Continuing Airworthiness and Maintenance

An aircraft leaves the factory airworthy: it conforms to its type design and is in a condition for safe operation. Continuing airworthiness is everything that keeps it so for the rest of its life. Parts wear, structure fatigues and corrodes, defects appear, and experience across the fleet reveals problems the designers did not foresee. Scheduled maintenance, inspections, mandatory corrective actions and faithful records together decide whether the certificate of airworthiness remains valid on the day of each flight.

Pilots meet continuing airworthiness at both ends of their careers. A private owner must know which inspections are due, what maintenance the rules allow a pilot to do, and how to read the aircraft's records; an airline commander checks the technical log and the release to service before every departure. The certificates themselves are described in aircraft certificates and flight permits, and the technical log and the MEL in technical log, MEL and deferred defects.

Continuing Airworthiness

Responsibility is set out in law. In the EU, continuing airworthiness is regulated by Regulation (EU) No 1321/2014. The owner of an aircraft is responsible for its continuing airworthiness, and may contract the management tasks to an approved continuing airworthiness management organisation (CAMO). In commercial air transport the operator, not the individual owner, is responsible, and the continuing airworthiness of its aircraft is managed by an approved CAMO. In the United States, 14 CFR 91.403 makes the owner or operator primarily responsible for maintaining the aircraft in an airworthy condition, including compliance with airworthiness directives.

The pilot in command has a separate and personal duty. Under 14 CFR 91.7 the PIC decides whether the aircraft is in condition for safe flight and must discontinue the flight when an unairworthy mechanical, electrical or structural condition arises. An inspection signed off last month proves the aircraft was airworthy then, not today, and "the company told me to fly it" is no defence.

Airworthiness Directives

An airworthiness directive (AD) is a mandatory instruction issued when an unsafe condition has been found in an aircraft, engine, propeller, part or appliance and is likely to exist or develop in others of the same design. It states the aircraft or parts affected, the inspection, modification or replacement required, and the time or flight hours within which it must be done. Some ADs are one-off actions; others are repetitive inspections with a recurring interval.

EASA issues or adopts ADs for aircraft on the registers of its Member States, and the FAA issues them under 14 CFR Part 39. An AD takes precedence over the MEL where they conflict. Under 14 CFR 91.417(a)(2)(v) the maintenance records must show the current status of applicable ADs, including the method of compliance, the AD number and revision date and, for recurring ADs, when the next action is due. A pilot checking whether ADs have been complied with does so by reviewing those records, and a European airworthiness review examines AD compliance before an ARC is issued.

Maintenance Programme Philosophies

Each aircraft is maintained to a maintenance programme that lists its tasks and their intervals. ATPL texts distinguish three ways of deciding when a component is dealt with:

Hard time is the most conservative and the most wasteful, because serviceable parts are discarded. On-condition maintenance is less conservative but depends on reliable inspection, as does engine condition monitoring, which tracks trends in engine parameters flight after flight.

Annual and 100-Hour Inspections

For light aircraft under FAA rules the backbone of the programme is set by 14 CFR 91.409:

• The annual inspection is required for every aircraft within the preceding 12 calendar months. It is performed to the scope of Part 43 Appendix D by a mechanic holding an inspection authorization (IA). There is no grace period: an aircraft whose annual has expired may be flown to a maintenance base only under a special flight permit issued under 14 CFR 21.197.
• The 100-hour inspection is required in addition when the aircraft carries any person other than a crewmember for hire, or is provided by a flight instructor for paid flight instruction. A certificated airframe and powerplant (A&P) mechanic may perform it. The 100 hours may be exceeded by up to 10 hours only to reach a place where the inspection can be done, and the excess counts towards the next interval: an aircraft due at 2,350.0 hours remains due next at 2,450.0 however late it arrived.

An annual inspection satisfies the 100-hour requirement, but a 100-hour inspection does not replace the annual. An owner may instead use an approved progressive inspection programme, and large aeroplanes and turbojet or turboprop multi-engine aeroplanes follow one of the inspection programmes of 91.409(e) and (f).

In Europe, light aircraft follow the maintenance programme approved or declared for each aircraft, and the yearly airworthiness review leading to the airworthiness review certificate (ARC) plays the role the FAA gives to the annual inspection.

Altimeter and Transponder Checks

Two FAA inspections are tied to instrument flight and to the transponder. 14 CFR 91.411 allows IFR flight in controlled airspace only if each static pressure system, altimeter and automatic pressure altitude reporting system has been tested and inspected within the preceding 24 calendar months, to Part 43 Appendix E. This altimeter and static system inspection is commonly done at the same visit as the transponder test. The static system must also be retested after it has been opened and closed, unless the only work was draining the system or using the alternate static source (see pitot-static system).

14 CFR 91.413 applies the same 24-calendar-month interval to the ATC transponder, to the tests of Part 43 Appendix F, and it applies to any use of the transponder, VFR as well as IFR.

