Aircraft Maintenance Checks and Programs

An airliner is never far from its next maintenance check. Every task that keeps it airworthy, from looking at a tire to inspecting a wing spar, is written into an aircraft maintenance program (AMP) with an interval in flight hours, flight cycles or calendar time. Operators group the tasks into checks: short ones done on the line between flights, such as the transit and daily checks, and longer ones done in a hangar, traditionally called A checks, C checks and, for the heaviest structural work, D checks. The letters are industry conventions. What the law requires is the program, and the intervals in it differ from type to type and from operator to operator.

Pilots meet this system at both ends: the commander reads the technical log for a valid release to service before each flight, and the crew's walkaround is the last look at the aircraft before it moves. This article covers the airline system under EASA rules, with the FAA's equivalents; airworthiness directives and the maintenance philosophies are covered in continuing airworthiness and maintenance.

The approved maintenance program

ICAO Annex 6, Part I, includes standards for the operator's maintenance program. In the EU each aircraft is maintained to an approved AMP under the continuing airworthiness regulation, Regulation (EU) No 1321/2014, and in commercial air transport the continuing airworthiness of the operator's aircraft, program included, is managed by an approved continuing airworthiness management organization (CAMO). Part-M applies above 2,730 kg maximum takeoff mass. Light aircraft at or below that mass generally follow the simpler Part-ML, under which the program may be declared rather than approved. Unlike the Annex, the EU rules for CAMOs do not require human factors principles in the program's design.

The program's core comes from the type certificate holder's instructions for continued airworthiness:

• The maintenance review board report (MRBR) sets the minimum scheduled tasks and intervals for the airframe, engines, systems and components. For the 737 MAX, the FAA describes the systems and powerplant tasks as developed from maintenance significant items through the MSG-3 analysis process, following its AC 121-22C.
• The manufacturer's maintenance planning document (MPD) gathers the inspection requirements for the operator's planning. Boeing's MPD for the 737 MAX lists the MRBR tasks and, in its section 9, the certification maintenance requirements.
• The airworthiness limitations are approved with the type design. For the A320 family, the data sheet lists safe-life airworthiness limitation items, damage-tolerant items, certification maintenance requirements (CMRs), system equipment maintenance requirements and fuel airworthiness limitations.

To these the operator adds the airworthiness directives that apply, its own tasks, and what its reliability data show. Each task carries its own interval, often as a pair of limits with the words "whichever occurs first", and components are handled as hard time, on-condition or condition monitoring items. Programs change with the aircraft: after Boeing changed the 737 MAX flight control computer software, a revised MSG-3 analysis added an operational check of the MCAS discrete, with an interval of 6,000 flight hours.

Line and base maintenance

European rules distinguish line maintenance from base maintenance, and for an airline both are done by organizations approved under Part-145. Line maintenance keeps the aircraft in service from day to day: the transit, daily and weekly checks, minor scheduled tasks and the rectification of the defects the crew report, usually on the apron between flights. Base maintenance is the heavier scheduled work, such as a C check, for which the aircraft is taken out of service. An operator may do its own maintenance only if it holds the Part-145 approval itself; otherwise it contracts the work to an approved organization.

Who may sign follows the same split. Under Part-66, a category A license lets a mechanic release minor scheduled line maintenance and simple defect rectification, limited to the tasks endorsed on the certification authorization and to work the holder has carried out personally. Category B1 covers structure, powerplant and mechanical and electrical systems, and B2 avionic and electrical systems. A category C engineer certifies scheduled base maintenance by issuing a single certificate of release to service for the complete aircraft, once B1 and B2 support staff have signed for the tasks in their specialties; the C engineer's main job is to make sure that all the required maintenance has been called up and signed for.

In the United States the work is done to 14 CFR Part 43 by certificated airframe and powerplant mechanics, by repair stations certificated under 14 CFR Part 145 and by manufacturers, while airlines maintain their fleets under their own programs.

Daily, weekly and A checks

The lightest scheduled checks are fitted around the flying. A transit check is made during a turnaround; the daily check, repeated at a short calendar interval set by the program, covers a little more; a weekly check adds items that change too slowly to look at every day. Typical tasks are a walk-round inspection for damage, leaks and missing parts, a look at tires, brakes and wheel wells, and servicing: tire pressures, engine oil, hydraulic fluid and accumulator pressures.

An A check is the first and most frequent of the letter checks. It is more thorough than a daily check but still short: general visual inspections of the airframe, engines and systems, lubrication, operational tests of systems, and a chance to clear deferred defects that need more time than a turnaround gives. Depending on its content and the operator's organization, an A check may be done at a line station or in a hangar. Some programs also have a B check, between the A and the C check in depth. Because each task has its own interval, the operator decides how to package the tasks into checks that suit its flying.

