Aircraft maintenance is the overhaul, repair, inspection or modification of an aircraft or aircraft component.

Maintenance includes the installation or removal of a component from an aircraft or aircraft sub assembly, but does not include:

Elementary work, such as removing and replacing tires, inspection plates, spark plugs, checking cylinder compression, etc servicing, such as refuelling, washing windows. 

Any work done on an aircraft or aircraft component as part of the manufacturing process, prior to issue of a certificate of airworthiness or other certification document

Maintenance may include such tasks as ensuring compliance with :

1.An airworthiness directive (commonly abbreviated as AD) is a notification to owners and operators of certified aircraft that a known safety deficiency  with a particular model of aircraft, engine, avionics or other system exists and must be corrected.

If a certified aircraft has outstanding airworthiness directives that have not been complied with, the aircraft is not considered airworthy.Thus, it is mandatory for an aircraft operator to comply with an AD.

ADs usually result from service difficulty reporting by operators or from the results of aircraft accident investigations. They are issued either by the national civil aviation authority of the country of aircraft manufacture or of aircraft registration. When ADs are issued by the country of registration they are almost always coordinated with the civil aviation authority of the country of manufacture to ensure that conflicting ADs are not issued.

In detail, the purpose an AD is to notify aircraft owners:

that the aircraft may have an unsafe condition, or

  • that the aircraft may not be in conformity with its type certificate or of other conditions that affect the aircraft's airworthiness, or
  • that there are mandatory actions that must be carried out to ensure continued safe operation, or
  • that, in some urgent cases, the aircraft must not be flown until a corrective action plan is designed and carried out
           i) Those of an emergency nature requiring immediate compliance prior to further flight, and
           ii) Those of a less urgent nature requiring compliance within a specified period of time.

ADs are mandatory in most jurisdictions and often contain dates or aircraft flying hours by which compliance must be completed.

ADs may be divided into two categories: