any necessary repair work is carried out.

Every component is given a docket that states when and whereto it has to be delivered in a serviceable condition in order for the aircraft to be finished on time. Once the aircraft has been stripped to its skeleton, the outer skin with ten of thousands rivets and the load-bearing structure of the aircraft can be scrutinized for damage and corrosion. Here, non-destructive testing methods such as eddy-current, ultrasonic and x-raying are used to detect the tiniest hairline cracks in the aircraft structure or its retaining elements.

Given the time requirements of this check, many airlines use the opportunity in order to also make major cabin modifications on the aircraft, which would otherwise require an amount of time that would have to put the aircraft out of service without the need for an inspection. This may include new seats, entertainment system, carpeting, etc.

Such a check can generally take up to 50.000 man-hours and 4-6 weeks to complete, depending on the aircraft and the number of technicians involved. It also requires the most space of all maintenance checks, and as such must be performed at a suitable maintenance base.

Celebrating a succesful D-check on a B747 at HAITEC
The requirements and the tremendous effort involved in this maintenance check make it by far the most expensive with a total cost up to 8 to 10 million dollars. Because of the nature and cost of such a check, most airlines — especially those with a large fleet — have to plan D-checks for their aircraft years in advance. Often older aircraft being phased out of a particular airline's fleet are either stored or scrapped upon reaching their next D-check due to the high costs involved in comparison to the aircraft's value. On average a commercial aircraft undergoes three D-checks before being retired.

The D-check includes the lower checks, i.e. A-, B-, C- and Daily checks.

Examples of D-check items include:

• Inspect stabilizer attach bolts
• Inspect floor beams
• Detailed inspection of wing box structure

Monarch MRO

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