Preventive Maintenance by Pilots

14 CFR Part 43 (Maintenance) defines who may maintain an aircraft. Section 43.3(g) allows the holder of a pilot certificate issued under Part 61 to perform preventive maintenance on an aircraft that pilot owns or operates, provided it is not used under Parts 121, 129 or 135. The tasks are those listed in Part 43 Appendix A, paragraph (c): simple, preservative work such as changing oil, replacing tyres, servicing wheel bearings, replacing safety belts, and cleaning, gapping and replacing spark plugs. Overhauling a magneto, replacing a cylinder or adjusting control cable tension is not on the list and needs a mechanic.

Every such task needs a maintenance record entry under 43.9: a description of the work, the date of completion, and the pilot's name, signature, certificate number and kind of certificate.

In Europe, Part-ML, the simplified continuing airworthiness rules for light aircraft, allows a pilot-owner holding a valid pilot licence to carry out a limited list of simple maintenance tasks on the aircraft and to sign the certificate of release to service for them.

Maintenance Organisations and Manuals

In commercial air transport, maintenance is carried out by an organisation approved under Part-145, the Part-145 approved maintenance organisation. An operator may do its own maintenance only if it holds the approval itself. Within the organisation, only authorised certifying staff may sign the release to service. The FAA's counterpart for maintenance organisations is the repair station certificated under 14 CFR Part 145.

The work is done to the manufacturer's data. The aircraft maintenance manual (AMM) gives the procedures, limits, tests and servicing specifications for each system, and related manuals cover structural repairs, component overhaul and illustrated parts. Pilots meet the AMM indirectly: after a heavy or overweight landing, a severe turbulence encounter or a tail strike, it is the AMM that specifies the conditional inspection before the aircraft flies again.

Duplicate Inspections and Functional Checks

Some errors are too dangerous to leave to one pair of eyes. British airworthiness requirements, which ATPL texts still quote, define a vital point as any point on an aircraft where a single incorrect assembly could lead to catastrophe, such as control rod end fittings, control cable terminations and engine controls. After any disturbance of a flight control system, whether adjustment, repair, replacement, overhaul or modification, a duplicate inspection is required before flight: one qualified person inspects and certifies the system, then a second inspects it independently and certifies. Nothing may be disturbed between the two, and anything disturbed afterwards is inspected in duplicate again.

The checks include cable tension, the safety and locking of turnbuckles, pins and fittings, the range and freedom of movement, friction and backlash, and above all the operating sense check: someone moves the controls while an observer confirms that each surface moves in the correct direction. Controls rigged in reverse have caused accidents, which is why pilots also check full and free movement, in the correct sense, before every flight.

Systems are proved by functional tests. A pressurised aircraft's cabin receives an initial proof pressure test before delivery, and the pressurisation leak rate test is repeated at the intervals in the maintenance manual, after an actual or suspected malfunction, and after repairs or modifications to the pressure hull.

Some work needs proof in the air. Under 14 CFR 91.407(b), an aircraft whose maintenance may have appreciably changed its flight characteristics or substantially affected its operation in flight must be test flown, by a pilot holding at least a private pilot certificate, before it carries passengers. This maintenance test flight carries no passengers.

Release to Service and Records

No maintenance is complete until it is certified. In Europe the certificate of release to service (CRS), also called the maintenance release, is signed by certifying staff to state that the work was done as required; in airline operations it is entered in the technical log, and the commander checks for a valid release before accepting the aircraft. Under 14 CFR 91.407(a) the FAA's equivalent is return to service: an aircraft that has undergone maintenance may not be operated until an authorised person has approved it for return to service and the maintenance record entry has been made.

Maintenance records are the aircraft's history. Under 14 CFR 91.417 they fall into two groups. Records of individual maintenance and inspections are kept until the work is repeated or superseded, or for one year. The status records are kept and transferred with the aircraft when it is sold: total time in service of the airframe, engines and propellers, the status of life-limited parts, the time since the last overhaul of items overhauled on a time basis, the current inspection status, the status of ADs, and records of major alterations.

Time in Service and TBO

For maintenance records, 14 CFR 1.1 defines time in service as the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing. Many light aircraft record it in practice with the tachometer time of the engine's hour meter. The tach meter counts engine revolutions against a reference rpm near cruise power, so it runs at roughly clock speed in cruise and more slowly during taxi, descent and idle. Rental is often charged on a separate Hobbs meter, which records elapsed clock time, typically while the engine is running, and so reads more than the tachometer.

Time between overhauls (TBO) is the engine manufacturer's recommended interval between major overhauls. For aircraft operated privately under Part 91 it is a recommendation, not a limit, and the engine's condition, shown by inspections and trend monitoring, decides its airworthiness. Commercial operators follow the intervals in their approved maintenance programme.

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.