C checks and heavy maintenance

A C check is base maintenance. The aircraft is withdrawn from service and put into a hangar, panels, floor boards and linings are removed as the work cards require, and the airframe is inspected zone by zone, including structure and systems that cannot be reached on the line. Functional tests, lubrication, hard-time component changes, airworthiness directives, modifications and cabin work are usually planned into the same visit. The defects found are raised on non-routine work cards and rectified before release. The heaviest checks, often called D checks or heavy maintenance visits, go further into the structure, looking for fatigue cracking and corrosion where the damage-tolerant inspection tasks of the airworthiness limitations require.

A heavy check is only as good as what is done with its findings. Flight 261's last jackscrew end play check, at a C check in 1997, first found 0.040 in, the maximum allowed, and a non-routine work card called for a new nut; a repeat three days later found 0.033 in, and the aircraft returned to service without the end play ever being measured again.

Not every inspection is scheduled. After a heavy or overweight landing, severe turbulence, a lightning strike, a bird strike or a tail strike, the aircraft maintenance manual calls for a conditional inspection, and it is the crew's report that sets it off. After a 737 NG tail strike, the tail skid's cartridge warning placard is checked as soon as possible, before the skid fairing can re-extend under gravity and spoil the reading. Accidents also change programs: after a fan blade failed on Southwest Airlines flight 1380 in 2018, airworthiness directives required ultrasonic inspections of CFM56-7B fan blades, repeated from late 2018 every 1,600 cycles.

Pilot checks and the technical log

The flight crew's exterior inspection, the walkaround, is the last look at the aircraft before flight. Airbus gives its objective as a global assessment of the aircraft's status, with any missing part or panel checked against the configuration deviation list. On the A320 a short safety exterior inspection comes first, and after a transit stop without a crew change only its items marked with an asterisk are repeated. At the landing gear the crew check the tires for cuts, wear and cracks, that the safety pins are removed, the brake wear pins with the parking brake on and the strut extension, and report any difference between the two main gears (see pre-flight preparation and routine checks).

Before accepting the aircraft the commander reads the technical log: a valid certificate of release to service, the maintenance statement showing which scheduled maintenance is next due, and the deferred defects with their MEL conditions. The log is kept for 36 months after the last entry. On the A320 the crew also press the ECAM recall button for at least three seconds to see the cautions and warnings of the previous flight.

After the flight the crew are maintenance's main sensor. Under CAT.GEN.MPA.100, crew members report to the commander any fault, malfunction or incident that could affect safe operation, and defects go into the technical log. Some indications have set thresholds: on the A320, maintenance action is due after landing if, among other cases, one brake exceeds 900 °C or if the average brake temperatures of the left and right gears differ by 200 °C or more.

In the United States, 14 CFR 121.563 makes the pilot in command of an airliner ensure that every mechanical irregularity during flight time is entered in the maintenance log at the end of that flight time. In any aircraft, 14 CFR 91.7 leaves the decision whether it is in condition for safe flight to the pilot in command.

Certificate of release to service

No maintenance is finished until it is certified. The certificate of release to service (CRS) is the statement, signed by authorized certifying staff, that the work was carried out as required and that, for that work, the aircraft is ready for release to service. Part-145 sets out its content in AMC 145.A.50(b), and in an aircraft technical log an abbreviated certificate may be used instead of the full statement. Unlike the ICAO standard, the EU rule also requires the release to include any limitations to airworthiness or operations.

Critical tasks get a second look. Part-145 requires an error-capturing method for critical maintenance tasks, a requirement more exacting than ICAO's. The classic case, from the British requirements that ATPL texts still quote, is the duplicate inspection after any disturbance of the flight controls: a second qualified person inspects the system independently, including the operating sense of the controls, before flight.

The FAA's term is approval for return to service. Under 14 CFR 91.407, an aircraft that has undergone maintenance may not be operated until an authorized person has approved it for return to service and the maintenance record entry has been made. If the work may have appreciably changed its flight characteristics, it is test flown before it carries passengers.

FAA inspection programs

The FAA's general operating rules treat small aircraft differently. Under 14 CFR 91.409, every aircraft needs an annual inspection within the preceding 12 calendar months. One used to carry passengers for hire, or provided by an instructor for paid flight instruction, also needs a 100-hour inspection, which may be exceeded by no more than 10 hours to reach a place where it can be done, with the excess counted toward the next. An owner may instead use an approved progressive inspection program. The emergency locator transmitter has its own clock: under 91.207 it is inspected within 12 calendar months of the last inspection. In Europe, the yearly airworthiness review leading to the airworthiness review certificate (ARC) plays a comparable role for light aircraft.

The annual and 100-hour inspections do not apply to large airplanes (over 12,500 lb), turbojets or turbopropeller-powered multi-engine airplanes, which follow instead one of the inspection programs set out in 91.409(e) and (f). Airlines under Part 121 maintain their fleets under a continuous airworthiness maintenance program (CAMP), the counterpart of the EASA approved program; letter checks are packages within it, as in Europe.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.